patch, 5 partsCreate 'expire' and 'repack' verbs for git-multi-pack-index

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..822d83c845 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,14 @@ verify::
 	When given as the verb, verify the contents of the MIDX file
 	at `<dir>/packs/multi-pack-index`.
 
+expire::
+	When given as the verb, delete the pack-files that are tracked
+	by the MIDX file at `<dir>/packs/multi-pack-index` but have
+	no objects referenced by the MIDX. All objects in these pack-
+	files have another copy in a more-recently modified pack-file.
+	Rewrite the MIDX file afterward to remove all references to
+	these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index fca70f8e4f..145de3a46c 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,7 +5,7 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -44,6 +44,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 730ff84dff..bb825ef816 100644
--- a/midx.c
+++ b/midx.c
@@ -1025,3 +1025,8 @@ int verify_midx_file(const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 774f652530..e3a2b740b5 100644
--- a/midx.h
+++ b/midx.h
@@ -49,6 +49,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
+int expire_midx_packs(const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 70926b5bc0..948effc1ee 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -348,4 +348,51 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 3bd7183a53..50e4cd7270 100644
--- a/midx.c
+++ b/midx.c
@@ -751,7 +751,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -765,6 +766,7 @@ int write_midx_file(const char *object_dir)
 	uint32_t nr_entries, num_large_offsets = 0;
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -773,7 +775,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
@@ -787,7 +792,24 @@ int write_midx_file(const char *object_dir)
 	ALLOC_ARRAY(packs.perm, packs.alloc_perm);
 
 	if (packs.m) {
+		int drop_index = 0, missing_drops = 0;
 		for (i = 0; i < packs.m->num_packs; i++) {
+			if (packs_to_drop && drop_index < packs_to_drop->nr) {
+				int cmp = strcmp(packs.m->pack_names[i],
+						 packs_to_drop->items[drop_index].string);
+
+				if (!cmp) {
+					drop_index++;
+					continue;
+				} else if (cmp > 0) {
+					error(_("did not see pack-file %s to drop"),
+					      packs_to_drop->items[drop_index].string);
+					drop_index++;
+					i--;
+					missing_drops++;
+				}
+			}
+
 			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
 			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
 			ALLOC_GROW(packs.perm, packs.nr + 1, packs.alloc_perm);
@@ -798,6 +820,12 @@ int write_midx_file(const char *object_dir)
 			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
 			packs.nr++;
 		}
+
+		if (packs_to_drop && (drop_index < packs_to_drop->nr || missing_drops)) {
+			error(_("did not see all pack-files to drop"));
+			result = 1;
+			goto cleanup;
+		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
@@ -932,7 +960,12 @@ cleanup:
 	free(packs.perm);
 	free(entries);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1034,5 +1067,51 @@ int verify_midx_file(const char *object_dir)
 
 int expire_midx_packs(const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	size_t dirlen;
+	struct strbuf buf = STRBUF_INIT;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	strbuf_addstr(&buf, object_dir);
+	strbuf_addstr(&buf, "/pack/");
+	dirlen = buf.len;
+
+	for (i = 0; i < m->num_packs; i++) {
+		if (count[i])
+			continue;
+
+		if (m->packs[i]) {
+			close_pack(m->packs[i]);
+			m->packs[i] = NULL;
+		}
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+
+		strbuf_setlen(&buf, dirlen);
+		strbuf_addstr(&buf, m->pack_names[i]);
+		unlink(buf.buf);
+
+		strip_suffix_mem(buf.buf, &buf.len, "idx");
+		strbuf_addstr(&buf, "pack");
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 948effc1ee..210279a3cf 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -395,4 +395,19 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

diff --git a/midx.c b/midx.c
index bb825ef816..3bd7183a53 100644
--- a/midx.c
+++ b/midx.c
@@ -380,9 +380,11 @@ static size_t write_midx_header(struct hashfile *f,
 struct pack_list {
 	struct packed_git **list;
 	char **names;
+	uint32_t *perm;
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
+	uint32_t alloc_perm;
 	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
@@ -398,6 +400,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
 		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->perm, packs->nr + 1, packs->alloc_perm);
 
 		packs->list[packs->nr] = add_packed_git(full_path,
 							full_path_len,
@@ -417,6 +420,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 			return;
 		}
 
+		packs->perm[packs->nr] = packs->nr;
 		packs->names[packs->nr] = xstrdup(file_name);
 		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
@@ -443,7 +447,7 @@ static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *p
 	ALLOC_ARRAY(pairs, nr_packs);
 
 	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
+		pairs[i].pack_int_id = perm[i];
 		pairs[i].pack_name = pack_names[i];
 	}
 
@@ -755,7 +759,6 @@ int write_midx_file(const char *object_dir)
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct pack_list packs;
-	uint32_t *pack_perm = NULL;
 	uint64_t written = 0;
 	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
 	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
@@ -774,18 +777,22 @@ int write_midx_file(const char *object_dir)
 
 	packs.nr = 0;
 	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
+	packs.alloc_perm = packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
+	packs.perm = NULL;
 	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	ALLOC_ARRAY(packs.perm, packs.alloc_perm);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
 			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
 			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.perm, packs.nr + 1, packs.alloc_perm);
 
+			packs.perm[packs.nr] = i;
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
 			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
@@ -802,10 +809,9 @@ int write_midx_file(const char *object_dir)
 		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
 					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
+	sort_packs_by_name(packs.names, packs.nr, packs.perm);
 
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.list, packs.perm, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -923,8 +929,8 @@ cleanup:
 
 	free(packs.list);
 	free(packs.names);
+	free(packs.perm);
 	free(entries);
-	free(pack_perm);
 	free(midx_name);
 	return 0;
 }

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 822d83c845..e43e7da71e 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -39,6 +39,18 @@ expire::
 	Rewrite the MIDX file afterward to remove all references to
 	these pack-files.
 
+repack::
+	When given as the verb, collect a batch of pack-files whose
+	size are all at most the size given by --batch-size, but
+	whose sizes sum to larger than --batch-size. The batch is
+	selected by greedily adding small pack-files starting with
+	the oldest pack-files that fit the size. Create a new pack-
+	file containing the objects the multi-pack-index indexes
+	into thos pack-files, and rewrite the multi-pack-index to
+	contain that pack-file. A later run of 'git multi-pack-index
+	expire' will delete the pack-files that were part of this
+	batch.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 145de3a46c..d87a2235e3 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	static struct option builtin_multi_pack_index_options[] = {
 		OPT_FILENAME(0, "object-dir", &opts.object_dir,
 		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,6 +43,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' verb"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
diff --git a/midx.c b/midx.c
index 50e4cd7270..4caf148464 100644
--- a/midx.c
+++ b/midx.c
@@ -1115,3 +1115,8 @@ int expire_midx_packs(const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index e3a2b740b5..394a21ee96 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
 int expire_midx_packs(const char *object_dir);
+int midx_repack(const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 210279a3cf..c23e930a5d 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -410,4 +410,15 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack does not create any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 4caf148464..3718e78132 100644
--- a/midx.c
+++ b/midx.c
@@ -8,6 +8,7 @@
 #include "sha1-lookup.h"
 #include "midx.h"
 #include "progress.h"
+#include "run-command.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -1116,7 +1117,113 @@ int expire_midx_packs(const char *object_dir)
 	return result;
 }
 
-int midx_repack(const char *object_dir, size_t batch_size)
+struct time_and_id {
+	timestamp_t mtime;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
 {
+	const struct time_and_id *a, *b;
+
+	a = (const struct time_and_id *)a_;
+	b = (const struct time_and_id *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
 	return 0;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	int result = 0;
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct time_and_id *pack_ti;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf base_name = STRBUF_INIT;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	include_pack = xcalloc(m->num_packs, sizeof(unsigned char));
+	pack_ti = xcalloc(m->num_packs, sizeof(struct time_and_id));
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_ti[i].pack_int_id = i;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		pack_ti[i].mtime = m->packs[i]->mtime;
+	}
+	QSORT(pack_ti, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_ti[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+
+		if (!p)
+			continue;
+		if (p->pack_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += p->pack_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	if (total_size < batch_size || packs_to_repack < 2)
+		goto cleanup;
+
+	argv_array_push(&cmd.args, "pack-objects");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	argv_array_push(&cmd.args, base_name.buf);
+	strbuf_release(&base_name);
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
+		xwrite(cmd.in, "\n", 1);
+	}
+	close(cmd.in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, m, NULL);
+	m = NULL;
+
+cleanup:
+	if (m)
+		close_midx(m);
+	free(include_pack);
+	free(pack_ti);
+	return result;
+}
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index c23e930a5d..3cc9c918d5 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -421,4 +421,29 @@ test_expect_success 'repack does not create any packs' '
 	)
 '
 
+test_expect_success 'repack creates a new pack' '
+	(
+		cd dup &&
+		SECOND_SMALLEST_SIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 2 | tail -n 1) &&
+		BATCH_SIZE=$(($SECOND_SMALLEST_SIZE + 1)) &&
+		git multi-pack-index repack --batch-size=$BATCH_SIZE &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 5 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 5 midx-list
+	)
+'
+
+test_expect_success 'expire removes repacked packs' '
+	(
+		cd dup &&
+		ls -S .git/objects/pack/*pack | head -n 3 >expect &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/*pack >actual &&
+		test_cmp expect actual &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 3 midx-list
+	)
+'
+
 test_done

builtin/repack.c, packfile.c, packfile.h

diff --git a/builtin/repack.c b/builtin/repack.c
index 45583683ee..3d445b34b4 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
 
diff --git a/packfile.c b/packfile.c
index d1e6683ffe..bacecb4d0d 100644
--- a/packfile.c
+++ b/packfile.c
@@ -352,6 +352,34 @@ void close_all_packs(struct raw_object_store *o)
 	}
 }
 
+void unlink_pack_path(const char *pack_name, int force_delete)
+{
+	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	int i;
+	struct strbuf buf = STRBUF_INIT;
+	size_t plen;
+
+	strbuf_addstr(&buf, pack_name);
+	strip_suffix_mem(buf.buf, &buf.len, ".pack");
+	plen = buf.len;
+
+	if (!force_delete) {
+		strbuf_addstr(&buf, ".keep");
+		if (!access(buf.buf, F_OK)) {
+			strbuf_release(&buf);
+			return;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		strbuf_setlen(&buf, plen);
+		strbuf_addstr(&buf, exts[i]);
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+}
+
 /*
  * The LRU pack is the one with the oldest MRU window, preferring packs
  * with no used windows, or the oldest mtime if it has no windows allocated.
diff --git a/packfile.h b/packfile.h
index 6c4037605d..5b7bcdb1dd 100644
--- a/packfile.h
+++ b/packfile.h
@@ -86,6 +86,13 @@ extern void unuse_pack(struct pack_window **);
 extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Unlink the .pack and associated extension files.
+ * Does not unlink if 'force_delete' is false and the pack-file is
+ * marked as ".keep".
+ */
+extern void unlink_pack_path(const char *pack_name, int force_delete);
+
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,
  * and can provide at least 8 bytes of data.

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..1af406aca2 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,13 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
 
 
 EXAMPLES

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 1af406aca2..6186c4c936 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index fca70f8e4f..145de3a46c 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,7 +5,7 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -44,6 +44,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 730ff84dff..bb825ef816 100644
--- a/midx.c
+++ b/midx.c
@@ -1025,3 +1025,8 @@ int verify_midx_file(const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 774f652530..e3a2b740b5 100644
--- a/midx.h
+++ b/midx.h
@@ -49,6 +49,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
+int expire_midx_packs(const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 70926b5bc0..948effc1ee 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -348,4 +348,51 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 6186c4c936..cc63531cc0 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -36,6 +36,17 @@ expire::
 	have no objects referenced by the MIDX. Rewrite the MIDX file
 	afterward to remove all references to these pack-files.
 
