patch, 7 partsFirst class shallow clone

diff --git a/transport.h b/transport.h
index ea70ea7..3178dc9 100644
--- a/transport.h
+++ b/transport.h
@@ -182,10 +182,4 @@ void transport_print_push_status(const char *dest, struct ref *refs,
 
 typedef void alternate_ref_fn(const struct ref *, void *);
 extern void for_each_alternate_ref(alternate_ref_fn, void *);
-
-struct send_pack_args;
-extern int send_pack(struct send_pack_args *args,
-		     int fd[], struct child_process *conn,
-		     struct ref *remote_refs,
-		     struct extra_have_objects *extra_have);
 #endif

Documentation/technical/pack-protocol.txt, builtin/receive-pack.c, commit.h, shallow.c, upload-pack.c

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index b898e97..eb8edd1 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -161,6 +161,7 @@ MUST peel the ref if it's an annotated tag.
 
 ----
   advertised-refs  =  (no-refs / list-of-refs)
+		      *shallow
 		      flush-pkt
 
   no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
@@ -174,6 +175,8 @@ MUST peel the ref if it's an annotated tag.
   other-tip        =  obj-id SP refname LF
   other-peeled     =  obj-id SP refname "^{}" LF
 
+  shallow          =  PKT-LINE("shallow" SP obj-id)
+
   capability-list  =  capability *(SP capability)
   capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
   LC_ALPHA         =  %x61-7A
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index e3eb5fc..2d8e19b 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -176,6 +176,8 @@ static void write_head_info(void)
 	if (!sent_capabilities)
 		show_ref("capabilities^{}", null_sha1);
 
+	advertise_shallow_grafts(1);
+
 	/* EOF */
 	packet_flush(1);
 }
@@ -990,9 +992,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	if (!enter_repo(dir, 0))
 		die("'%s' does not appear to be a git repository", dir);
 
-	if (is_repository_shallow())
-		die("attempt to push into a shallow repository");
-
 	git_config(receive_pack_config, NULL);
 
 	if (0 <= transfer_unpack_limit)
diff --git a/commit.h b/commit.h
index 4d452dc..e0688c3 100644
--- a/commit.h
+++ b/commit.h
@@ -187,6 +187,7 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 		int depth, int shallow_flag, int not_shallow_flag);
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
+extern void advertise_shallow_grafts(int);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/shallow.c b/shallow.c
index cbe2526..ccdfefc 100644
--- a/shallow.c
+++ b/shallow.c
@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "commit.h"
 #include "tag.h"
+#include "pkt-line.h"
 
 static int is_shallow = -1;
 static struct stat shallow_stat;
@@ -146,3 +147,18 @@ void check_shallow_file_for_update(void)
 		   )
 		die("shallow file was changed during fetch");
 }
+
+static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
+{
+	int fd = *(int*)cb;
+	if (graft->nr_parent == -1)
+		packet_write(fd, "shallow %s\n", sha1_to_hex(graft->sha1));
+	return 0;
+}
+
+void advertise_shallow_grafts(int fd)
+{
+	if (!is_repository_shallow())
+		return;
+	for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
+}
diff --git a/upload-pack.c b/upload-pack.c
index 127e59a..6cefe43 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -766,6 +766,7 @@ static void upload_pack(void)
 		reset_timeout();
 		head_ref_namespaced(send_ref, NULL);
 		for_each_namespaced_ref(send_ref, NULL);
+		advertise_shallow_grafts(1);
 		packet_flush(1);
 	} else {
 		head_ref_namespaced(mark_our_ref, NULL);
@@ -837,8 +838,6 @@ int main(int argc, char **argv)
 
 	if (!enter_repo(dir, strict))
 		die("'%s' does not appear to be a git repository", dir);
-	if (is_repository_shallow())
-		die("attempt to fetch/clone from a shallow repository");
 	git_config(upload_pack_config, NULL);
 	upload_pack();
 	return 0;

builtin/fetch-pack.c, builtin/send-pack.c, cache.h, connect.c, remote-curl.c, transport.c

diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index aba4465..080e599 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -144,7 +144,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 				   args.verbose ? CONNECT_VERBOSE : 0);
 	}
 
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL);
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, NULL);
 
 	ref = fetch_pack(&args, fd, conn, ref, dest,
 			 sought, nr_sought, pack_lockfile_ptr);
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 152c4ea..f0ad0ce 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -207,7 +207,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 
 	memset(&extra_have, 0, sizeof(extra_have));
 
-	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have);
+	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have, NULL);
 
 	transport_verify_remote_names(nr_refspecs, refspecs);
 
diff --git a/cache.h b/cache.h
index dd0fb33..7665e03 100644
--- a/cache.h
+++ b/cache.h
@@ -1091,6 +1091,7 @@ struct extra_have_objects {
 };
 extern struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 				     struct ref **list, unsigned int flags,
+				     struct extra_have_objects *,
 				     struct extra_have_objects *);
 extern int server_supports(const char *feature);
 extern int parse_feature_request(const char *features, const char *feature);
diff --git a/connect.c b/connect.c
index a0783d4..1cba424 100644
--- a/connect.c
+++ b/connect.c
@@ -64,7 +64,8 @@ static void die_initial_contact(int got_at_least_one_head)
  */
 struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			      struct ref **list, unsigned int flags,
-			      struct extra_have_objects *extra_have)
+			      struct extra_have_objects *extra_have,
+			      struct extra_have_objects *shallow_points)
 {
 	int got_at_least_one_head = 0;
 
@@ -89,6 +90,15 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 		if (len > 4 && !prefixcmp(buffer, "ERR "))
 			die("remote error: %s", buffer + 4);
 
+		if (len == 48 && !prefixcmp(buffer, "shallow ")) {
+			if (get_sha1_hex(buffer + 8, old_sha1))
+				die("protocol error: expected shallow sha, got '%s'", buffer + 8);
+			if (!shallow_points)
+				die("repository on the other end cannot be shallow");
+			add_extra_have(shallow_points, old_sha1);
+			continue;
+		}
+
 		if (len < 42 || get_sha1_hex(buffer, old_sha1) || buffer[40] != ' ')
 			die("protocol error: expected sha/ref, got '%s'", buffer);
 		name = buffer + 41;
diff --git a/remote-curl.c b/remote-curl.c
index 5b3ce9e..c329bd3 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -86,7 +86,7 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 {
 	struct ref *list = NULL;
 	get_remote_heads(-1, heads->buf, heads->len, &list,
-			 for_push ? REF_NORMAL : 0, NULL);
+			 for_push ? REF_NORMAL : 0, NULL, NULL);
 	return list;
 }
 
diff --git a/transport.c b/transport.c
index e15db98..10a8cb8 100644
--- a/transport.c
+++ b/transport.c
@@ -509,7 +509,7 @@ static struct ref *get_refs_via_connect(struct transport *transport, int for_pus
 
 	connect_setup(transport, for_push, 0);
 	get_remote_heads(data->fd[0], NULL, 0, &refs,
-			 for_push ? REF_NORMAL : 0, &data->extra_have);
+			 for_push ? REF_NORMAL : 0, &data->extra_have, NULL);
 	data->got_remote_heads = 1;
 
 	return refs;
@@ -539,7 +539,8 @@ static int fetch_refs_via_pack(struct transport *transport,
 
 	if (!data->got_remote_heads) {
 		connect_setup(transport, 0, 0);
-		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0,
+				 NULL, NULL);
 		data->got_remote_heads = 1;
 	}
 
@@ -799,7 +800,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		struct ref *tmp_refs;
 		connect_setup(transport, 1, 0);
 
-		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL, NULL);
 		data->got_remote_heads = 1;
 	}
 

commit.h, fetch-pack.c, shallow.c

diff --git a/commit.h b/commit.h
index e0688c3..678fa20 100644
--- a/commit.h
+++ b/commit.h
@@ -188,6 +188,9 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
 extern void advertise_shallow_grafts(int);
+extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
+extern void setup_alternate_shallow(struct lock_file *shallow_lock,
+				    const char **alternate_shallow_file);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/fetch-pack.c b/fetch-pack.c
index abe5ffb..dc71a2b 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -185,36 +185,6 @@ static void consume_shallow_list(struct fetch_pack_args *args, int fd)
 	}
 }
 
-struct write_shallow_data {
-	struct strbuf *out;
-	int use_pack_protocol;
-	int count;
-};
-
-static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
-{
-	struct write_shallow_data *data = cb_data;
-	const char *hex = sha1_to_hex(graft->sha1);
-	data->count++;
-	if (data->use_pack_protocol)
-		packet_buf_write(data->out, "shallow %s", hex);
-	else {
-		strbuf_addstr(data->out, hex);
-		strbuf_addch(data->out, '\n');
-	}
-	return 0;
-}
-
-static int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
-{
-	struct write_shallow_data data;
-	data.out = out;
-	data.use_pack_protocol = use_pack_protocol;
-	data.count = 0;
-	for_each_commit_graft(write_one_shallow, &data);
-	return data.count;
-}
-
 static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 {
 	int len;
@@ -795,27 +765,6 @@ static int cmp_ref_by_name(const void *a_, const void *b_)
 	return strcmp(a->name, b->name);
 }
 
-static void setup_alternate_shallow(void)
-{
-	struct strbuf sb = STRBUF_INIT;
-	int fd;
-
-	check_shallow_file_for_update();
-	fd = hold_lock_file_for_update(&shallow_lock, git_path("shallow"),
-				       LOCK_DIE_ON_ERROR);
-	if (write_shallow_commits(&sb, 0)) {
-		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
-			die_errno("failed to write to %s", shallow_lock.filename);
-		alternate_shallow_file = shallow_lock.filename;
-	} else
-		/*
-		 * is_repository_shallow() sees empty string as "no
-		 * shallow file".
-		 */
-		alternate_shallow_file = "";
-	strbuf_release(&sb);
-}
-
 static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
@@ -896,7 +845,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
 	if (args->depth > 0)
-		setup_alternate_shallow();
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file);
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
diff --git a/shallow.c b/shallow.c
index ccdfefc..ee9edd4 100644
--- a/shallow.c
+++ b/shallow.c
@@ -162,3 +162,56 @@ void advertise_shallow_grafts(int fd)
 		return;
 	for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
 }
+
+struct write_shallow_data {
+	struct strbuf *out;
+	int use_pack_protocol;
+	int count;
+};
+
+static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
+{
+	struct write_shallow_data *data = cb_data;
+	const char *hex = sha1_to_hex(graft->sha1);
+	data->count++;
+	if (data->use_pack_protocol)
+		packet_buf_write(data->out, "shallow %s", hex);
+	else {
+		strbuf_addstr(data->out, hex);
+		strbuf_addch(data->out, '\n');
+	}
+	return 0;
+}
+
+int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
+{
+	struct write_shallow_data data;
+	data.out = out;
+	data.use_pack_protocol = use_pack_protocol;
+	data.count = 0;
+	for_each_commit_graft(write_one_shallow, &data);
+	return data.count;
+}
+
+void setup_alternate_shallow(struct lock_file *shallow_lock,
+			     const char **alternate_shallow_file)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int fd;
+
+	check_shallow_file_for_update();
+	fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
+				       LOCK_DIE_ON_ERROR);
+	if (write_shallow_commits(&sb, 0)) {
+		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
+			die_errno("failed to write to %s",
+				  shallow_lock->filename);
+		*alternate_shallow_file = shallow_lock->filename;
+	} else
+		/*
+		 * is_repository_shallow() sees empty string as "no
+		 * shallow file".
+		 */
+		*alternate_shallow_file = "";
+	strbuf_release(&sb);
+}

builtin/fetch-pack.c, commit.h, fetch-pack.c, fetch-pack.h, shallow.c, t/t5536-fetch-shallow.sh, transport.c

diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 080e599..b89d753 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -37,6 +37,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 	char **pack_lockfile_ptr = NULL;
 	struct child_process *conn;
 	struct fetch_pack_args args;
+	struct extra_have_objects shallow;
 
 	packet_trace_identity("fetch-pack");
 
@@ -144,10 +145,11 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 				   args.verbose ? CONNECT_VERBOSE : 0);
 	}
 