+repack::
+	Collect a batch of pack-files whose size are all at most the
+	size given by --batch-size, but whose sizes sum to larger
+	than --batch-size. The batch is selected by greedily adding
+	small pack-files starting with the oldest pack-files that fit
+	the size. Create a new pack-file containing the objects the
+	multi-pack-index indexes into those pack-files, and rewrite
+	the multi-pack-index to contain that pack-file. A later run
+	of 'git multi-pack-index expire' will delete the pack-files
+	that were part of this batch.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 145de3a46c..d87a2235e3 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	static struct option builtin_multi_pack_index_options[] = {
 		OPT_FILENAME(0, "object-dir", &opts.object_dir,
 		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,6 +43,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' verb"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
diff --git a/midx.c b/midx.c
index 043cd1fd97..127c43f7b0 100644
--- a/midx.c
+++ b/midx.c
@@ -1110,3 +1110,8 @@ int expire_midx_packs(const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index e3a2b740b5..394a21ee96 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
 int expire_midx_packs(const char *object_dir);
+int midx_repack(const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 210279a3cf..f675621080 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -410,4 +410,15 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack with minimum size does not alter existing packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 3bd7183a53..043cd1fd97 100644
--- a/midx.c
+++ b/midx.c
@@ -751,7 +751,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -765,6 +766,7 @@ int write_midx_file(const char *object_dir)
 	uint32_t nr_entries, num_large_offsets = 0;
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -773,7 +775,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
@@ -787,7 +792,25 @@ int write_midx_file(const char *object_dir)
 	ALLOC_ARRAY(packs.perm, packs.alloc_perm);
 
 	if (packs.m) {
+		int drop_index = 0, missing_drops = 0;
 		for (i = 0; i < packs.m->num_packs; i++) {
+			if (packs_to_drop && drop_index < packs_to_drop->nr) {
+				int cmp = strcmp(packs.m->pack_names[i],
+						 packs_to_drop->items[drop_index].string);
+
+				if (!cmp) {
+					drop_index++;
+					continue;
+				} else if (cmp > 0) {
+					error(_("did not see pack-file %s to drop"),
+					      packs_to_drop->items[drop_index].string);
+					drop_index++;
+					i--;
+					missing_drops++;
+					continue;
+				}
+			}
+
 			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
 			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
 			ALLOC_GROW(packs.perm, packs.nr + 1, packs.alloc_perm);
@@ -798,6 +821,12 @@ int write_midx_file(const char *object_dir)
 			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
 			packs.nr++;
 		}
+
+		if (packs_to_drop && (drop_index < packs_to_drop->nr || missing_drops)) {
+			error(_("did not see all pack-files to drop"));
+			result = 1;
+			goto cleanup;
+		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
@@ -932,7 +961,12 @@ int write_midx_file(const char *object_dir)
 	free(packs.perm);
 	free(entries);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1034,5 +1068,45 @@ int verify_midx_file(const char *object_dir)
 
 int expire_midx_packs(const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+		FREE_AND_NULL(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 948effc1ee..210279a3cf 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -395,4 +395,19 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 127c43f7b0..f6bc111438 100644
--- a/midx.c
+++ b/midx.c
@@ -8,6 +8,7 @@
 #include "sha1-lookup.h"
 #include "midx.h"
 #include "progress.h"
+#include "run-command.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -1111,7 +1112,113 @@ int expire_midx_packs(const char *object_dir)
 	return result;
 }
 
-int midx_repack(const char *object_dir, size_t batch_size)
+struct time_and_id {
+	timestamp_t mtime;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
 {
+	const struct time_and_id *a, *b;
+
+	a = (const struct time_and_id *)a_;
+	b = (const struct time_and_id *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
 	return 0;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	int result = 0;
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct time_and_id *pack_ti;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf base_name = STRBUF_INIT;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	include_pack = xcalloc(m->num_packs, sizeof(unsigned char));
+	pack_ti = xcalloc(m->num_packs, sizeof(struct time_and_id));
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_ti[i].pack_int_id = i;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		pack_ti[i].mtime = m->packs[i]->mtime;
+	}
+	QSORT(pack_ti, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_ti[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+
+		if (!p)
+			continue;
+		if (p->pack_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += p->pack_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	if (total_size < batch_size || packs_to_repack < 2)
+		goto cleanup;
+
+	argv_array_push(&cmd.args, "pack-objects");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	argv_array_push(&cmd.args, base_name.buf);
+	strbuf_release(&base_name);
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
+		xwrite(cmd.in, "\n", 1);
+	}
+	close(cmd.in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, m, NULL);
+	m = NULL;
+
+cleanup:
+	if (m)
+		close_midx(m);
+	free(include_pack);
+	free(pack_ti);
+	return result;
+}
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index f675621080..3f5e9ea653 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -421,4 +421,29 @@ test_expect_success 'repack with minimum size does not alter existing packs' '
 	)
 '
 
+test_expect_success 'repack creates a new pack' '
+	(
+		cd dup &&
+		SECOND_SMALLEST_SIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 2 | tail -n 1) &&
+		BATCH_SIZE=$(($SECOND_SMALLEST_SIZE + 1)) &&
+		git multi-pack-index repack --batch-size=$BATCH_SIZE &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 5 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 5 midx-list
+	)
+'
+
+test_expect_success 'expire removes repacked packs' '
+	(
+		cd dup &&
+		ls -S .git/objects/pack/*pack | head -n 3 >expect &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/*pack >actual &&
+		test_cmp expect actual &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 3 midx-list
+	)
+'
+
 test_done

diff --git a/midx.c b/midx.c
index bb825ef816..3bd7183a53 100644
--- a/midx.c
+++ b/midx.c
@@ -380,9 +380,11 @@ static size_t write_midx_header(struct hashfile *f,
 struct pack_list {
 	struct packed_git **list;
 	char **names;
+	uint32_t *perm;
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
+	uint32_t alloc_perm;
 	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
@@ -398,6 +400,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
 		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->perm, packs->nr + 1, packs->alloc_perm);
 
 		packs->list[packs->nr] = add_packed_git(full_path,
 							full_path_len,
@@ -417,6 +420,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 			return;
 		}
 
+		packs->perm[packs->nr] = packs->nr;
 		packs->names[packs->nr] = xstrdup(file_name);
 		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
@@ -443,7 +447,7 @@ static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *p
 	ALLOC_ARRAY(pairs, nr_packs);
 
 	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
+		pairs[i].pack_int_id = perm[i];
 		pairs[i].pack_name = pack_names[i];
 	}
 
@@ -755,7 +759,6 @@ int write_midx_file(const char *object_dir)
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct pack_list packs;
-	uint32_t *pack_perm = NULL;
 	uint64_t written = 0;
 	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
 	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
@@ -774,18 +777,22 @@ int write_midx_file(const char *object_dir)
 
 	packs.nr = 0;
 	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
+	packs.alloc_perm = packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
+	packs.perm = NULL;
 	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	ALLOC_ARRAY(packs.perm, packs.alloc_perm);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
 			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
 			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.perm, packs.nr + 1, packs.alloc_perm);
 
+			packs.perm[packs.nr] = i;
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
 			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
@@ -802,10 +809,9 @@ int write_midx_file(const char *object_dir)
 		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
 					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
+	sort_packs_by_name(packs.names, packs.nr, packs.perm);
 
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.list, packs.perm, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -923,8 +929,8 @@ int write_midx_file(const char *object_dir)
 
 	free(packs.list);
 	free(packs.names);
+	free(packs.perm);
 	free(entries);
-	free(pack_perm);
 	free(midx_name);
 	return 0;
 }

builtin/repack.c, packfile.c, packfile.h

diff --git a/builtin/repack.c b/builtin/repack.c
index 45583683ee..3d445b34b4 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
 
diff --git a/packfile.c b/packfile.c
index d1e6683ffe..bacecb4d0d 100644
--- a/packfile.c
+++ b/packfile.c
@@ -352,6 +352,34 @@ void close_all_packs(struct raw_object_store *o)
 	}
 }
 
+void unlink_pack_path(const char *pack_name, int force_delete)
+{
+	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	int i;
+	struct strbuf buf = STRBUF_INIT;
+	size_t plen;
+
+	strbuf_addstr(&buf, pack_name);
+	strip_suffix_mem(buf.buf, &buf.len, ".pack");
+	plen = buf.len;
+
+	if (!force_delete) {
+		strbuf_addstr(&buf, ".keep");
+		if (!access(buf.buf, F_OK)) {
+			strbuf_release(&buf);
+			return;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		strbuf_setlen(&buf, plen);
+		strbuf_addstr(&buf, exts[i]);
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+}
+
 /*
  * The LRU pack is the one with the oldest MRU window, preferring packs
  * with no used windows, or the oldest mtime if it has no windows allocated.
diff --git a/packfile.h b/packfile.h
index 6c4037605d..5b7bcdb1dd 100644
--- a/packfile.h
+++ b/packfile.h
@@ -86,6 +86,13 @@ extern void unuse_pack(struct pack_window **);
 extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Unlink the .pack and associated extension files.
+ * Does not unlink if 'force_delete' is false and the pack-file is
+ * marked as ".keep".
+ */
+extern void unlink_pack_path(const char *pack_name, int force_delete);
+
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,
  * and can provide at least 8 bytes of data.

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 1af406aca2..6186c4c936 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index fca70f8e4f..145de3a46c 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,7 +5,7 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -44,6 +44,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 730ff84dff..bb825ef816 100644
--- a/midx.c
+++ b/midx.c
@@ -1025,3 +1025,8 @@ int verify_midx_file(const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 774f652530..e3a2b740b5 100644
--- a/midx.h
+++ b/midx.h
@@ -49,6 +49,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
+int expire_midx_packs(const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 70926b5bc0..948effc1ee 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -348,4 +348,51 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

diff --git a/midx.c b/midx.c
index bb825ef816..f087bbbe82 100644
--- a/midx.c
+++ b/midx.c
@@ -383,7 +383,6 @@ struct pack_list {
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
-	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
 
@@ -418,7 +417,6 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		}
 
 		packs->names[packs->nr] = xstrdup(file_name);
-		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
 	}
 }
@@ -762,6 +760,7 @@ int write_midx_file(const char *object_dir)
 	uint32_t nr_entries, num_large_offsets = 0;
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
+	int pack_name_concat_len = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -777,7 +776,6 @@ int write_midx_file(const char *object_dir)
 	packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
-	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
 
@@ -788,7 +786,6 @@ int write_midx_file(const char *object_dir)
 
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
-			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
 			packs.nr++;
 		}
 	}
@@ -798,10 +795,6 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	if (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
-		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
-					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
-
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	sort_packs_by_name(packs.names, packs.nr, pack_perm);
 
@@ -814,6 +807,13 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	for (i = 0; i < packs.nr; i++)
+		pack_name_concat_len += strlen(packs.names[i]) + 1;
+
+	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
+		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
+					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
+
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 	FREE_AND_NULL(midx_name);
@@ -831,7 +831,7 @@ int write_midx_file(const char *object_dir)
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + packs.pack_name_concat_len;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + pack_name_concat_len;
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 7ae5275c25..6ccbec3f19 100644
--- a/midx.c
+++ b/midx.c
@@ -33,6 +33,8 @@
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
+#define PACK_EXPIRED UINT_MAX
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
@@ -381,6 +383,7 @@ struct pack_info {
 	uint32_t orig_pack_int_id;
 	char *pack_name;
 	struct packed_git *p;
+	unsigned expired : 1;
 };
 
 static int pack_info_compare(const void *_a, const void *_b)
@@ -428,6 +431,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		packs->info[packs->nr].pack_name = xstrdup(file_name);
 		packs->info[packs->nr].orig_pack_int_id = packs->nr;
+		packs->info[packs->nr].expired = 0;
 		packs->nr++;
 	}
 }
@@ -587,13 +591,17 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(info[i].pack_name) + 1;
+		size_t writelen;
+
+		if (info[i].expired)
+			continue;
 
 		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
 			    info[i - 1].pack_name,
 			    info[i].pack_name);
 
+		writelen = strlen(info[i].pack_name) + 1;
 		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
@@ -675,6 +683,11 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
+		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+			BUG("object %s is in an expired pack with int-id %d",
+			    oid_to_hex(&obj->oid),
+			    obj->pack_int_id);
+
 		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
@@ -721,7 +734,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -737,6 +751,8 @@ int write_midx_file(const char *object_dir)
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
 	int pack_name_concat_len = 0;
+	int dropped_packs = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -745,7 +761,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc = packs.m ? packs.m->num_packs : 16;
@@ -759,13 +778,14 @@ int write_midx_file(const char *object_dir)
 			packs.info[packs.nr].orig_pack_int_id = i;
 			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
 			packs.info[packs.nr].p = NULL;
+			packs.info[packs.nr].expired = 0;
 			packs.nr++;
 		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
 
-	if (packs.m && packs.nr == packs.m->num_packs)
+	if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
 	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
@@ -779,13 +799,48 @@ int write_midx_file(const char *object_dir)
 
 	QSORT(packs.info, packs.nr, pack_info_compare);
 
+	if (packs_to_drop && packs_to_drop->nr) {
+		int drop_index = 0;
+		int missing_drops = 0;
+
+		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(packs.info[i].pack_name,
+					 packs_to_drop->items[drop_index].string);
+
+			if (!cmp) {
+				drop_index++;
+				packs.info[i].expired = 1;
+			} else if (cmp > 0) {
+				error(_("did not see pack-file %s to drop"),
+				      packs_to_drop->items[drop_index].string);
+				drop_index++;
+				missing_drops++;
+				i--;
+			} else {
+				packs.info[i].expired = 0;
+			}
+		}
+
+		if (missing_drops) {
+			result = 1;
+			goto cleanup;
+		}
+	}
+
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	for (i = 0; i < packs.nr; i++) {
-		pack_perm[packs.info[i].orig_pack_int_id] = i;
+		if (packs.info[i].expired) {
+			dropped_packs++;
+			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
+		} else {
+			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
+		}
 	}
 
-	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	for (i = 0; i < packs.nr; i++) {
+		if (!packs.info[i].expired)
+			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	}
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -801,7 +856,7 @@ int write_midx_file(const char *object_dir)
 	cur_chunk = 0;
 	num_chunks = large_offsets_needed ? 5 : 4;
 