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, NULL);
+	memset(&shallow, 0, sizeof(shallow));
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, &shallow);
 
 	ref = fetch_pack(&args, fd, conn, ref, dest,
-			 sought, nr_sought, pack_lockfile_ptr);
+			 sought, nr_sought, &shallow, pack_lockfile_ptr);
 	if (pack_lockfile) {
 		printf("lock %s\n", pack_lockfile);
 		fflush(stdout);
diff --git a/commit.h b/commit.h
index 678fa20..7faf0e4 100644
--- a/commit.h
+++ b/commit.h
@@ -188,9 +188,13 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
 extern void advertise_shallow_grafts(int);
-extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
+extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+				 struct extra_have_objects *extra,
+				 int remove_unused_grafts);
 extern void setup_alternate_shallow(struct lock_file *shallow_lock,
-				    const char **alternate_shallow_file);
+				    const char **alternate_shallow_file,
+				    struct extra_have_objects *extra,
+				    int rewrite);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/fetch-pack.c b/fetch-pack.c
index dc71a2b..68b95a5 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -311,7 +311,7 @@ static int find_common(struct fetch_pack_args *args,
 	}
 
 	if (is_repository_shallow())
-		write_shallow_commits(&req_buf, 1);
+		write_shallow_commits(&req_buf, 1, NULL, 0);
 	if (args->depth > 0)
 		packet_buf_write(&req_buf, "deepen %d", args->depth);
 	packet_buf_flush(&req_buf);
@@ -769,6 +769,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
 				 struct ref **sought, int nr_sought,
+				 struct extra_have_objects *shallow,
 				 char **pack_lockfile)
 {
 	struct ref *ref = copy_ref_list(orig_ref);
@@ -844,8 +845,9 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
-	if (args->depth > 0)
-		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file);
+	if (args->depth > 0 || shallow->nr)
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
+					shallow, 0);
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
@@ -922,6 +924,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       const struct ref *ref,
 		       const char *dest,
 		       struct ref **sought, int nr_sought,
+		       struct extra_have_objects *shallow,
 		       char **pack_lockfile)
 {
 	struct ref *ref_cpy;
@@ -934,14 +937,28 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		packet_flush(fd[1]);
 		die("no matching remote head");
 	}
-	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought, pack_lockfile);
+	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought,
+				shallow, pack_lockfile);
 
 	if (alternate_shallow_file) {
 		if (*alternate_shallow_file == '\0') { /* --unshallow */
 			unlink_or_warn(git_path("shallow"));
 			rollback_lock_file(&shallow_lock);
-		} else
+		} else {
+			/*
+			 *  The server is a shallow clone and it sends
+			 *  us all of its shallow grafts. Some may be
+			 *  needed if the server sends objects down to
+			 *  the bottom. Remove all unused grafts.
+			 */
+			if (shallow->nr) {
+				reprepare_packed_git();
+				setup_alternate_shallow(&shallow_lock,
+							&alternate_shallow_file,
+							shallow, 1);
+			}
 			commit_lock_file(&shallow_lock);
+		}
 	}
 
 	reprepare_packed_git();
diff --git a/fetch-pack.h b/fetch-pack.h
index 40f08ba..c55bf1c 100644
--- a/fetch-pack.h
+++ b/fetch-pack.h
@@ -32,6 +32,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       const char *dest,
 		       struct ref **sought,
 		       int nr_sought,
+		       struct extra_have_objects *shallow,
 		       char **pack_lockfile);
 
 #endif
diff --git a/shallow.c b/shallow.c
index ee9edd4..d909d95 100644
--- a/shallow.c
+++ b/shallow.c
@@ -183,26 +183,60 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	return 0;
 }
 
-int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
+int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+			  struct extra_have_objects *extra,
+			  int remove_unused_grafts)
 {
 	struct write_shallow_data data;
+	int i;
 	data.out = out;
 	data.use_pack_protocol = use_pack_protocol;
 	data.count = 0;
 	for_each_commit_graft(write_one_shallow, &data);
+	if (!extra)
+		return data.count;
+
+	for (i = 0; i < extra->nr; i++) {
+		if (!remove_unused_grafts && has_sha1_file(extra->array[i]))
+			/*
+			 * The server may have even shorter history
+			 * than the client (e.g. the client has
+			 * A-B-C-D but the server only has C-D). We do
+			 * NOT want to cut client's history down to
+			 * C-D simply because the server is set up
+			 * so. If we don't have "C" and the server
+			 * sends C-D to us, then we set up a graft at
+			 * "C".
+			 */
+			continue;
+		if (remove_unused_grafts && !has_sha1_file(extra->array[i]))
+			continue;
+		strbuf_addstr(out, sha1_to_hex(extra->array[i]));
+		strbuf_addch(out, '\n');
+		data.count++;
+	}
 	return data.count;
 }
 
 void setup_alternate_shallow(struct lock_file *shallow_lock,
-			     const char **alternate_shallow_file)
+			     const char **alternate_shallow_file,
+			     struct extra_have_objects *extra,
+			     int rewrite)
 {
 	struct strbuf sb = STRBUF_INIT;
 	int fd;
 
-	check_shallow_file_for_update();
-	fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
-				       LOCK_DIE_ON_ERROR);
-	if (write_shallow_commits(&sb, 0)) {
+	if (!rewrite)
+		fd = hold_lock_file_for_update(shallow_lock,
+					       git_path("shallow"),
+					       LOCK_DIE_ON_ERROR);
+	else
+		fd = shallow_lock->fd;
+	if (write_shallow_commits(&sb, 0, extra, rewrite)) {
+		if (rewrite && (lseek(fd, 0, SEEK_SET) == -1 ||
+				ftruncate(fd, 0) == -1))
+			die_errno("unable to truncate the new shallow file %s",
+				  shallow_lock->filename);
 		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
 			die_errno("failed to write to %s",
 				  shallow_lock->filename);
diff --git a/t/t5536-fetch-shallow.sh b/t/t5536-fetch-shallow.sh
new file mode 100755
index 0000000..599928f
--- /dev/null
+++ b/t/t5536-fetch-shallow.sh
@@ -0,0 +1,75 @@
+#!/bin/sh
+
+test_description='fetch/clone from a shallow clone'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit 1 &&
+	test_commit 2 &&
+	test_commit 3 &&
+	test_commit 4
+'
+
+test_expect_success 'setup shallow clone' '
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'clone from shallow clone' '
+	git clone --no-local shallow shallow2 &&
+	(
+	cd shallow2 &&
+	git fsck &&
+	git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch from shallow clone' '
+	(
+	cd shallow &&
+	test_commit 5
+	) &&
+	(
+	cd shallow2 &&
+	git fetch &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch --depth from shallow clone' '
+	(
+	cd shallow &&
+	test_commit 6
+	) &&
+	(
+	cd shallow2 &&
+	git fetch --depth=2 &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+6
+5
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_done
diff --git a/transport.c b/transport.c
index 10a8cb8..17fef16 100644
--- a/transport.c
+++ b/transport.c
@@ -453,6 +453,7 @@ struct git_transport_data {
 	int fd[2];
 	unsigned got_remote_heads : 1;
 	struct extra_have_objects extra_have;
+	struct extra_have_objects shallow;
 };
 