-	written = write_midx_header(f, num_chunks, packs.nr);
+	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
 	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
@@ -902,7 +957,12 @@ int write_midx_file(const char *object_dir)
 	free(entries);
 	free(pack_perm);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1004,5 +1064,45 @@ int verify_midx_file(const char *object_dir)
 
 int expire_midx_packs(const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+		FREE_AND_NULL(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 948effc1ee..f55a60a89c 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -395,4 +395,22 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual &&
+		ls .git/objects/pack/ | grep idx >expect-idx &&
+		test-tool read-midx .git/objects | grep idx >actual-midx &&
+		test_cmp expect-idx actual-midx
+	)
+'
+
 test_done

diff --git a/midx.c b/midx.c
index f087bbbe82..7ae5275c25 100644
--- a/midx.c
+++ b/midx.c
@@ -377,12 +377,23 @@ static size_t write_midx_header(struct hashfile *f,
 	return MIDX_HEADER_SIZE;
 }
 
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+};
+
+static int pack_info_compare(const void *_a, const void *_b)
+{
+	struct pack_info *a = (struct pack_info *)_a;
+	struct pack_info *b = (struct pack_info *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
 struct pack_list {
-	struct packed_git **list;
-	char **names;
+	struct pack_info *info;
 	uint32_t nr;
-	uint32_t alloc_list;
-	uint32_t alloc_names;
+	uint32_t alloc;
 	struct multi_pack_index *m;
 };
 
@@ -395,66 +406,32 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		if (packs->m && midx_contains_pack(packs->m, file_name))
 			return;
 
-		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
-		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->info, packs->nr + 1, packs->alloc);
 
-		packs->list[packs->nr] = add_packed_git(full_path,
-							full_path_len,
-							0);
+		packs->info[packs->nr].p = add_packed_git(full_path,
+							  full_path_len,
+							  0);
 
-		if (!packs->list[packs->nr]) {
+		if (!packs->info[packs->nr].p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}
 
-		if (open_pack_index(packs->list[packs->nr])) {
+		if (open_pack_index(packs->info[packs->nr].p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(packs->list[packs->nr]);
-			FREE_AND_NULL(packs->list[packs->nr]);
+			close_pack(packs->info[packs->nr].p);
+			FREE_AND_NULL(packs->info[packs->nr].p);
 			return;
 		}
 
-		packs->names[packs->nr] = xstrdup(file_name);
+		packs->info[packs->nr].pack_name = xstrdup(file_name);
+		packs->info[packs->nr].orig_pack_int_id = packs->nr;
 		packs->nr++;
 	}
 }
 
-struct pack_pair {
-	uint32_t pack_int_id;
-	char *pack_name;
-};
-
-static int pack_pair_compare(const void *_a, const void *_b)
-{
-	struct pack_pair *a = (struct pack_pair *)_a;
-	struct pack_pair *b = (struct pack_pair *)_b;
-	return strcmp(a->pack_name, b->pack_name);
-}
-
-static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
-{
-	uint32_t i;
-	struct pack_pair *pairs;
-
-	ALLOC_ARRAY(pairs, nr_packs);
-
-	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
-		pairs[i].pack_name = pack_names[i];
-	}
-
-	QSORT(pairs, nr_packs, pack_pair_compare);
-
-	for (i = 0; i < nr_packs; i++) {
-		pack_names[i] = pairs[i].pack_name;
-		perm[pairs[i].pack_int_id] = i;
-	}
-
-	free(pairs);
-}
-
 struct pack_midx_entry {
 	struct object_id oid;
 	uint32_t pack_int_id;
@@ -480,7 +457,6 @@ static int midx_oid_compare(const void *_a, const void *_b)
 }
 
 static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
-				      uint32_t *pack_perm,
 				      struct pack_midx_entry *e,
 				      uint32_t pos)
 {
@@ -488,7 +464,7 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 		return 1;
 
 	nth_midxed_object_oid(&e->oid, m, pos);
-	e->pack_int_id = pack_perm[nth_midxed_pack_int_id(m, pos)];
+	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
 	e->offset = nth_midxed_offset(m, pos);
 
 	/* consider objects in midx to be from "old" packs */
@@ -522,8 +498,7 @@ static void fill_pack_entry(uint32_t pack_int_id,
  * of a packfile containing the object).
  */
 static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
-						  struct packed_git **p,
-						  uint32_t *perm,
+						  struct pack_info *info,
 						  uint32_t nr_packs,
 						  uint32_t *nr_objects)
 {
@@ -534,7 +509,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	uint32_t start_pack = m ? m->num_packs : 0;
 
 	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
-		total_objects += p[cur_pack]->num_objects;
+		total_objects += info[cur_pack].p->num_objects;
 
 	/*
 	 * As we de-duplicate by fanout value, we expect the fanout
@@ -559,7 +534,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				nth_midxed_pack_midx_entry(m, perm,
+				nth_midxed_pack_midx_entry(m,
 							   &entries_by_fanout[nr_fanout],
 							   cur_object);
 				nr_fanout++;
@@ -570,12 +545,12 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 			uint32_t start = 0, end;
 
 			if (cur_fanout)
-				start = get_pack_fanout(p[cur_pack], cur_fanout - 1);
-			end = get_pack_fanout(p[cur_pack], cur_fanout);
+				start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
+			end = get_pack_fanout(info[cur_pack].p, cur_fanout);
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				fill_pack_entry(perm[cur_pack], p[cur_pack], cur_object, &entries_by_fanout[nr_fanout]);
+				fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
 				nr_fanout++;
 			}
 		}
@@ -604,7 +579,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 }
 
 static size_t write_midx_pack_names(struct hashfile *f,
-				    char **pack_names,
+				    struct pack_info *info,
 				    uint32_t num_packs)
 {
 	uint32_t i;
@@ -612,14 +587,14 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(pack_names[i]) + 1;
+		size_t writelen = strlen(info[i].pack_name) + 1;
 
-		if (i && strcmp(pack_names[i], pack_names[i - 1]) <= 0)
+		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    pack_names[i - 1],
-			    pack_names[i]);
+			    info[i - 1].pack_name,
+			    info[i].pack_name);
 
-		hashwrite(f, pack_names[i], writelen);
+		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -690,6 +665,7 @@ static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
 }
 
 static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
+					uint32_t *perm,
 					struct pack_midx_entry *objects, uint32_t nr_objects)
 {
 	struct pack_midx_entry *list = objects;
@@ -699,7 +675,7 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		hashwrite_be32(f, obj->pack_int_id);
+		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
@@ -772,20 +748,17 @@ int write_midx_file(const char *object_dir)
 	packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
-	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
-	packs.list = NULL;
-	packs.names = NULL;
-	ALLOC_ARRAY(packs.list, packs.alloc_list);
-	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	packs.alloc = packs.m ? packs.m->num_packs : 16;
+	packs.info = NULL;
+	ALLOC_ARRAY(packs.info, packs.alloc);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
-			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
-			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.info, packs.nr + 1, packs.alloc);
 
-			packs.list[packs.nr] = NULL;
-			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].orig_pack_int_id = i;
+			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].p = NULL;
 			packs.nr++;
 		}
 	}
@@ -795,10 +768,7 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
-
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -807,8 +777,15 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	QSORT(packs.info, packs.nr, pack_info_compare);
+
+	ALLOC_ARRAY(pack_perm, packs.nr);
+	for (i = 0; i < packs.nr; i++) {
+		pack_perm[packs.info[i].orig_pack_int_id] = i;
+	}
+
 	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.names[i]) + 1;
+		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -879,7 +856,7 @@ int write_midx_file(const char *object_dir)
 
 		switch (chunk_ids[i]) {
 			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, packs.names, packs.nr);
+				written += write_midx_pack_names(f, packs.info, packs.nr);
 				break;
 
 			case MIDX_CHUNKID_OIDFANOUT:
@@ -891,7 +868,7 @@ int write_midx_file(const char *object_dir)
 				break;
 
 			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, entries, nr_entries);
+				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
 				break;
 
 			case MIDX_CHUNKID_LARGEOFFSETS:
@@ -914,15 +891,14 @@ int write_midx_file(const char *object_dir)
 
 cleanup:
 	for (i = 0; i < packs.nr; i++) {
-		if (packs.list[i]) {
-			close_pack(packs.list[i]);
-			free(packs.list[i]);
+		if (packs.info[i].p) {
+			close_pack(packs.info[i].p);
+			free(packs.info[i].p);
 		}
-		free(packs.names[i]);
+		free(packs.info[i].pack_name);
 	}
 
-	free(packs.list);
-	free(packs.names);
+	free(packs.info);
 	free(entries);
 	free(pack_perm);
 	free(midx_name);

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 30ff4430ab..f0f16690e6 100644
--- a/midx.c
+++ b/midx.c
@@ -8,6 +8,7 @@
 #include "sha1-lookup.h"
 #include "midx.h"
 #include "progress.h"
+#include "run-command.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -1107,7 +1108,113 @@ int expire_midx_packs(const char *object_dir)
 	return result;
 }
 
-int midx_repack(const char *object_dir, size_t batch_size)
+struct time_and_id {
+	timestamp_t mtime;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
 {
+	const struct time_and_id *a, *b;
+
+	a = (const struct time_and_id *)a_;
+	b = (const struct time_and_id *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
 	return 0;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	int result = 0;
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct time_and_id *pack_ti;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf base_name = STRBUF_INIT;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	include_pack = xcalloc(m->num_packs, sizeof(unsigned char));
+	pack_ti = xcalloc(m->num_packs, sizeof(struct time_and_id));
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_ti[i].pack_int_id = i;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		pack_ti[i].mtime = m->packs[i]->mtime;
+	}
+	QSORT(pack_ti, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_ti[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+
+		if (!p)
+			continue;
+		if (p->pack_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += p->pack_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	if (total_size < batch_size || packs_to_repack < 2)
+		goto cleanup;
+
+	argv_array_push(&cmd.args, "pack-objects");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	argv_array_push(&cmd.args, base_name.buf);
+	strbuf_release(&base_name);
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
+		xwrite(cmd.in, "\n", 1);
+	}
+	close(cmd.in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, m, NULL);
+	m = NULL;
+
+cleanup:
+	if (m)
+		close_midx(m);
+	free(include_pack);
+	free(pack_ti);
+	return result;
+}
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index d64767600a..99afb5ec51 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -424,4 +424,29 @@ test_expect_success 'repack with minimum size does not alter existing packs' '
 	)
 '
 
+test_expect_success 'repack creates a new pack' '
+	(
+		cd dup &&
+		SECOND_SMALLEST_SIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 2 | tail -n 1) &&
+		BATCH_SIZE=$(($SECOND_SMALLEST_SIZE + 1)) &&
+		git multi-pack-index repack --batch-size=$BATCH_SIZE &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 5 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 5 midx-list
+	)
+'
+
+test_expect_success 'expire removes repacked packs' '
+	(
+		cd dup &&
+		ls -S .git/objects/pack/*pack | head -n 3 >expect &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/*pack >actual &&
+		test_cmp expect actual &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 3 midx-list
+	)
+'
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 99afb5ec51..1213549d25 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -449,4 +449,36 @@ test_expect_success 'expire removes repacked packs' '
 	)
 '
 
+test_expect_success 'expire works when adding new packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/a-pack <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/z-pack <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index expire &&
+		ls .git/objects/pack/ | grep idx >expect &&
+		test-tool read-midx .git/objects | grep idx >actual &&
+		test_cmp expect actual &&
+		git multi-pack-index verify
+	)
+'
+
 test_done

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 6186c4c936..cc63531cc0 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -36,6 +36,17 @@ expire::
 	have no objects referenced by the MIDX. Rewrite the MIDX file
 	afterward to remove all references to these pack-files.
 
+repack::
+	Collect a batch of pack-files whose size are all at most the
+	size given by --batch-size, but whose sizes sum to larger
+	than --batch-size. The batch is selected by greedily adding
+	small pack-files starting with the oldest pack-files that fit
+	the size. Create a new pack-file containing the objects the
+	multi-pack-index indexes into those pack-files, and rewrite
+	the multi-pack-index to contain that pack-file. A later run
+	of 'git multi-pack-index expire' will delete the pack-files
+	that were part of this batch.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 145de3a46c..d87a2235e3 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	static struct option builtin_multi_pack_index_options[] = {
 		OPT_FILENAME(0, "object-dir", &opts.object_dir,
 		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,6 +43,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' verb"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
diff --git a/midx.c b/midx.c
index 6ccbec3f19..30ff4430ab 100644
--- a/midx.c
+++ b/midx.c
@@ -1106,3 +1106,8 @@ int expire_midx_packs(const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index e3a2b740b5..394a21ee96 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
 int expire_midx_packs(const char *object_dir);
+int midx_repack(const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index f55a60a89c..d64767600a 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -413,4 +413,15 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack with minimum size does not alter existing packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..1af406aca2 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,13 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
 
 
 EXAMPLES

builtin/repack.c, packfile.c, packfile.h

diff --git a/builtin/repack.c b/builtin/repack.c
index 45583683ee..3d445b34b4 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
 
diff --git a/packfile.c b/packfile.c
index d1e6683ffe..bacecb4d0d 100644
--- a/packfile.c
+++ b/packfile.c
@@ -352,6 +352,34 @@ void close_all_packs(struct raw_object_store *o)
 	}
 }
 
+void unlink_pack_path(const char *pack_name, int force_delete)
+{
+	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	int i;
+	struct strbuf buf = STRBUF_INIT;
+	size_t plen;
+
+	strbuf_addstr(&buf, pack_name);
+	strip_suffix_mem(buf.buf, &buf.len, ".pack");
+	plen = buf.len;
+
+	if (!force_delete) {
+		strbuf_addstr(&buf, ".keep");
+		if (!access(buf.buf, F_OK)) {
+			strbuf_release(&buf);
+			return;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		strbuf_setlen(&buf, plen);
+		strbuf_addstr(&buf, exts[i]);
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+}
+
 /*
  * The LRU pack is the one with the oldest MRU window, preferring packs
  * with no used windows, or the oldest mtime if it has no windows allocated.
diff --git a/packfile.h b/packfile.h
index 6c4037605d..5b7bcdb1dd 100644
--- a/packfile.h
+++ b/packfile.h
@@ -86,6 +86,13 @@ extern void unuse_pack(struct pack_window **);
 extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Unlink the .pack and associated extension files.
+ * Does not unlink if 'force_delete' is false and the pack-file is
+ * marked as ".keep".
+ */
+extern void unlink_pack_path(const char *pack_name, int force_delete);
+
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,
  * and can provide at least 8 bytes of data.

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..1af406aca2 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,13 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
 
 
 EXAMPLES

diff --git a/midx.c b/midx.c
index bb825ef816..f087bbbe82 100644
--- a/midx.c
+++ b/midx.c
@@ -383,7 +383,6 @@ struct pack_list {
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
-	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
 
@@ -418,7 +417,6 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		}
 
 		packs->names[packs->nr] = xstrdup(file_name);
-		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
 	}
 }
@@ -762,6 +760,7 @@ int write_midx_file(const char *object_dir)
 	uint32_t nr_entries, num_large_offsets = 0;
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
+	int pack_name_concat_len = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -777,7 +776,6 @@ int write_midx_file(const char *object_dir)
 	packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
-	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
 
@@ -788,7 +786,6 @@ int write_midx_file(const char *object_dir)
 
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
-			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
 			packs.nr++;
 		}
 	}
@@ -798,10 +795,6 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	if (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
-		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
-					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
-
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	sort_packs_by_name(packs.names, packs.nr, pack_perm);
 
@@ -814,6 +807,13 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	for (i = 0; i < packs.nr; i++)
+		pack_name_concat_len += strlen(packs.names[i]) + 1;
+
+	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
+		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
+					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
+
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 	FREE_AND_NULL(midx_name);
@@ -831,7 +831,7 @@ int write_midx_file(const char *object_dir)
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + packs.pack_name_concat_len;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + pack_name_concat_len;
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;

diff --git a/midx.c b/midx.c
index f087bbbe82..95c39106b2 100644
--- a/midx.c
+++ b/midx.c
@@ -377,12 +377,23 @@ static size_t write_midx_header(struct hashfile *f,
 	return MIDX_HEADER_SIZE;
 }
 
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+};
+
+static int pack_info_compare(const void *_a, const void *_b)
+{
+	struct pack_info *a = (struct pack_info *)_a;
+	struct pack_info *b = (struct pack_info *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
 struct pack_list {
-	struct packed_git **list;
-	char **names;
+	struct pack_info *info;
 	uint32_t nr;
-	uint32_t alloc_list;
-	uint32_t alloc_names;
+	uint32_t alloc;
 	struct multi_pack_index *m;
 };
 
@@ -395,66 +406,32 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		if (packs->m && midx_contains_pack(packs->m, file_name))
 			return;
 
-		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
-		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->info, packs->nr + 1, packs->alloc);
 
-		packs->list[packs->nr] = add_packed_git(full_path,
-							full_path_len,
-							0);
+		packs->info[packs->nr].p = add_packed_git(full_path,
+							  full_path_len,
+							  0);
 
-		if (!packs->list[packs->nr]) {
+		if (!packs->info[packs->nr].p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}
 
-		if (open_pack_index(packs->list[packs->nr])) {
+		if (open_pack_index(packs->info[packs->nr].p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(packs->list[packs->nr]);
-			FREE_AND_NULL(packs->list[packs->nr]);
+			close_pack(packs->info[packs->nr].p);
+			FREE_AND_NULL(packs->info[packs->nr].p);
 			return;
 		}
 
-		packs->names[packs->nr] = xstrdup(file_name);
+		packs->info[packs->nr].pack_name = xstrdup(file_name);
+		packs->info[packs->nr].orig_pack_int_id = packs->nr;
 		packs->nr++;
 	}
 }
 
-struct pack_pair {
-	uint32_t pack_int_id;
-	char *pack_name;
-};
-
-static int pack_pair_compare(const void *_a, const void *_b)
-{
-	struct pack_pair *a = (struct pack_pair *)_a;
-	struct pack_pair *b = (struct pack_pair *)_b;
-	return strcmp(a->pack_name, b->pack_name);
-}
-
-static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
-{
-	uint32_t i;
-	struct pack_pair *pairs;
-
-	ALLOC_ARRAY(pairs, nr_packs);
-
-	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
-		pairs[i].pack_name = pack_names[i];
-	}
-
-	QSORT(pairs, nr_packs, pack_pair_compare);
-
-	for (i = 0; i < nr_packs; i++) {
-		pack_names[i] = pairs[i].pack_name;
-		perm[pairs[i].pack_int_id] = i;
-	}
-
-	free(pairs);
-}
-
 struct pack_midx_entry {
 	struct object_id oid;
 	uint32_t pack_int_id;
@@ -480,7 +457,6 @@ static int midx_oid_compare(const void *_a, const void *_b)
 }
 
 static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
-				      uint32_t *pack_perm,
 				      struct pack_midx_entry *e,
 				      uint32_t pos)
 {
@@ -488,7 +464,7 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 		return 1;
 
 	nth_midxed_object_oid(&e->oid, m, pos);
-	e->pack_int_id = pack_perm[nth_midxed_pack_int_id(m, pos)];
+	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
 	e->offset = nth_midxed_offset(m, pos);
 
 	/* consider objects in midx to be from "old" packs */
@@ -522,8 +498,7 @@ static void fill_pack_entry(uint32_t pack_int_id,
  * of a packfile containing the object).
  */
 static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
-						  struct packed_git **p,
-						  uint32_t *perm,
+						  struct pack_info *info,
 						  uint32_t nr_packs,
 						  uint32_t *nr_objects)
 {
@@ -534,7 +509,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	uint32_t start_pack = m ? m->num_packs : 0;
 
 	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
-		total_objects += p[cur_pack]->num_objects;
+		total_objects += info[cur_pack].p->num_objects;
 
 	/*
 	 * As we de-duplicate by fanout value, we expect the fanout
@@ -559,7 +534,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				nth_midxed_pack_midx_entry(m, perm,
+				nth_midxed_pack_midx_entry(m,
 							   &entries_by_fanout[nr_fanout],
 							   cur_object);
 				nr_fanout++;
@@ -570,12 +545,12 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 			uint32_t start = 0, end;
 
 			if (cur_fanout)
-				start = get_pack_fanout(p[cur_pack], cur_fanout - 1);
-			end = get_pack_fanout(p[cur_pack], cur_fanout);
+				start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
+			end = get_pack_fanout(info[cur_pack].p, cur_fanout);
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				fill_pack_entry(perm[cur_pack], p[cur_pack], cur_object, &entries_by_fanout[nr_fanout]);
+				fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
 				nr_fanout++;
 			}
 		}
@@ -604,7 +579,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 }
 
 static size_t write_midx_pack_names(struct hashfile *f,
-				    char **pack_names,
+				    struct pack_info *info,
 				    uint32_t num_packs)
 {
 	uint32_t i;
@@ -612,14 +587,14 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(pack_names[i]) + 1;
+		size_t writelen = strlen(info[i].pack_name) + 1;
 
-		if (i && strcmp(pack_names[i], pack_names[i - 1]) <= 0)
+		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    pack_names[i - 1],
-			    pack_names[i]);
+			    info[i - 1].pack_name,
+			    info[i].pack_name);
 
-		hashwrite(f, pack_names[i], writelen);
+		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -690,6 +665,7 @@ static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
 }
 
 static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
+					uint32_t *perm,
 					struct pack_midx_entry *objects, uint32_t nr_objects)
 {
 	struct pack_midx_entry *list = objects;
@@ -699,7 +675,7 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		hashwrite_be32(f, obj->pack_int_id);
+		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
@@ -772,20 +748,17 @@ int write_midx_file(const char *object_dir)
 	packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
-	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
-	packs.list = NULL;
-	packs.names = NULL;
-	ALLOC_ARRAY(packs.list, packs.alloc_list);
-	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	packs.alloc = packs.m ? packs.m->num_packs : 16;
+	packs.info = NULL;
+	ALLOC_ARRAY(packs.info, packs.alloc);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
-			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
-			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.info, packs.nr + 1, packs.alloc);
 
-			packs.list[packs.nr] = NULL;
-			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].orig_pack_int_id = i;
+			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].p = NULL;
 			packs.nr++;
 		}
 	}
@@ -795,10 +768,7 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
-
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -807,8 +777,21 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	QSORT(packs.info, packs.nr, pack_info_compare);
+
+	/*
+	 * pack_perm stores a permutation between pack-int-ids from the
+	 * previous multi-pack-index to the new one we are writing:
+	 *
+	 * pack_perm[old_id] = new_id
+	 */
+	ALLOC_ARRAY(pack_perm, packs.nr);
+	for (i = 0; i < packs.nr; i++) {
+		pack_perm[packs.info[i].orig_pack_int_id] = i;
+	}
+
 	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.names[i]) + 1;
+		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -879,7 +862,7 @@ int write_midx_file(const char *object_dir)
 
 		switch (chunk_ids[i]) {
 			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, packs.names, packs.nr);
+				written += write_midx_pack_names(f, packs.info, packs.nr);
 				break;
 
 			case MIDX_CHUNKID_OIDFANOUT:
@@ -891,7 +874,7 @@ int write_midx_file(const char *object_dir)
 				break;
 
 			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, entries, nr_entries);
+				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
 				break;
 
 			case MIDX_CHUNKID_LARGEOFFSETS:
@@ -914,15 +897,14 @@ int write_midx_file(const char *object_dir)
 
 cleanup:
 	for (i = 0; i < packs.nr; i++) {
-		if (packs.list[i]) {
-			close_pack(packs.list[i]);
-			free(packs.list[i]);
+		if (packs.info[i].p) {
+			close_pack(packs.info[i].p);
+			free(packs.info[i].p);
 		}
-		free(packs.names[i]);
+		free(packs.info[i].pack_name);
 	}
 
-	free(packs.list);
-	free(packs.names);
+	free(packs.info);
 	free(entries);
 	free(pack_perm);
 	free(midx_name);

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 95c39106b2..299e9b2e8f 100644
--- a/midx.c
+++ b/midx.c
@@ -33,6 +33,8 @@
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
+#define PACK_EXPIRED UINT_MAX
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
@@ -381,6 +383,7 @@ struct pack_info {
 	uint32_t orig_pack_int_id;
 	char *pack_name;
 	struct packed_git *p;
+	unsigned expired : 1;
 };
 
 static int pack_info_compare(const void *_a, const void *_b)
@@ -428,6 +431,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		packs->info[packs->nr].pack_name = xstrdup(file_name);
 		packs->info[packs->nr].orig_pack_int_id = packs->nr;
+		packs->info[packs->nr].expired = 0;
 		packs->nr++;
 	}
 }
@@ -587,13 +591,17 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(info[i].pack_name) + 1;
+		size_t writelen;
+
+		if (info[i].expired)
+			continue;
 
 		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
 			    info[i - 1].pack_name,
 			    info[i].pack_name);
 
+		writelen = strlen(info[i].pack_name) + 1;
 		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
@@ -675,6 +683,11 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
+		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+			BUG("object %s is in an expired pack with int-id %d",
+			    oid_to_hex(&obj->oid),
+			    obj->pack_int_id);
+
 		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
@@ -721,7 +734,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -737,6 +751,8 @@ int write_midx_file(const char *object_dir)
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
 	int pack_name_concat_len = 0;
+	int dropped_packs = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -745,7 +761,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc = packs.m ? packs.m->num_packs : 16;
@@ -759,13 +778,14 @@ int write_midx_file(const char *object_dir)
 			packs.info[packs.nr].orig_pack_int_id = i;
 			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
 			packs.info[packs.nr].p = NULL;
+			packs.info[packs.nr].expired = 0;
 			packs.nr++;
 		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
 
-	if (packs.m && packs.nr == packs.m->num_packs)
+	if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
 	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
@@ -779,6 +799,34 @@ int write_midx_file(const char *object_dir)
 
 	QSORT(packs.info, packs.nr, pack_info_compare);
 