 static int set_git_option(struct git_transport_options *opts,
@@ -509,7 +510,8 @@ static struct ref *get_refs_via_connect(struct transport *transport, int for_pus
 
 	connect_setup(transport, for_push, 0);
 	get_remote_heads(data->fd[0], NULL, 0, &refs,
-			 for_push ? REF_NORMAL : 0, &data->extra_have, NULL);
+			 for_push ? REF_NORMAL : 0,
+			 &data->extra_have, &data->shallow);
 	data->got_remote_heads = 1;
 
 	return refs;
@@ -540,13 +542,13 @@ static int fetch_refs_via_pack(struct transport *transport,
 	if (!data->got_remote_heads) {
 		connect_setup(transport, 0, 0);
 		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0,
-				 NULL, NULL);
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
 
 	refs = fetch_pack(&args, data->fd, data->conn,
 			  refs_tmp ? refs_tmp : transport->remote_refs,
-			  dest, to_fetch, nr_heads,
+			  dest, to_fetch, nr_heads, &data->shallow,
 			  &transport->pack_lockfile);
 	close(data->fd[0]);
 	close(data->fd[1]);

Documentation/technical/pack-protocol.txt, builtin/receive-pack.c, send-pack.c, t/t5537-push-shallow.sh

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index eb8edd1..c73b62f 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -464,7 +464,9 @@ contain all the objects that the server will need to complete the new
 references.
 
 ----
-  update-request    =  command-list [pack-file]
+  update-request    =  *shallow command-list [pack-file]
+
+  shallow           =  PKT-LINE("shallow" SP obj-id)
 
   command-list      =  PKT-LINE(command NUL capability-list LF)
 		       *PKT-LINE(command LF)
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 2d8e19b..0537e26 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -41,6 +41,10 @@ static int auto_gc = 1;
 static const char *head_name;
 static void *head_name_to_free;
 static int sent_capabilities;
+static int shallow_changed;
+static const char* alternate_shallow_file;
+static struct lock_file shallow_lock;
+static struct extra_have_objects shallow;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -751,6 +755,13 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 	}
 }
 
+static void add_extra_have(struct extra_have_objects *extra, unsigned char *sha1)
+{
+	ALLOC_GROW(extra->array, extra->nr + 1, extra->alloc);
+	hashcpy(&(extra->array[extra->nr][0]), sha1);
+	extra->nr++;
+}
+
 static struct command *read_head_info(void)
 {
 	struct command *commands = NULL;
@@ -765,6 +776,17 @@ static struct command *read_head_info(void)
 		line = packet_read_line(0, &len);
 		if (!line)
 			break;
+
+		if (len == 48 && !prefixcmp(line, "shallow ")) {
+			if (get_sha1_hex(line + 8, old_sha1))
+				die("protocol error: expected shallow sha, got '%s'", line + 8);
+			if (!has_sha1_file(old_sha1)) {
+				add_extra_have(&shallow, old_sha1);
+				shallow_changed = 1;
+			}
+			continue;
+		}
+
 		if (len < 83 ||
 		    line[40] != ' ' ||
 		    line[81] != ' ' ||
@@ -827,6 +849,12 @@ static const char *unpack(int err_fd)
 			    ? transfer_fsck_objects
 			    : 0);
 
+	if (shallow_changed)
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					&shallow, 0);
+
+
 	hdr_err = parse_pack_header(&hdr);
 	if (hdr_err) {
 		if (err_fd > 0)
@@ -854,9 +882,8 @@ static const char *unpack(int err_fd)
 		child.err = err_fd;
 		child.git_cmd = 1;
 		code = run_command(&child);
-		if (!code)
-			return NULL;
-		return "unpack-objects abnormal exit";
+		if (code)
+			return "unpack-objects abnormal exit";
 	} else {
 		const char *keeper[7];
 		int s, status, i = 0;
@@ -887,12 +914,18 @@ static const char *unpack(int err_fd)
 		pack_lockfile = index_pack_lockfile(ip.out);
 		close(ip.out);
 		status = finish_command(&ip);
-		if (!status) {
-			reprepare_packed_git();
-			return NULL;
-		}
-		return "index-pack abnormal exit";
+		if (status)
+			return "index-pack abnormal exit";
+		reprepare_packed_git();
+	}
+
+	if (shallow_changed) {
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					&shallow, 1);
+		commit_lock_file(&shallow_lock);
 	}
+	return NULL;
 }
 
 static const char *unpack_with_sideband(void)
diff --git a/send-pack.c b/send-pack.c
index 7d172ef..81d4b1c 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -212,6 +212,9 @@ int send_pack(struct send_pack_args *args,
 		return 0;
 	}
 
+	if (!args->dry_run && !args->stateless_rpc)
+		advertise_shallow_grafts(out);
+
 	/*
 	 * Finally, tell the other end!
 	 */
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
new file mode 100755
index 0000000..30fdbc8
--- /dev/null
+++ b/t/t5537-push-shallow.sh
@@ -0,0 +1,67 @@
+#!/bin/sh
+
+test_description='push from/to a shallow clone'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit 1 &&
+	test_commit 2 &&
+	test_commit 3 &&
+	test_commit 4
+'
+
+test_expect_success 'setup shallow clone' '
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone' '
+	(
+	cd shallow &&
+	test_commit 5 &&
+	git push ../.git +master:refs/remotes/shallow/master
+	) &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+2
+1
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone, with grafted roots' '
+	git init shallow2 &&
+	(
+	cd shallow2 &&
+	test_commit a &&
+	test_commit b &&
+	test_commit c &&
+	git rev-parse b > .git/shallow &&
+	git log --format=%s >actual &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual &&
+	git push ../.git +master:refs/remotes/shallow2/master
+	) &&
+	git log --format=%s shallow2/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual
+'
+
+test_done

builtin/send-pack.c, send-pack.c, send-pack.h, t/t5537-push-shallow.sh, transport.c

diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index f0ad0ce..b177120 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -94,6 +94,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	int fd[2];
 	struct child_process *conn;
 	struct extra_have_objects extra_have;
+	struct extra_have_objects shallow;
 	struct ref *remote_refs, *local_refs;
 	int ret;
 	int helper_status = 0;
@@ -206,8 +207,10 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	}
 
 	memset(&extra_have, 0, sizeof(extra_have));
+	memset(&shallow, 0, sizeof(shallow));
 
-	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have, NULL);
+	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL,
+			 &extra_have, &shallow);
 
 	transport_verify_remote_names(nr_refspecs, refspecs);
 