+	if (packs_to_drop && packs_to_drop->nr) {
+		int drop_index = 0;
+		int missing_drops = 0;
+
+		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(packs.info[i].pack_name,
+					 packs_to_drop->items[drop_index].string);
+
+			if (!cmp) {
+				drop_index++;
+				packs.info[i].expired = 1;
+			} else if (cmp > 0) {
+				error(_("did not see pack-file %s to drop"),
+				      packs_to_drop->items[drop_index].string);
+				drop_index++;
+				missing_drops++;
+				i--;
+			} else {
+				packs.info[i].expired = 0;
+			}
+		}
+
+		if (missing_drops) {
+			result = 1;
+			goto cleanup;
+		}
+	}
+
 	/*
 	 * pack_perm stores a permutation between pack-int-ids from the
 	 * previous multi-pack-index to the new one we are writing:
@@ -787,11 +835,18 @@ int write_midx_file(const char *object_dir)
 	 */
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	for (i = 0; i < packs.nr; i++) {
-		pack_perm[packs.info[i].orig_pack_int_id] = i;
+		if (packs.info[i].expired) {
+			dropped_packs++;
+			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
+		} else {
+			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
+		}
 	}
 
-	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	for (i = 0; i < packs.nr; i++) {
+		if (!packs.info[i].expired)
+			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	}
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -807,7 +862,7 @@ int write_midx_file(const char *object_dir)
 	cur_chunk = 0;
 	num_chunks = large_offsets_needed ? 5 : 4;
 
-	written = write_midx_header(f, num_chunks, packs.nr);
+	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
 	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
@@ -908,7 +963,12 @@ int write_midx_file(const char *object_dir)
 	free(entries);
 	free(pack_perm);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1010,5 +1070,45 @@ int verify_midx_file(const char *object_dir)
 
 int expire_midx_packs(const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+		FREE_AND_NULL(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index a8528f7da0..65e85debec 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -397,4 +397,24 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual &&
+		ls .git/objects/pack/ | grep idx >expect-idx &&
+		test-tool read-midx .git/objects | grep idx >actual-midx &&
+		test_cmp expect-idx actual-midx &&
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
 test_done

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 1af406aca2..6186c4c936 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index fca70f8e4f..145de3a46c 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,7 +5,7 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -44,6 +44,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 730ff84dff..bb825ef816 100644
--- a/midx.c
+++ b/midx.c
@@ -1025,3 +1025,8 @@ int verify_midx_file(const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 774f652530..e3a2b740b5 100644
--- a/midx.h
+++ b/midx.h
@@ -49,6 +49,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
+int expire_midx_packs(const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 70926b5bc0..a8528f7da0 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -348,4 +348,53 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		test-tool genrandom "data" 4096 >large_file.txt &&
+		git update-index --add large_file.txt &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 768a7dff73..7d81bf943e 100644
--- a/midx.c
+++ b/midx.c
@@ -8,6 +8,7 @@
 #include "sha1-lookup.h"
 #include "midx.h"
 #include "progress.h"
+#include "run-command.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -1113,7 +1114,113 @@ int expire_midx_packs(const char *object_dir)
 	return result;
 }
 
-int midx_repack(const char *object_dir, size_t batch_size)
+struct time_and_id {
+	timestamp_t mtime;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
 {
+	const struct time_and_id *a, *b;
+
+	a = (const struct time_and_id *)a_;
+	b = (const struct time_and_id *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
 	return 0;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	int result = 0;
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct time_and_id *pack_ti;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf base_name = STRBUF_INIT;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	include_pack = xcalloc(m->num_packs, sizeof(unsigned char));
+	pack_ti = xcalloc(m->num_packs, sizeof(struct time_and_id));
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_ti[i].pack_int_id = i;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		pack_ti[i].mtime = m->packs[i]->mtime;
+	}
+	QSORT(pack_ti, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_ti[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+
+		if (!p)
+			continue;
+		if (p->pack_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += p->pack_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	if (total_size < batch_size || packs_to_repack < 2)
+		goto cleanup;
+
+	argv_array_push(&cmd.args, "pack-objects");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	argv_array_push(&cmd.args, base_name.buf);
+	strbuf_release(&base_name);
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
+		xwrite(cmd.in, "\n", 1);
+	}
+	close(cmd.in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, m, NULL);
+	m = NULL;
+
+cleanup:
+	if (m)
+		close_midx(m);
+	free(include_pack);
+	free(pack_ti);
+	return result;
+}
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index acc5e65ecc..d6c1353514 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -383,7 +383,8 @@ test_expect_success 'setup expire tests' '
 		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
 		refs/heads/E
 		EOF
-		git multi-pack-index write
+		git multi-pack-index write &&
+		cp -r .git/objects/pack .git/objects/pack-backup
 	)
 '
 
@@ -434,4 +435,32 @@ test_expect_success 'repack with minimum size does not alter existing packs' '
 	)
 '
 
+test_expect_success 'repack creates a new pack' '
+	(
+		cd dup &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 5 idx-list &&
+		THIRD_SMALLEST_SIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 3 | tail -n 1) &&
+		BATCH_SIZE=$(($THIRD_SMALLEST_SIZE + 1)) &&
+		git multi-pack-index repack --batch-size=$BATCH_SIZE &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 6 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 6 midx-list
+	)
+'
+
+test_expect_success 'expire removes repacked packs' '
+	(
+		cd dup &&
+		ls -al .git/objects/pack/*pack &&
+		ls -S .git/objects/pack/*pack | head -n 4 >expect &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/*pack >actual &&
+		test_cmp expect actual &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 4 midx-list
+	)
+'
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index d6c1353514..19b769eea0 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -463,4 +463,36 @@ test_expect_success 'expire removes repacked packs' '
 	)
 '
 
+test_expect_success 'expire works when adding new packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/a-pack <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/z-pack <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index expire &&
+		ls .git/objects/pack/ | grep idx >expect &&
+		test-tool read-midx .git/objects | grep idx >actual &&
+		test_cmp expect actual &&
+		git multi-pack-index verify
+	)
+'
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 19b769eea0..bcfa520401 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -495,4 +495,22 @@ test_expect_success 'expire works when adding new packs' '
 	)
 '
 
+test_expect_success 'expire respects .keep files' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-all <<-EOF &&
+		refs/heads/A
+		EOF
+		git multi-pack-index write &&
+		PACKA=$(ls .git/objects/pack/a-pack*\.pack | sed s/\.pack\$//) &&
+		touch $PACKA.keep &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/a-pack* | grep $PACKA >a-pack-files &&
+		test_line_count = 3 a-pack-files &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 2 midx-list
+	)
+'
+
+
 test_done

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 6186c4c936..de345c2400 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -36,6 +36,17 @@ expire::
 	have no objects referenced by the MIDX. Rewrite the MIDX file
 	afterward to remove all references to these pack-files.
 
+repack::
+	Create a new pack-file containing objects in small pack-files
+	referenced by the multi-pack-index. Select the pack-files by
+	examining packs from oldest-to-newest, adding a pack if its
+	size is below the batch size. Stop adding packs when the sum
+	of sizes of the added packs is above the batch size. If the
+	total size does not reach the batch size, then do nothing.
+	Rewrite the multi-pack-index to reference the new pack-file.
+	A later run of 'git multi-pack-index expire' will delete the
+	pack-files that were part of this batch.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 145de3a46c..c66239de33 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	static struct option builtin_multi_pack_index_options[] = {
 		OPT_FILENAME(0, "object-dir", &opts.object_dir,
 		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,6 +43,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' subcommand"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
@@ -47,5 +55,5 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	if (!strcmp(argv[0], "expire"))
 		return expire_midx_packs(opts.object_dir);
 
-	die(_("unrecognized verb: %s"), argv[0]);
+	die(_("unrecognized subcommand: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 299e9b2e8f..768a7dff73 100644
--- a/midx.c
+++ b/midx.c
@@ -1112,3 +1112,8 @@ int expire_midx_packs(const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index e3a2b740b5..394a21ee96 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
 int expire_midx_packs(const char *object_dir);
+int midx_repack(const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 65e85debec..acc5e65ecc 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -417,4 +417,21 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack with minimum size does not alter existing packs' '
+	(
+		cd dup &&
+		rm -rf .git/objects/pack &&
+		mv .git/objects/pack-backup .git/objects/pack &&
+		touch -m -t 201901010000 .git/objects/pack/pack-D* &&
+		touch -m -t 201901010001 .git/objects/pack/pack-C* &&
+		touch -m -t 201901010002 .git/objects/pack/pack-B* &&
+		touch -m -t 201901010003 .git/objects/pack/pack-A* &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

builtin/repack.c, packfile.c, packfile.h

diff --git a/builtin/repack.c b/builtin/repack.c
index 45583683ee..3d445b34b4 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
 
diff --git a/packfile.c b/packfile.c
index d1e6683ffe..bacecb4d0d 100644
--- a/packfile.c
+++ b/packfile.c
@@ -352,6 +352,34 @@ void close_all_packs(struct raw_object_store *o)
 	}
 }
 
+void unlink_pack_path(const char *pack_name, int force_delete)
+{
+	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	int i;
+	struct strbuf buf = STRBUF_INIT;
+	size_t plen;
+
+	strbuf_addstr(&buf, pack_name);
+	strip_suffix_mem(buf.buf, &buf.len, ".pack");
+	plen = buf.len;
+
+	if (!force_delete) {
+		strbuf_addstr(&buf, ".keep");
+		if (!access(buf.buf, F_OK)) {
+			strbuf_release(&buf);
+			return;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		strbuf_setlen(&buf, plen);
+		strbuf_addstr(&buf, exts[i]);
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+}
+
 /*
  * The LRU pack is the one with the oldest MRU window, preferring packs
  * with no used windows, or the oldest mtime if it has no windows allocated.
diff --git a/packfile.h b/packfile.h
index 6c4037605d..5b7bcdb1dd 100644
--- a/packfile.h
+++ b/packfile.h
@@ -86,6 +86,13 @@ extern void unuse_pack(struct pack_window **);
 extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Unlink the .pack and associated extension files.
+ * Does not unlink if 'force_delete' is false and the pack-file is
+ * marked as ".keep".
+ */
+extern void unlink_pack_path(const char *pack_name, int force_delete);
+
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,
  * and can provide at least 8 bytes of data.

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 1af406aca2..6186c4c936 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index fca70f8e4f..145de3a46c 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,7 +5,7 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -44,6 +44,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 730ff84dff..bb825ef816 100644
--- a/midx.c
+++ b/midx.c
@@ -1025,3 +1025,8 @@ int verify_midx_file(const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 774f652530..e3a2b740b5 100644
--- a/midx.h
+++ b/midx.h
@@ -49,6 +49,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
+int expire_midx_packs(const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 70926b5bc0..a8528f7da0 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -348,4 +348,53 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		test-tool genrandom "data" 4096 >large_file.txt &&
+		git update-index --add large_file.txt &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..1af406aca2 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,13 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
 
 
 EXAMPLES

diff --git a/midx.c b/midx.c
index bb825ef816..f087bbbe82 100644
--- a/midx.c
+++ b/midx.c
@@ -383,7 +383,6 @@ struct pack_list {
 	uint32_t nr;
 	uint32_t alloc_list;
 	uint32_t alloc_names;
-	size_t pack_name_concat_len;
 	struct multi_pack_index *m;
 };
 
@@ -418,7 +417,6 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		}
 
 		packs->names[packs->nr] = xstrdup(file_name);
-		packs->pack_name_concat_len += strlen(file_name) + 1;
 		packs->nr++;
 	}
 }
@@ -762,6 +760,7 @@ int write_midx_file(const char *object_dir)
 	uint32_t nr_entries, num_large_offsets = 0;
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
+	int pack_name_concat_len = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -777,7 +776,6 @@ int write_midx_file(const char *object_dir)
 	packs.alloc_names = packs.alloc_list;
 	packs.list = NULL;
 	packs.names = NULL;
-	packs.pack_name_concat_len = 0;
 	ALLOC_ARRAY(packs.list, packs.alloc_list);
 	ALLOC_ARRAY(packs.names, packs.alloc_names);
 
@@ -788,7 +786,6 @@ int write_midx_file(const char *object_dir)
 
 			packs.list[packs.nr] = NULL;
 			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
-			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
 			packs.nr++;
 		}
 	}
@@ -798,10 +795,6 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	if (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
-		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
-					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
-
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	sort_packs_by_name(packs.names, packs.nr, pack_perm);
 
@@ -814,6 +807,13 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	for (i = 0; i < packs.nr; i++)
+		pack_name_concat_len += strlen(packs.names[i]) + 1;
+
+	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
+		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
+					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
+
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 	FREE_AND_NULL(midx_name);
@@ -831,7 +831,7 @@ int write_midx_file(const char *object_dir)
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
-	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + packs.pack_name_concat_len;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + pack_name_concat_len;
 
 	cur_chunk++;
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 6186c4c936..233b2b7862 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -36,6 +36,23 @@ expire::
 	have no objects referenced by the MIDX. Rewrite the MIDX file
 	afterward to remove all references to these pack-files.
 
+repack::
+	Create a new pack-file containing objects in small pack-files
+	referenced by the multi-pack-index. If the size given by the
+	`--batch-size=<size>` argument is zero, then create a pack
+	containing all objects referenced by the multi-pack-index. For
+	a non-zero batch size, Select the pack-files by examining packs
+	from oldest-to-newest, computing the "expected size" by counting
+	the number of objects in the pack referenced by the
+	multi-pack-index, then divide by the total number of objects in
+	the pack and multiply by the pack size. We select packs with
+	expected size below the batch size until the set of packs have
+	total expected size at least the batch size. If the total size
+	does not reach the batch size, then do nothing. If a new pack-
+	file is created, rewrite the multi-pack-index to reference the
+	new pack-file. A later run of 'git multi-pack-index expire' will
+	delete the pack-files that were part of this batch.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 145de3a46c..c66239de33 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	static struct option builtin_multi_pack_index_options[] = {
 		OPT_FILENAME(0, "object-dir", &opts.object_dir,
 		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,6 +43,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' subcommand"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
@@ -47,5 +55,5 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	if (!strcmp(argv[0], "expire"))
 		return expire_midx_packs(opts.object_dir);
 
-	die(_("unrecognized verb: %s"), argv[0]);
+	die(_("unrecognized subcommand: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index 299e9b2e8f..768a7dff73 100644
--- a/midx.c
+++ b/midx.c
@@ -1112,3 +1112,8 @@ int expire_midx_packs(const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index e3a2b740b5..394a21ee96 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(const char *object_dir);
 int expire_midx_packs(const char *object_dir);
+int midx_repack(const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 65e85debec..26ae8b3f62 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -383,7 +383,8 @@ test_expect_success 'setup expire tests' '
 		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
 		refs/heads/E
 		EOF
-		git multi-pack-index write
+		git multi-pack-index write &&
+		cp -r .git/objects/pack .git/objects/pack-backup
 	)
 '
 
@@ -417,4 +418,21 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack with minimum size does not alter existing packs' '
+	(
+		cd dup &&
+		rm -rf .git/objects/pack &&
+		mv .git/objects/pack-backup .git/objects/pack &&
+		touch -m -t 201901010000 .git/objects/pack/pack-D* &&
+		touch -m -t 201901010001 .git/objects/pack/pack-C* &&
+		touch -m -t 201901010002 .git/objects/pack/pack-B* &&
+		touch -m -t 201901010003 .git/objects/pack/pack-A* &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 768a7dff73..01c6a05732 100644
--- a/midx.c
+++ b/midx.c
@@ -8,6 +8,7 @@
 #include "sha1-lookup.h"
 #include "midx.h"
 #include "progress.h"
+#include "run-command.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -1113,7 +1114,154 @@ int expire_midx_packs(const char *object_dir)
 	return result;
 }
 
-int midx_repack(const char *object_dir, size_t batch_size)
+struct repack_info {
+	timestamp_t mtime;
+	uint32_t referenced_objects;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
 {
+	const struct repack_info *a, *b;
+
+	a = (const struct repack_info *)a_;
+	b = (const struct repack_info *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
+	return 0;
+}
+
+static int fill_included_packs_all(struct multi_pack_index *m,
+				   unsigned char *include_pack)
+{
+	uint32_t i;
+
+	for (i = 0; i < m->num_packs; i++)
+		include_pack[i] = 1;
+
+	return m->num_packs < 2;
+}
+
+static int fill_included_packs_batch(struct multi_pack_index *m,
+				     unsigned char *include_pack,
+				     size_t batch_size)
+{
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct repack_info *pack_info = xcalloc(m->num_packs, sizeof(struct repack_info));
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_info[i].pack_int_id = i;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		pack_info[i].mtime = m->packs[i]->mtime;
+	}
+
+	for (i = 0; batch_size && i < m->num_objects; i++) {
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+		pack_info[pack_int_id].referenced_objects++;
+	}
+
+	QSORT(pack_info, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_info[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+		size_t expected_size;
+
+		if (!p)
+			continue;
+		if (open_pack_index(p) || !p->num_objects)
+			continue;
+
+		expected_size = (size_t)(p->pack_size
+					 * pack_info[i].referenced_objects);
+		expected_size /= p->num_objects;
+
+		if (expected_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += expected_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	free(pack_info);
+
+	if (total_size < batch_size || packs_to_repack < 2)
+		return 1;
+
 	return 0;
+}	
+
+int midx_repack(const char *object_dir, size_t batch_size)
+{
+	int result = 0;
+	uint32_t i;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	struct strbuf base_name = STRBUF_INIT;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	include_pack = xcalloc(m->num_packs, sizeof(unsigned char));
+
+	if (batch_size) {
+		if (fill_included_packs_batch(m, include_pack, batch_size))
+			goto cleanup;
+	} else if (fill_included_packs_all(m, include_pack))
+		goto cleanup;
+
+	argv_array_push(&cmd.args, "pack-objects");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	argv_array_push(&cmd.args, base_name.buf);
+	strbuf_release(&base_name);
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
+		xwrite(cmd.in, "\n", 1);
+	}
+	close(cmd.in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, m, NULL);
+	m = NULL;
+
+cleanup:
+	if (m)
+		close_midx(m);
+	free(include_pack);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 26ae8b3f62..d6c1353514 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -435,4 +435,32 @@ test_expect_success 'repack with minimum size does not alter existing packs' '
 	)
 '
 
+test_expect_success 'repack creates a new pack' '
+	(
+		cd dup &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 5 idx-list &&
+		THIRD_SMALLEST_SIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 3 | tail -n 1) &&
+		BATCH_SIZE=$(($THIRD_SMALLEST_SIZE + 1)) &&
+		git multi-pack-index repack --batch-size=$BATCH_SIZE &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 6 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 6 midx-list
+	)
+'
+
+test_expect_success 'expire removes repacked packs' '
+	(
+		cd dup &&
+		ls -al .git/objects/pack/*pack &&
+		ls -S .git/objects/pack/*pack | head -n 4 >expect &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/*pack >actual &&
+		test_cmp expect actual &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 4 midx-list
+	)
+'
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index d6c1353514..19b769eea0 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -463,4 +463,36 @@ test_expect_success 'expire removes repacked packs' '
 	)
 '
 
+test_expect_success 'expire works when adding new packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/a-pack <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git multi-pack-index write &&
+		git pack-objects --revs .git/objects/pack/z-pack <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index expire &&
+		ls .git/objects/pack/ | grep idx >expect &&
+		test-tool read-midx .git/objects | grep idx >actual &&
+		test_cmp expect actual &&
+		git multi-pack-index verify
+	)
+'
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 19b769eea0..bcfa520401 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -495,4 +495,22 @@ test_expect_success 'expire works when adding new packs' '
 	)
 '
 
+test_expect_success 'expire respects .keep files' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-all <<-EOF &&
+		refs/heads/A
+		EOF
+		git multi-pack-index write &&
+		PACKA=$(ls .git/objects/pack/a-pack*\.pack | sed s/\.pack\$//) &&
+		touch $PACKA.keep &&
+		git multi-pack-index expire &&
+		ls -S .git/objects/pack/a-pack* | grep $PACKA >a-pack-files &&
+		test_line_count = 3 a-pack-files &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 2 midx-list
+	)
+'
+
+
 test_done

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index bcfa520401..0f116b4b92 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -512,5 +512,24 @@ test_expect_success 'expire respects .keep files' '
 	)
 '
 
+test_expect_success 'repack --batch-size=0 repacks everything' '
+	(
+		cd dup &&
+		rm .git/objects/pack/*.keep &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 2 idx-list &&
+		git multi-pack-index repack --batch-size=0 &&
+		ls .git/objects/pack/*idx >idx-list &&
+		test_line_count = 3 idx-list &&
+		test-tool read-midx .git/objects | grep idx >midx-list &&
+		test_line_count = 3 midx-list &&
+		git multi-pack-index expire &&
+		ls -al .git/objects/pack/*idx >idx-list &&
+		test_line_count = 1 idx-list &&
+		git multi-pack-index repack --batch-size=0 &&
+		ls -al .git/objects/pack/*idx >new-idx-list &&
+		test_cmp idx-list new-idx-list
+	)
+'
 
 test_done

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 95c39106b2..299e9b2e8f 100644
--- a/midx.c
+++ b/midx.c
@@ -33,6 +33,8 @@
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
+#define PACK_EXPIRED UINT_MAX
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
@@ -381,6 +383,7 @@ struct pack_info {
 	uint32_t orig_pack_int_id;
 	char *pack_name;
 	struct packed_git *p;
+	unsigned expired : 1;
 };
 
 static int pack_info_compare(const void *_a, const void *_b)
@@ -428,6 +431,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		packs->info[packs->nr].pack_name = xstrdup(file_name);
 		packs->info[packs->nr].orig_pack_int_id = packs->nr;
+		packs->info[packs->nr].expired = 0;
 		packs->nr++;
 	}
 }
@@ -587,13 +591,17 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(info[i].pack_name) + 1;
+		size_t writelen;
+
+		if (info[i].expired)
+			continue;
 
 		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
 			    info[i - 1].pack_name,
 			    info[i].pack_name);
 
+		writelen = strlen(info[i].pack_name) + 1;
 		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
@@ -675,6 +683,11 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
+		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+			BUG("object %s is in an expired pack with int-id %d",
+			    oid_to_hex(&obj->oid),
+			    obj->pack_int_id);
+
 		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
@@ -721,7 +734,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -737,6 +751,8 @@ int write_midx_file(const char *object_dir)
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
 	int pack_name_concat_len = 0;
+	int dropped_packs = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -745,7 +761,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc = packs.m ? packs.m->num_packs : 16;
@@ -759,13 +778,14 @@ int write_midx_file(const char *object_dir)
 			packs.info[packs.nr].orig_pack_int_id = i;
 			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
 			packs.info[packs.nr].p = NULL;
+			packs.info[packs.nr].expired = 0;
 			packs.nr++;
 		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
 
-	if (packs.m && packs.nr == packs.m->num_packs)
+	if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
 	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
@@ -779,6 +799,34 @@ int write_midx_file(const char *object_dir)
 
 	QSORT(packs.info, packs.nr, pack_info_compare);
 
+	if (packs_to_drop && packs_to_drop->nr) {
+		int drop_index = 0;
+		int missing_drops = 0;
+
+		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(packs.info[i].pack_name,
+					 packs_to_drop->items[drop_index].string);
+
+			if (!cmp) {
+				drop_index++;
+				packs.info[i].expired = 1;
+			} else if (cmp > 0) {
+				error(_("did not see pack-file %s to drop"),
+				      packs_to_drop->items[drop_index].string);
+				drop_index++;
+				missing_drops++;
+				i--;
+			} else {
+				packs.info[i].expired = 0;
+			}
+		}
+
+		if (missing_drops) {
+			result = 1;
+			goto cleanup;
+		}
+	}
+
 	/*
 	 * pack_perm stores a permutation between pack-int-ids from the
 	 * previous multi-pack-index to the new one we are writing:
@@ -787,11 +835,18 @@ int write_midx_file(const char *object_dir)
 	 */
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	for (i = 0; i < packs.nr; i++) {
-		pack_perm[packs.info[i].orig_pack_int_id] = i;
+		if (packs.info[i].expired) {
+			dropped_packs++;
+			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
+		} else {
+			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
+		}
 	}
 
-	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	for (i = 0; i < packs.nr; i++) {
+		if (!packs.info[i].expired)
+			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	}
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -807,7 +862,7 @@ int write_midx_file(const char *object_dir)
 	cur_chunk = 0;
 	num_chunks = large_offsets_needed ? 5 : 4;
 
-	written = write_midx_header(f, num_chunks, packs.nr);
+	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
 	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
@@ -908,7 +963,12 @@ int write_midx_file(const char *object_dir)
 	free(entries);
 	free(pack_perm);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1010,5 +1070,45 @@ int verify_midx_file(const char *object_dir)
 
 int expire_midx_packs(const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+		FREE_AND_NULL(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index a8528f7da0..65e85debec 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -397,4 +397,24 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual &&
+		ls .git/objects/pack/ | grep idx >expect-idx &&
+		test-tool read-midx .git/objects | grep idx >actual-midx &&
+		test_cmp expect-idx actual-midx &&
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
 test_done

diff --git a/midx.c b/midx.c
index f087bbbe82..95c39106b2 100644
--- a/midx.c
+++ b/midx.c
@@ -377,12 +377,23 @@ static size_t write_midx_header(struct hashfile *f,
 	return MIDX_HEADER_SIZE;
 }
 
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+};
+
+static int pack_info_compare(const void *_a, const void *_b)
+{
+	struct pack_info *a = (struct pack_info *)_a;
+	struct pack_info *b = (struct pack_info *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
 struct pack_list {
-	struct packed_git **list;
-	char **names;
+	struct pack_info *info;
 	uint32_t nr;
-	uint32_t alloc_list;
-	uint32_t alloc_names;
+	uint32_t alloc;
 	struct multi_pack_index *m;
 };
 
@@ -395,66 +406,32 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		if (packs->m && midx_contains_pack(packs->m, file_name))
 			return;
 
-		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
-		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->info, packs->nr + 1, packs->alloc);
 
-		packs->list[packs->nr] = add_packed_git(full_path,
-							full_path_len,
-							0);
+		packs->info[packs->nr].p = add_packed_git(full_path,
+							  full_path_len,
+							  0);
 