@@ -227,7 +230,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	set_ref_status_for_push(remote_refs, args.send_mirror,
 		args.force_update);
 
-	ret = send_pack(&args, fd, conn, remote_refs, &extra_have);
+	ret = send_pack(&args, fd, conn, remote_refs, &extra_have, &shallow);
 
 	if (helper_status)
 		print_helper_status(remote_refs);
diff --git a/send-pack.c b/send-pack.c
index 81d4b1c..08de681 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -27,14 +27,19 @@ static int feed_object(const unsigned char *sha1, int fd, int negative)
 /*
  * Make a pack stream and spit it out into file descriptor fd
  */
-static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *extra, struct send_pack_args *args)
+static int pack_objects(int fd, struct ref *refs,
+			struct extra_have_objects *extra,
+			struct extra_have_objects *extra_shallow,
+			struct send_pack_args *args)
 {
 	/*
 	 * The child becomes pack-objects --revs; we feed
 	 * the revision parameters to it via its stdin and
 	 * let its stdout go back to the other end.
 	 */
-	const char *argv[] = {
+	const char *av[] = {
+		"--shallow-file",
+		NULL,
 		"pack-objects",
 		"--all-progress-implied",
 		"--revs",
@@ -45,10 +50,27 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 		NULL,
 		NULL,
 	};
+	const char **argv;
 	struct child_process po;
+	static struct lock_file shallow_lock;
+	const char *alternate_shallow_file = NULL;
 	int i;
 
-	i = 4;
+	if (extra_shallow->nr) {
+		memset(&shallow_lock, 0, sizeof(shallow_lock));
+		/* just to load up .git/shallow if exists */
+		is_repository_shallow();
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					extra_shallow,
+					0);
+		av[1] = alternate_shallow_file;
+		argv = av;
+		i = 6;
+	} else {
+		argv = &av[2];
+		i = 4;
+	}
 	if (args->use_thin_pack)
 		argv[i++] = "--thin";
 	if (args->use_ofs_delta)
@@ -100,6 +122,10 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 
 	if (finish_command(&po))
 		return -1;
+
+	if (extra_shallow->nr)
+		rollback_lock_file(&shallow_lock);
+
 	return 0;
 }
 
@@ -176,7 +202,8 @@ static int sideband_demux(int in, int out, void *data)
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
-	      struct extra_have_objects *extra_have)
+	      struct extra_have_objects *extra_have,
+	      struct extra_have_objects *extra_shallow)
 {
 	int in = fd[0];
 	int out = fd[1];
@@ -294,7 +321,8 @@ int send_pack(struct send_pack_args *args,
 	}
 
 	if (new_refs && cmds_sent) {
-		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
+		if (pack_objects(out, remote_refs, extra_have, extra_shallow,
+				 args) < 0) {
 			for (ref = remote_refs; ref; ref = ref->next)
 				ref->status = REF_STATUS_NONE;
 			if (args->stateless_rpc)
diff --git a/send-pack.h b/send-pack.h
index 05d7ab1..1c8a343 100644
--- a/send-pack.h
+++ b/send-pack.h
@@ -16,6 +16,8 @@ struct send_pack_args {
 
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
-	      struct ref *remote_refs, struct extra_have_objects *extra_have);
+	      struct ref *remote_refs,
+	      struct extra_have_objects *extra_have,
+	      struct extra_have_objects *extra_shallow);
 
 #endif
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
index 30fdbc8..76d3d51 100755
--- a/t/t5537-push-shallow.sh
+++ b/t/t5537-push-shallow.sh
@@ -64,4 +64,22 @@ EOF
 	test_cmp expect actual
 '
 
+test_expect_success 'push from shallow to shallow' '
+	(
+	cd shallow &&
+	git push ../shallow2/.git +master:refs/remotes/shallow/master
+	) &&
+	(
+	cd shallow2 &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
 test_done
diff --git a/transport.c b/transport.c
index 17fef16..d592e8d 100644
--- a/transport.c
+++ b/transport.c
@@ -802,7 +802,8 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		struct ref *tmp_refs;
 		connect_setup(transport, 1, 0);
 
-		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL,
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
 
@@ -817,7 +818,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 	args.porcelain = !!(flags & TRANSPORT_PUSH_PORCELAIN);
 
 	ret = send_pack(&args, data->fd, data->conn, remote_refs,
-			&data->extra_have);
+			&data->extra_have, &data->shallow);
 
 	close(data->fd[1]);
 	close(data->fd[0]);

diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 152c4ea..d4cbf64 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	    (send_all && args.send_mirror))
 		usage(send_pack_usage);
 
+	if (is_repository_shallow())
+		die("attempt to push from a shallow repository");
+
 	if (remote_name) {
 		remote = remote_get(remote_name);
 		if (!remote_has_url(remote, dest)) {

Documentation/technical/pack-protocol.txt, builtin/receive-pack.c, commit.h, shallow.c, upload-pack.c

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index b898e97..eb8edd1 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -161,6 +161,7 @@ MUST peel the ref if it's an annotated tag.
 
 ----
   advertised-refs  =  (no-refs / list-of-refs)
+		      *shallow
 		      flush-pkt
 
   no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
@@ -174,6 +175,8 @@ MUST peel the ref if it's an annotated tag.
   other-tip        =  obj-id SP refname LF
   other-peeled     =  obj-id SP refname "^{}" LF
 
+  shallow          =  PKT-LINE("shallow" SP obj-id)
+
   capability-list  =  capability *(SP capability)
   capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
   LC_ALPHA         =  %x61-7A
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index e3eb5fc..6ffe526 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -176,6 +176,8 @@ static void write_head_info(void)
 	if (!sent_capabilities)
 		show_ref("capabilities^{}", null_sha1);
 
+	advertise_shallow_grafts(1);
+
 	/* EOF */
 	packet_flush(1);
 }
@@ -990,7 +992,7 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	if (!enter_repo(dir, 0))
 		die("'%s' does not appear to be a git repository", dir);
 