-		if (!packs->list[packs->nr]) {
+		if (!packs->info[packs->nr].p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}
 
-		if (open_pack_index(packs->list[packs->nr])) {
+		if (open_pack_index(packs->info[packs->nr].p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(packs->list[packs->nr]);
-			FREE_AND_NULL(packs->list[packs->nr]);
+			close_pack(packs->info[packs->nr].p);
+			FREE_AND_NULL(packs->info[packs->nr].p);
 			return;
 		}
 
-		packs->names[packs->nr] = xstrdup(file_name);
+		packs->info[packs->nr].pack_name = xstrdup(file_name);
+		packs->info[packs->nr].orig_pack_int_id = packs->nr;
 		packs->nr++;
 	}
 }
 
-struct pack_pair {
-	uint32_t pack_int_id;
-	char *pack_name;
-};
-
-static int pack_pair_compare(const void *_a, const void *_b)
-{
-	struct pack_pair *a = (struct pack_pair *)_a;
-	struct pack_pair *b = (struct pack_pair *)_b;
-	return strcmp(a->pack_name, b->pack_name);
-}
-
-static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
-{
-	uint32_t i;
-	struct pack_pair *pairs;
-
-	ALLOC_ARRAY(pairs, nr_packs);
-
-	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
-		pairs[i].pack_name = pack_names[i];
-	}
-
-	QSORT(pairs, nr_packs, pack_pair_compare);
-
-	for (i = 0; i < nr_packs; i++) {
-		pack_names[i] = pairs[i].pack_name;
-		perm[pairs[i].pack_int_id] = i;
-	}
-
-	free(pairs);
-}
-
 struct pack_midx_entry {
 	struct object_id oid;
 	uint32_t pack_int_id;
@@ -480,7 +457,6 @@ static int midx_oid_compare(const void *_a, const void *_b)
 }
 
 static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
-				      uint32_t *pack_perm,
 				      struct pack_midx_entry *e,
 				      uint32_t pos)
 {
@@ -488,7 +464,7 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 		return 1;
 
 	nth_midxed_object_oid(&e->oid, m, pos);
-	e->pack_int_id = pack_perm[nth_midxed_pack_int_id(m, pos)];
+	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
 	e->offset = nth_midxed_offset(m, pos);
 
 	/* consider objects in midx to be from "old" packs */
@@ -522,8 +498,7 @@ static void fill_pack_entry(uint32_t pack_int_id,
  * of a packfile containing the object).
  */
 static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
-						  struct packed_git **p,
-						  uint32_t *perm,
+						  struct pack_info *info,
 						  uint32_t nr_packs,
 						  uint32_t *nr_objects)
 {
@@ -534,7 +509,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	uint32_t start_pack = m ? m->num_packs : 0;
 
 	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
-		total_objects += p[cur_pack]->num_objects;
+		total_objects += info[cur_pack].p->num_objects;
 
 	/*
 	 * As we de-duplicate by fanout value, we expect the fanout
@@ -559,7 +534,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				nth_midxed_pack_midx_entry(m, perm,
+				nth_midxed_pack_midx_entry(m,
 							   &entries_by_fanout[nr_fanout],
 							   cur_object);
 				nr_fanout++;
@@ -570,12 +545,12 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 			uint32_t start = 0, end;
 
 			if (cur_fanout)
-				start = get_pack_fanout(p[cur_pack], cur_fanout - 1);
-			end = get_pack_fanout(p[cur_pack], cur_fanout);
+				start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
+			end = get_pack_fanout(info[cur_pack].p, cur_fanout);
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				fill_pack_entry(perm[cur_pack], p[cur_pack], cur_object, &entries_by_fanout[nr_fanout]);
+				fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
 				nr_fanout++;
 			}
 		}
@@ -604,7 +579,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 }
 
 static size_t write_midx_pack_names(struct hashfile *f,
-				    char **pack_names,
+				    struct pack_info *info,
 				    uint32_t num_packs)
 {
 	uint32_t i;
@@ -612,14 +587,14 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(pack_names[i]) + 1;
+		size_t writelen = strlen(info[i].pack_name) + 1;
 
-		if (i && strcmp(pack_names[i], pack_names[i - 1]) <= 0)
+		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    pack_names[i - 1],
-			    pack_names[i]);
+			    info[i - 1].pack_name,
+			    info[i].pack_name);
 
-		hashwrite(f, pack_names[i], writelen);
+		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -690,6 +665,7 @@ static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
 }
 
 static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
+					uint32_t *perm,
 					struct pack_midx_entry *objects, uint32_t nr_objects)
 {
 	struct pack_midx_entry *list = objects;
@@ -699,7 +675,7 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		hashwrite_be32(f, obj->pack_int_id);
+		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
@@ -772,20 +748,17 @@ int write_midx_file(const char *object_dir)
 	packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
-	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
-	packs.list = NULL;
-	packs.names = NULL;
-	ALLOC_ARRAY(packs.list, packs.alloc_list);
-	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	packs.alloc = packs.m ? packs.m->num_packs : 16;
+	packs.info = NULL;
+	ALLOC_ARRAY(packs.info, packs.alloc);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
-			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
-			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.info, packs.nr + 1, packs.alloc);
 
-			packs.list[packs.nr] = NULL;
-			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].orig_pack_int_id = i;
+			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].p = NULL;
 			packs.nr++;
 		}
 	}
@@ -795,10 +768,7 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
-
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -807,8 +777,21 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	QSORT(packs.info, packs.nr, pack_info_compare);
+
+	/*
+	 * pack_perm stores a permutation between pack-int-ids from the
+	 * previous multi-pack-index to the new one we are writing:
+	 *
+	 * pack_perm[old_id] = new_id
+	 */
+	ALLOC_ARRAY(pack_perm, packs.nr);
+	for (i = 0; i < packs.nr; i++) {
+		pack_perm[packs.info[i].orig_pack_int_id] = i;
+	}
+
 	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.names[i]) + 1;
+		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -879,7 +862,7 @@ int write_midx_file(const char *object_dir)
 
 		switch (chunk_ids[i]) {
 			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, packs.names, packs.nr);
+				written += write_midx_pack_names(f, packs.info, packs.nr);
 				break;
 
 			case MIDX_CHUNKID_OIDFANOUT:
@@ -891,7 +874,7 @@ int write_midx_file(const char *object_dir)
 				break;
 
 			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, entries, nr_entries);
+				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
 				break;
 
 			case MIDX_CHUNKID_LARGEOFFSETS:
@@ -914,15 +897,14 @@ int write_midx_file(const char *object_dir)
 
 cleanup:
 	for (i = 0; i < packs.nr; i++) {
-		if (packs.list[i]) {
-			close_pack(packs.list[i]);
-			free(packs.list[i]);
+		if (packs.info[i].p) {
+			close_pack(packs.info[i].p);
+			free(packs.info[i].p);
 		}
-		free(packs.names[i]);
+		free(packs.info[i].pack_name);
 	}
 
-	free(packs.list);
-	free(packs.names);
+	free(packs.info);
 	free(entries);
 	free(pack_perm);
 	free(midx_name);

builtin/repack.c, packfile.c, packfile.h

diff --git a/builtin/repack.c b/builtin/repack.c
index 67f8978043..1f9e6fad1b 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
 
diff --git a/packfile.c b/packfile.c
index 060de420d1..683ce5674c 100644
--- a/packfile.c
+++ b/packfile.c
@@ -352,6 +352,34 @@ void close_all_packs(struct raw_object_store *o)
 	}
 }
 
+void unlink_pack_path(const char *pack_name, int force_delete)
+{
+	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	int i;
+	struct strbuf buf = STRBUF_INIT;
+	size_t plen;
+
+	strbuf_addstr(&buf, pack_name);
+	strip_suffix_mem(buf.buf, &buf.len, ".pack");
+	plen = buf.len;
+
+	if (!force_delete) {
+		strbuf_addstr(&buf, ".keep");
+		if (!access(buf.buf, F_OK)) {
+			strbuf_release(&buf);
+			return;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(exts); i++) {
+		strbuf_setlen(&buf, plen);
+		strbuf_addstr(&buf, exts[i]);
+		unlink(buf.buf);
+	}
+
+	strbuf_release(&buf);
+}
+
 /*
  * The LRU pack is the one with the oldest MRU window, preferring packs
  * with no used windows, or the oldest mtime if it has no windows allocated.
diff --git a/packfile.h b/packfile.h
index 12baa6118a..09f5222113 100644
--- a/packfile.h
+++ b/packfile.h
@@ -94,6 +94,13 @@ void unuse_pack(struct pack_window **);
 void clear_delta_base_cache(void);
 struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Unlink the .pack and associated extension files.
+ * Does not unlink if 'force_delete' is false and the pack-file is
+ * marked as ".keep".
+ */
+extern void unlink_pack_path(const char *pack_name, int force_delete);
+
 /*
  * Make sure that a pointer access into an mmap'd index file is within bounds,
  * and can provide at least 8 bytes of data.

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index f7778a2c85..1af406aca2 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,13 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
 
 
 EXAMPLES

diff --git a/midx.c b/midx.c
index 62404620ad..6d4b84e243 100644
--- a/midx.c
+++ b/midx.c
@@ -427,12 +427,23 @@ static size_t write_midx_header(struct hashfile *f,
 	return MIDX_HEADER_SIZE;
 }
 
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+};
+
+static int pack_info_compare(const void *_a, const void *_b)
+{
+	struct pack_info *a = (struct pack_info *)_a;
+	struct pack_info *b = (struct pack_info *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
 struct pack_list {
-	struct packed_git **list;
-	char **names;
+	struct pack_info *info;
 	uint32_t nr;
-	uint32_t alloc_list;
-	uint32_t alloc_names;
+	uint32_t alloc;
 	struct multi_pack_index *m;
 };
 
@@ -445,66 +456,32 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 		if (packs->m && midx_contains_pack(packs->m, file_name))
 			return;
 
-		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
-		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+		ALLOC_GROW(packs->info, packs->nr + 1, packs->alloc);
 
-		packs->list[packs->nr] = add_packed_git(full_path,
-							full_path_len,
-							0);
+		packs->info[packs->nr].p = add_packed_git(full_path,
+							  full_path_len,
+							  0);
 
-		if (!packs->list[packs->nr]) {
+		if (!packs->info[packs->nr].p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}
 
-		if (open_pack_index(packs->list[packs->nr])) {
+		if (open_pack_index(packs->info[packs->nr].p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(packs->list[packs->nr]);
-			FREE_AND_NULL(packs->list[packs->nr]);
+			close_pack(packs->info[packs->nr].p);
+			FREE_AND_NULL(packs->info[packs->nr].p);
 			return;
 		}
 
-		packs->names[packs->nr] = xstrdup(file_name);
+		packs->info[packs->nr].pack_name = xstrdup(file_name);
+		packs->info[packs->nr].orig_pack_int_id = packs->nr;
 		packs->nr++;
 	}
 }
 
-struct pack_pair {
-	uint32_t pack_int_id;
-	char *pack_name;
-};
-
-static int pack_pair_compare(const void *_a, const void *_b)
-{
-	struct pack_pair *a = (struct pack_pair *)_a;
-	struct pack_pair *b = (struct pack_pair *)_b;
-	return strcmp(a->pack_name, b->pack_name);
-}
-
-static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
-{
-	uint32_t i;
-	struct pack_pair *pairs;
-
-	ALLOC_ARRAY(pairs, nr_packs);
-
-	for (i = 0; i < nr_packs; i++) {
-		pairs[i].pack_int_id = i;
-		pairs[i].pack_name = pack_names[i];
-	}
-
-	QSORT(pairs, nr_packs, pack_pair_compare);
-
-	for (i = 0; i < nr_packs; i++) {
-		pack_names[i] = pairs[i].pack_name;
-		perm[pairs[i].pack_int_id] = i;
-	}
-
-	free(pairs);
-}
-
 struct pack_midx_entry {
 	struct object_id oid;
 	uint32_t pack_int_id;
@@ -530,7 +507,6 @@ static int midx_oid_compare(const void *_a, const void *_b)
 }
 
 static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
-				      uint32_t *pack_perm,
 				      struct pack_midx_entry *e,
 				      uint32_t pos)
 {
@@ -538,7 +514,7 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 		return 1;
 
 	nth_midxed_object_oid(&e->oid, m, pos);
-	e->pack_int_id = pack_perm[nth_midxed_pack_int_id(m, pos)];
+	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
 	e->offset = nth_midxed_offset(m, pos);
 
 	/* consider objects in midx to be from "old" packs */
@@ -572,8 +548,7 @@ static void fill_pack_entry(uint32_t pack_int_id,
  * of a packfile containing the object).
  */
 static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
-						  struct packed_git **p,
-						  uint32_t *perm,
+						  struct pack_info *info,
 						  uint32_t nr_packs,
 						  uint32_t *nr_objects)
 {
@@ -584,7 +559,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 	uint32_t start_pack = m ? m->num_packs : 0;
 
 	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
-		total_objects += p[cur_pack]->num_objects;
+		total_objects += info[cur_pack].p->num_objects;
 