-	if (is_repository_shallow())
+	if (is_repository_shallow() && stateless_rpc)
 		die("attempt to push into a shallow repository");
 
 	git_config(receive_pack_config, NULL);
diff --git a/commit.h b/commit.h
index 4d452dc..e0688c3 100644
--- a/commit.h
+++ b/commit.h
@@ -187,6 +187,7 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 		int depth, int shallow_flag, int not_shallow_flag);
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
+extern void advertise_shallow_grafts(int);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/shallow.c b/shallow.c
index cbe2526..ccdfefc 100644
--- a/shallow.c
+++ b/shallow.c
@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "commit.h"
 #include "tag.h"
+#include "pkt-line.h"
 
 static int is_shallow = -1;
 static struct stat shallow_stat;
@@ -146,3 +147,18 @@ void check_shallow_file_for_update(void)
 		   )
 		die("shallow file was changed during fetch");
 }
+
+static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
+{
+	int fd = *(int*)cb;
+	if (graft->nr_parent == -1)
+		packet_write(fd, "shallow %s\n", sha1_to_hex(graft->sha1));
+	return 0;
+}
+
+void advertise_shallow_grafts(int fd)
+{
+	if (!is_repository_shallow())
+		return;
+	for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
+}
diff --git a/upload-pack.c b/upload-pack.c
index 127e59a..c377a3e 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -766,6 +766,7 @@ static void upload_pack(void)
 		reset_timeout();
 		head_ref_namespaced(send_ref, NULL);
 		for_each_namespaced_ref(send_ref, NULL);
+		advertise_shallow_grafts(1);
 		packet_flush(1);
 	} else {
 		head_ref_namespaced(mark_our_ref, NULL);
@@ -837,8 +838,9 @@ int main(int argc, char **argv)
 
 	if (!enter_repo(dir, strict))
 		die("'%s' does not appear to be a git repository", dir);
-	if (is_repository_shallow())
-		die("attempt to fetch/clone from a shallow repository");
+	if (is_repository_shallow() && stateless_rpc)
+		die("attempt to push into a shallow repository");
+
 	git_config(upload_pack_config, NULL);
 	upload_pack();
 	return 0;

builtin/fetch-pack.c, builtin/send-pack.c, cache.h, connect.c, remote-curl.c, transport.c

diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index aba4465..080e599 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -144,7 +144,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 				   args.verbose ? CONNECT_VERBOSE : 0);
 	}
 
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL);
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, NULL);
 
 	ref = fetch_pack(&args, fd, conn, ref, dest,
 			 sought, nr_sought, pack_lockfile_ptr);
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index d4cbf64..4be5931 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -210,7 +210,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 
 	memset(&extra_have, 0, sizeof(extra_have));
 
-	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have);
+	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have, NULL);
 
 	transport_verify_remote_names(nr_refspecs, refspecs);
 
diff --git a/cache.h b/cache.h
index dd0fb33..7f17228 100644
--- a/cache.h
+++ b/cache.h
@@ -1091,7 +1091,8 @@ struct extra_have_objects {
 };
 extern struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 				     struct ref **list, unsigned int flags,
-				     struct extra_have_objects *);
+				     struct extra_have_objects *have,
+				     struct extra_have_objects *shallow);
 extern int server_supports(const char *feature);
 extern int parse_feature_request(const char *features, const char *feature);
 extern const char *server_feature_value(const char *feature, int *len_ret);
diff --git a/connect.c b/connect.c
index a0783d4..1cba424 100644
--- a/connect.c
+++ b/connect.c
@@ -64,7 +64,8 @@ static void die_initial_contact(int got_at_least_one_head)
  */
 struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 			      struct ref **list, unsigned int flags,
-			      struct extra_have_objects *extra_have)
+			      struct extra_have_objects *extra_have,
+			      struct extra_have_objects *shallow_points)
 {
 	int got_at_least_one_head = 0;
 
@@ -89,6 +90,15 @@ struct ref **get_remote_heads(int in, char *src_buf, size_t src_len,
 		if (len > 4 && !prefixcmp(buffer, "ERR "))
 			die("remote error: %s", buffer + 4);
 
+		if (len == 48 && !prefixcmp(buffer, "shallow ")) {
+			if (get_sha1_hex(buffer + 8, old_sha1))
+				die("protocol error: expected shallow sha, got '%s'", buffer + 8);
+			if (!shallow_points)
+				die("repository on the other end cannot be shallow");
+			add_extra_have(shallow_points, old_sha1);
+			continue;
+		}
+
 		if (len < 42 || get_sha1_hex(buffer, old_sha1) || buffer[40] != ' ')
 			die("protocol error: expected sha/ref, got '%s'", buffer);
 		name = buffer + 41;
diff --git a/remote-curl.c b/remote-curl.c
index 5b3ce9e..c329bd3 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -86,7 +86,7 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 {
 	struct ref *list = NULL;
 	get_remote_heads(-1, heads->buf, heads->len, &list,
-			 for_push ? REF_NORMAL : 0, NULL);
+			 for_push ? REF_NORMAL : 0, NULL, NULL);
 	return list;
 }
 
diff --git a/transport.c b/transport.c
index e15db98..10a8cb8 100644
--- a/transport.c
+++ b/transport.c
@@ -509,7 +509,7 @@ static struct ref *get_refs_via_connect(struct transport *transport, int for_pus
 
 	connect_setup(transport, for_push, 0);
 	get_remote_heads(data->fd[0], NULL, 0, &refs,
-			 for_push ? REF_NORMAL : 0, &data->extra_have);
+			 for_push ? REF_NORMAL : 0, &data->extra_have, NULL);
 	data->got_remote_heads = 1;
 
 	return refs;
@@ -539,7 +539,8 @@ static int fetch_refs_via_pack(struct transport *transport,
 
 	if (!data->got_remote_heads) {
 		connect_setup(transport, 0, 0);
-		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0,
+				 NULL, NULL);
 		data->got_remote_heads = 1;
 	}
 
@@ -799,7 +800,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		struct ref *tmp_refs;
 		connect_setup(transport, 1, 0);
 
-		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL, NULL);
 		data->got_remote_heads = 1;
 	}
 

commit.h, fetch-pack.c, shallow.c

diff --git a/commit.h b/commit.h
index e0688c3..678fa20 100644
--- a/commit.h
+++ b/commit.h
@@ -188,6 +188,9 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
 extern void advertise_shallow_grafts(int);
+extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
+extern void setup_alternate_shallow(struct lock_file *shallow_lock,
+				    const char **alternate_shallow_file);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/fetch-pack.c b/fetch-pack.c
index abe5ffb..dc71a2b 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -185,36 +185,6 @@ static void consume_shallow_list(struct fetch_pack_args *args, int fd)
 	}
 }
 
-struct write_shallow_data {
-	struct strbuf *out;
-	int use_pack_protocol;
-	int count;
-};
-
-static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
-{
-	struct write_shallow_data *data = cb_data;
-	const char *hex = sha1_to_hex(graft->sha1);
-	data->count++;
-	if (data->use_pack_protocol)
-		packet_buf_write(data->out, "shallow %s", hex);
-	else {
-		strbuf_addstr(data->out, hex);
-		strbuf_addch(data->out, '\n');
-	}
-	return 0;
-}
-
-static int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
-{
-	struct write_shallow_data data;
-	data.out = out;
-	data.use_pack_protocol = use_pack_protocol;
-	data.count = 0;
-	for_each_commit_graft(write_one_shallow, &data);
-	return data.count;
-}
-
 static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 {
 	int len;
@@ -795,27 +765,6 @@ static int cmp_ref_by_name(const void *a_, const void *b_)
 	return strcmp(a->name, b->name);
 }
 
-static void setup_alternate_shallow(void)
-{
-	struct strbuf sb = STRBUF_INIT;
-	int fd;
-
-	check_shallow_file_for_update();
-	fd = hold_lock_file_for_update(&shallow_lock, git_path("shallow"),
-				       LOCK_DIE_ON_ERROR);
-	if (write_shallow_commits(&sb, 0)) {
-		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
-			die_errno("failed to write to %s", shallow_lock.filename);
-		alternate_shallow_file = shallow_lock.filename;
-	} else
-		/*
-		 * is_repository_shallow() sees empty string as "no
-		 * shallow file".
-		 */
-		alternate_shallow_file = "";
-	strbuf_release(&sb);
-}
-
 static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
@@ -896,7 +845,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
 	if (args->depth > 0)
-		setup_alternate_shallow();
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file);
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
diff --git a/shallow.c b/shallow.c
index ccdfefc..ee9edd4 100644
--- a/shallow.c
+++ b/shallow.c
@@ -162,3 +162,56 @@ void advertise_shallow_grafts(int fd)
 		return;
 	for_each_commit_graft(advertise_shallow_grafts_cb, &fd);
 }
+
+struct write_shallow_data {
+	struct strbuf *out;
+	int use_pack_protocol;
+	int count;
+};
+
+static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
+{
+	struct write_shallow_data *data = cb_data;
+	const char *hex = sha1_to_hex(graft->sha1);
+	data->count++;
+	if (data->use_pack_protocol)
+		packet_buf_write(data->out, "shallow %s", hex);
+	else {
+		strbuf_addstr(data->out, hex);
+		strbuf_addch(data->out, '\n');
+	}
+	return 0;
+}
+
+int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
+{
+	struct write_shallow_data data;
+	data.out = out;
+	data.use_pack_protocol = use_pack_protocol;
+	data.count = 0;
+	for_each_commit_graft(write_one_shallow, &data);
+	return data.count;
+}
+
+void setup_alternate_shallow(struct lock_file *shallow_lock,
+			     const char **alternate_shallow_file)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int fd;
+
+	check_shallow_file_for_update();
+	fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
+				       LOCK_DIE_ON_ERROR);
+	if (write_shallow_commits(&sb, 0)) {
+		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
+			die_errno("failed to write to %s",
+				  shallow_lock->filename);
+		*alternate_shallow_file = shallow_lock->filename;
+	} else
+		/*
+		 * is_repository_shallow() sees empty string as "no
+		 * shallow file".
+		 */
+		*alternate_shallow_file = "";
+	strbuf_release(&sb);
+}

builtin/fetch-pack.c, commit.h, fetch-pack.c, fetch-pack.h, shallow.c, t/t5536-fetch-shallow.sh, transport.c

diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 080e599..f6a6d76 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -37,6 +37,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 	char **pack_lockfile_ptr = NULL;
 	struct child_process *conn;
 	struct fetch_pack_args args;
+	struct extra_have_objects shallow;
 
 	packet_trace_identity("fetch-pack");
 
@@ -144,10 +145,12 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 				   args.verbose ? CONNECT_VERBOSE : 0);
 	}
 
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, NULL);
+	memset(&shallow, 0, sizeof(shallow));
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL,
+			 args.stateless_rpc ? NULL : &shallow);
 
 	ref = fetch_pack(&args, fd, conn, ref, dest,
-			 sought, nr_sought, pack_lockfile_ptr);
+			 sought, nr_sought, &shallow, pack_lockfile_ptr);
 	if (pack_lockfile) {
 		printf("lock %s\n", pack_lockfile);
 		fflush(stdout);
diff --git a/commit.h b/commit.h
index 678fa20..97fd54e 100644
--- a/commit.h
+++ b/commit.h
@@ -188,9 +188,32 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 extern void check_shallow_file_for_update(void);
 extern void set_alternate_shallow_file(const char *path);
 extern void advertise_shallow_grafts(int);
-extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol);
+
+/*
+ * Ignore graft points that cut out existing "good" parts of our
+ * history. Graft points pointing to incomplete commit islands are
+ * _not_ ignore.
+ */
+#define WRITE_SHALLOW_NO_CUT         1
+/*
+ * Remove graft points that are not attached to any SHA-1 in current
+ * history.
+ */
+#define WRITE_SHALLOW_NO_UNUSED      2
+/*
+ * Not really write_shallow_commits() flag, this lets
+ * setup_alternate_shallow know this is the "final" setup before
+ * committing to .git/shallow
+ */
+#define WRITE_SHALLOW_REWRITE        4
+
+extern int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+				 struct extra_have_objects *extra,
+				 unsigned int flags);
 extern void setup_alternate_shallow(struct lock_file *shallow_lock,
-				    const char **alternate_shallow_file);
+				    const char **alternate_shallow_file,
+				    struct extra_have_objects *extra,
+				    unsigned int flags);
 
 int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit *);
diff --git a/fetch-pack.c b/fetch-pack.c
index dc71a2b..f337526 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -311,7 +311,7 @@ static int find_common(struct fetch_pack_args *args,
 	}
 
 	if (is_repository_shallow())
-		write_shallow_commits(&req_buf, 1);
+		write_shallow_commits(&req_buf, 1, NULL, WRITE_SHALLOW_NO_CUT);
 	if (args->depth > 0)
 		packet_buf_write(&req_buf, "deepen %d", args->depth);
 	packet_buf_flush(&req_buf);
@@ -769,6 +769,7 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 				 int fd[2],
 				 const struct ref *orig_ref,
 				 struct ref **sought, int nr_sought,
+				 struct extra_have_objects *shallow,
 				 char **pack_lockfile)
 {
 	struct ref *ref = copy_ref_list(orig_ref);
@@ -844,8 +845,10 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 
 	if (args->stateless_rpc)
 		packet_flush(fd[1]);
-	if (args->depth > 0)
-		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file);
+	if (args->depth > 0 || shallow->nr)
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
+					shallow,
+					WRITE_SHALLOW_NO_CUT);
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
@@ -922,6 +925,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       const struct ref *ref,
 		       const char *dest,
 		       struct ref **sought, int nr_sought,
+		       struct extra_have_objects *shallow,
 		       char **pack_lockfile)
 {
 	struct ref *ref_cpy;
@@ -934,14 +938,30 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		packet_flush(fd[1]);
 		die("no matching remote head");
 	}
-	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought, pack_lockfile);
+	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought,
+				shallow, pack_lockfile);
 