 	/*
 	 * As we de-duplicate by fanout value, we expect the fanout
@@ -609,7 +584,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				nth_midxed_pack_midx_entry(m, perm,
+				nth_midxed_pack_midx_entry(m,
 							   &entries_by_fanout[nr_fanout],
 							   cur_object);
 				nr_fanout++;
@@ -620,12 +595,12 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 			uint32_t start = 0, end;
 
 			if (cur_fanout)
-				start = get_pack_fanout(p[cur_pack], cur_fanout - 1);
-			end = get_pack_fanout(p[cur_pack], cur_fanout);
+				start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
+			end = get_pack_fanout(info[cur_pack].p, cur_fanout);
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				fill_pack_entry(perm[cur_pack], p[cur_pack], cur_object, &entries_by_fanout[nr_fanout]);
+				fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
 				nr_fanout++;
 			}
 		}
@@ -654,7 +629,7 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 }
 
 static size_t write_midx_pack_names(struct hashfile *f,
-				    char **pack_names,
+				    struct pack_info *info,
 				    uint32_t num_packs)
 {
 	uint32_t i;
@@ -662,14 +637,14 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(pack_names[i]) + 1;
+		size_t writelen = strlen(info[i].pack_name) + 1;
 
-		if (i && strcmp(pack_names[i], pack_names[i - 1]) <= 0)
+		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
-			    pack_names[i - 1],
-			    pack_names[i]);
+			    info[i - 1].pack_name,
+			    info[i].pack_name);
 
-		hashwrite(f, pack_names[i], writelen);
+		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
 
@@ -740,6 +715,7 @@ static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
 }
 
 static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
+					uint32_t *perm,
 					struct pack_midx_entry *objects, uint32_t nr_objects)
 {
 	struct pack_midx_entry *list = objects;
@@ -749,7 +725,7 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		hashwrite_be32(f, obj->pack_int_id);
+		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
@@ -822,20 +798,17 @@ int write_midx_file(const char *object_dir)
 	packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
-	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
-	packs.alloc_names = packs.alloc_list;
-	packs.list = NULL;
-	packs.names = NULL;
-	ALLOC_ARRAY(packs.list, packs.alloc_list);
-	ALLOC_ARRAY(packs.names, packs.alloc_names);
+	packs.alloc = packs.m ? packs.m->num_packs : 16;
+	packs.info = NULL;
+	ALLOC_ARRAY(packs.info, packs.alloc);
 
 	if (packs.m) {
 		for (i = 0; i < packs.m->num_packs; i++) {
-			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
-			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+			ALLOC_GROW(packs.info, packs.nr + 1, packs.alloc);
 
-			packs.list[packs.nr] = NULL;
-			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].orig_pack_int_id = i;
+			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
+			packs.info[packs.nr].p = NULL;
 			packs.nr++;
 		}
 	}
@@ -845,10 +818,7 @@ int write_midx_file(const char *object_dir)
 	if (packs.m && packs.nr == packs.m->num_packs)
 		goto cleanup;
 
-	ALLOC_ARRAY(pack_perm, packs.nr);
-	sort_packs_by_name(packs.names, packs.nr, pack_perm);
-
-	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
 
 	for (i = 0; i < nr_entries; i++) {
 		if (entries[i].offset > 0x7fffffff)
@@ -857,8 +827,21 @@ int write_midx_file(const char *object_dir)
 			large_offsets_needed = 1;
 	}
 
+	QSORT(packs.info, packs.nr, pack_info_compare);
+
+	/*
+	 * pack_perm stores a permutation between pack-int-ids from the
+	 * previous multi-pack-index to the new one we are writing:
+	 *
+	 * pack_perm[old_id] = new_id
+	 */
+	ALLOC_ARRAY(pack_perm, packs.nr);
+	for (i = 0; i < packs.nr; i++) {
+		pack_perm[packs.info[i].orig_pack_int_id] = i;
+	}
+
 	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.names[i]) + 1;
+		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -929,7 +912,7 @@ int write_midx_file(const char *object_dir)
 
 		switch (chunk_ids[i]) {
 			case MIDX_CHUNKID_PACKNAMES:
-				written += write_midx_pack_names(f, packs.names, packs.nr);
+				written += write_midx_pack_names(f, packs.info, packs.nr);
 				break;
 
 			case MIDX_CHUNKID_OIDFANOUT:
@@ -941,7 +924,7 @@ int write_midx_file(const char *object_dir)
 				break;
 
 			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, entries, nr_entries);
+				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
 				break;
 
 			case MIDX_CHUNKID_LARGEOFFSETS:
@@ -964,15 +947,14 @@ int write_midx_file(const char *object_dir)
 
 cleanup:
 	for (i = 0; i < packs.nr; i++) {
-		if (packs.list[i]) {
-			close_pack(packs.list[i]);
-			free(packs.list[i]);
+		if (packs.info[i].p) {
+			close_pack(packs.info[i].p);
+			free(packs.info[i].p);
 		}
-		free(packs.names[i]);
+		free(packs.info[i].pack_name);
 	}
 
-	free(packs.list);
-	free(packs.names);
+	free(packs.info);
 	free(entries);
 	free(pack_perm);
 	free(midx_name);

midx.c, t/t5319-multi-pack-index.sh

diff --git a/midx.c b/midx.c
index 6d4b84e243..9b0b4c1520 100644
--- a/midx.c
+++ b/midx.c
@@ -34,6 +34,8 @@
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000
 
+#define PACK_EXPIRED UINT_MAX
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
@@ -431,6 +433,7 @@ struct pack_info {
 	uint32_t orig_pack_int_id;
 	char *pack_name;
 	struct packed_git *p;
+	unsigned expired : 1;
 };
 
 static int pack_info_compare(const void *_a, const void *_b)
@@ -478,6 +481,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 
 		packs->info[packs->nr].pack_name = xstrdup(file_name);
 		packs->info[packs->nr].orig_pack_int_id = packs->nr;
+		packs->info[packs->nr].expired = 0;
 		packs->nr++;
 	}
 }
@@ -637,13 +641,17 @@ static size_t write_midx_pack_names(struct hashfile *f,
 	size_t written = 0;
 
 	for (i = 0; i < num_packs; i++) {
-		size_t writelen = strlen(info[i].pack_name) + 1;
+		size_t writelen;
+
+		if (info[i].expired)
+			continue;
 
 		if (i && strcmp(info[i].pack_name, info[i - 1].pack_name) <= 0)
 			BUG("incorrect pack-file order: %s before %s",
 			    info[i - 1].pack_name,
 			    info[i].pack_name);
 
+		writelen = strlen(info[i].pack_name) + 1;
 		hashwrite(f, info[i].pack_name, writelen);
 		written += writelen;
 	}
@@ -725,6 +733,11 @@ static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_nee
 	for (i = 0; i < nr_objects; i++) {
 		struct pack_midx_entry *obj = list++;
 
+		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+			BUG("object %s is in an expired pack with int-id %d",
+			    oid_to_hex(&obj->oid),
+			    obj->pack_int_id);
+
 		hashwrite_be32(f, perm[obj->pack_int_id]);
 
 		if (large_offset_needed && obj->offset >> 31)
@@ -771,7 +784,8 @@ static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_off
 	return written;
 }
 
-int write_midx_file(const char *object_dir)
+static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
+			       struct string_list *packs_to_drop)
 {
 	unsigned char cur_chunk, num_chunks = 0;
 	char *midx_name;
@@ -787,6 +801,8 @@ int write_midx_file(const char *object_dir)
 	struct pack_midx_entry *entries = NULL;
 	int large_offsets_needed = 0;
 	int pack_name_concat_len = 0;
+	int dropped_packs = 0;
+	int result = 0;
 
 	midx_name = get_midx_filename(object_dir);
 	if (safe_create_leading_directories(midx_name)) {
@@ -795,7 +811,10 @@ int write_midx_file(const char *object_dir)
 			  midx_name);
 	}
 
-	packs.m = load_multi_pack_index(object_dir, 1);
+	if (m)
+		packs.m = m;
+	else
+		packs.m = load_multi_pack_index(object_dir, 1);
 
 	packs.nr = 0;
 	packs.alloc = packs.m ? packs.m->num_packs : 16;
@@ -809,13 +828,14 @@ int write_midx_file(const char *object_dir)
 			packs.info[packs.nr].orig_pack_int_id = i;
 			packs.info[packs.nr].pack_name = xstrdup(packs.m->pack_names[i]);
 			packs.info[packs.nr].p = NULL;
+			packs.info[packs.nr].expired = 0;
 			packs.nr++;
 		}
 	}
 
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
 
-	if (packs.m && packs.nr == packs.m->num_packs)
+	if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
 	entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);
@@ -829,6 +849,34 @@ int write_midx_file(const char *object_dir)
 
 	QSORT(packs.info, packs.nr, pack_info_compare);
 
+	if (packs_to_drop && packs_to_drop->nr) {
+		int drop_index = 0;
+		int missing_drops = 0;
+
+		for (i = 0; i < packs.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(packs.info[i].pack_name,
+					 packs_to_drop->items[drop_index].string);
+
+			if (!cmp) {
+				drop_index++;
+				packs.info[i].expired = 1;
+			} else if (cmp > 0) {
+				error(_("did not see pack-file %s to drop"),
+				      packs_to_drop->items[drop_index].string);
+				drop_index++;
+				missing_drops++;
+				i--;
+			} else {
+				packs.info[i].expired = 0;
+			}
+		}
+
+		if (missing_drops) {
+			result = 1;
+			goto cleanup;
+		}
+	}
+
 	/*
 	 * pack_perm stores a permutation between pack-int-ids from the
 	 * previous multi-pack-index to the new one we are writing:
@@ -837,11 +885,18 @@ int write_midx_file(const char *object_dir)
 	 */
 	ALLOC_ARRAY(pack_perm, packs.nr);
 	for (i = 0; i < packs.nr; i++) {
-		pack_perm[packs.info[i].orig_pack_int_id] = i;
+		if (packs.info[i].expired) {
+			dropped_packs++;
+			pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
+		} else {
+			pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
+		}
 	}
 
-	for (i = 0; i < packs.nr; i++)
-		pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	for (i = 0; i < packs.nr; i++) {
+		if (!packs.info[i].expired)
+			pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
+	}
 
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
@@ -857,7 +912,7 @@ int write_midx_file(const char *object_dir)
 	cur_chunk = 0;
 	num_chunks = large_offsets_needed ? 5 : 4;
 
-	written = write_midx_header(f, num_chunks, packs.nr);
+	written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);
 
 	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
 	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
@@ -958,7 +1013,12 @@ int write_midx_file(const char *object_dir)
 	free(entries);
 	free(pack_perm);
 	free(midx_name);
-	return 0;
+	return result;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	return write_midx_internal(object_dir, NULL, NULL);
 }
 
 void clear_midx_file(struct repository *r)
@@ -1125,5 +1185,44 @@ int verify_midx_file(struct repository *r, const char *object_dir)
 
 int expire_midx_packs(struct repository *r, const char *object_dir)
 {
-	return 0;
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 0;
+
+	count = xcalloc(m->num_packs, sizeof(uint32_t));
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+	}
+
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(r, m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, m, &packs_to_drop);
+
+	string_list_clear(&packs_to_drop, 0);
+	return result;
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 1b2d32f475..12570fe7ac 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -412,4 +412,24 @@ test_expect_success 'expire does not remove any packs' '
 	)
 '
 
+test_expect_success 'expire removes unreferenced packs' '
+	(
+		cd dup &&
+		git pack-objects --revs .git/objects/pack/pack-combined <<-EOF &&
+		refs/heads/A
+		^refs/heads/C
+		EOF
+		git multi-pack-index write &&
+		ls .git/objects/pack | grep -v -e pack-[AB] >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual &&
+		ls .git/objects/pack/ | grep idx >expect-idx &&
+		test-tool read-midx .git/objects | grep idx >actual-midx &&
+		test_cmp expect-idx actual-midx &&
+		git multi-pack-index verify &&
+		git fsck
+	)
+'
+
 test_done

Documentation/git-multi-pack-index.txt, builtin/multi-pack-index.c, midx.c, midx.h, t/t5319-multi-pack-index.sh

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 1af406aca2..6186c4c936 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 72dfd3dadc..ad10d40512 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -6,7 +6,7 @@
 #include "trace2.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -47,6 +47,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(the_repository, opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(the_repository, opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }
diff --git a/midx.c b/midx.c
index e7e1fe4d65..3b7da1a360 100644
--- a/midx.c
+++ b/midx.c
@@ -1140,3 +1140,8 @@ int verify_midx_file(struct repository *r, const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(struct repository *r, const char *object_dir)
+{
+	return 0;
+}
diff --git a/midx.h b/midx.h
index 3eb29731f2..505f1431b7 100644
--- a/midx.h
+++ b/midx.h
@@ -50,6 +50,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct repository *r, const char *object_dir);
+int expire_midx_packs(struct repository *r, const char *object_dir);
 
 void close_midx(struct multi_pack_index *m);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 1ebf19ec3c..1b2d32f475 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -363,4 +363,53 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		test-tool genrandom "data" 4096 >large_file.txt &&
+		git update-index --add large_file.txt &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done