 	if (alternate_shallow_file) {
 		if (*alternate_shallow_file == '\0') { /* --unshallow */
 			unlink_or_warn(git_path("shallow"));
 			rollback_lock_file(&shallow_lock);
-		} else
+		} else {
+			/*
+			 *  The server is a shallow clone and it sends
+			 *  us all of its shallow grafts. Some may be
+			 *  needed if the server sends objects down to
+			 *  the bottom. Remove all unused grafts.
+			 */
+			if (shallow->nr) {
+				reprepare_packed_git();
+				setup_alternate_shallow(&shallow_lock,
+							&alternate_shallow_file,
+							shallow,
+							WRITE_SHALLOW_NO_CUT |
+							WRITE_SHALLOW_REWRITE);
+			}
 			commit_lock_file(&shallow_lock);
+		}
 	}
 
 	reprepare_packed_git();
diff --git a/fetch-pack.h b/fetch-pack.h
index 40f08ba..c55bf1c 100644
--- a/fetch-pack.h
+++ b/fetch-pack.h
@@ -32,6 +32,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 		       const char *dest,
 		       struct ref **sought,
 		       int nr_sought,
+		       struct extra_have_objects *shallow,
 		       char **pack_lockfile);
 
 #endif
diff --git a/shallow.c b/shallow.c
index ee9edd4..337f676 100644
--- a/shallow.c
+++ b/shallow.c
@@ -2,6 +2,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "pkt-line.h"
+#include "connected.h"
 
 static int is_shallow = -1;
 static struct stat shallow_stat;
@@ -183,26 +184,103 @@ static int write_one_shallow(const struct commit_graft *graft, void *cb_data)
 	return 0;
 }
 
-int write_shallow_commits(struct strbuf *out, int use_pack_protocol)
+static int sha1_iterate(void *cb, unsigned char *sha1)
+{
+	if (is_null_sha1(cb))
+		return 1;
+	hashcpy(sha1, cb);
+	hashclr(cb);
+	return 0;
+}
+
+struct connectivity_cache_item {
+	unsigned char sha1[20];
+	int good;
+};
+
+static int good_connectivity_from(const unsigned char *sha1)
+{
+	static struct connectivity_cache_item *cache;
+	static int nr, alloc;
+	int i;
+	unsigned char sha1_cb[20];
+
+	for (i = 0; i < nr; i++)
+		if (!hashcmp(cache[i].sha1, sha1))
+			return cache[i].good;
+
+	ALLOC_GROW(cache, nr + 1, alloc);
+	hashcpy(sha1_cb, sha1);
+	hashcpy(cache[nr].sha1, sha1);
+	cache[nr].good = !check_everything_connected(sha1_iterate, 1, sha1_cb);
+	nr++;
+	return cache[nr - 1].good;
+}
+
+int write_shallow_commits(struct strbuf *out, int use_pack_protocol,
+			  struct extra_have_objects *extra,
+			  unsigned int flags)
 {
 	struct write_shallow_data data;
+	int i;
 	data.out = out;
 	data.use_pack_protocol = use_pack_protocol;
 	data.count = 0;
 	for_each_commit_graft(write_one_shallow, &data);
+	if (!extra)
+		return data.count;
+
+	for (i = 0; i < extra->nr; i++) {
+		if ((flags & WRITE_SHALLOW_NO_CUT) &&
+		    good_connectivity_from(extra->array[i]))
+			/*
+			 * The server may have even shorter history
+			 * than the client (e.g. the client has
+			 * A-B-C-D but the server only has C-D). We do
+			 * NOT want to cut client's history down to
+			 * C-D simply because the server is set up
+			 * so. If we don't have "C" and the server
+			 * sends C-D to us, then we set up a graft at
+			 * "C".
+			 *
+			 * If the client has (broken)-B-C-D, then "C"
+			 * _is_ accepted. The -B-C-D is an incomplete
+			 * island that normally has no connectivity to
+			 * the rest of the repo.
+			 */
+			continue;
+		if ((flags & WRITE_SHALLOW_NO_UNUSED) &&
+		    !has_sha1_file(extra->array[i]))
+			continue;
+		strbuf_addstr(out, sha1_to_hex(extra->array[i]));
+		strbuf_addch(out, '\n');
+		data.count++;
+	}
 	return data.count;
 }
 
 void setup_alternate_shallow(struct lock_file *shallow_lock,
-			     const char **alternate_shallow_file)
+			     const char **alternate_shallow_file,
+			     struct extra_have_objects *extra,
+			     unsigned int flags)
 {
 	struct strbuf sb = STRBUF_INIT;
+	int rewrite = flags & WRITE_SHALLOW_REWRITE;
 	int fd;
 
-	check_shallow_file_for_update();
-	fd = hold_lock_file_for_update(shallow_lock, git_path("shallow"),
-				       LOCK_DIE_ON_ERROR);
-	if (write_shallow_commits(&sb, 0)) {
+	if (!rewrite)
+		fd = hold_lock_file_for_update(shallow_lock,
+					       git_path("shallow"),
+					       LOCK_DIE_ON_ERROR);
+	else {
+		flags |= WRITE_SHALLOW_NO_UNUSED;
+		fd = shallow_lock->fd;
+	}
+	if (write_shallow_commits(&sb, 0, extra, flags)) {
+		if (rewrite && (lseek(fd, 0, SEEK_SET) == -1 ||
+				ftruncate(fd, 0) == -1))
+			die_errno("unable to truncate the new shallow file %s",
+				  shallow_lock->filename);
 		if (write_in_full(fd, sb.buf, sb.len) != sb.len)
 			die_errno("failed to write to %s",
 				  shallow_lock->filename);
diff --git a/t/t5536-fetch-shallow.sh b/t/t5536-fetch-shallow.sh
new file mode 100755
index 0000000..15a8208
--- /dev/null
+++ b/t/t5536-fetch-shallow.sh
@@ -0,0 +1,105 @@
+#!/bin/sh
+
+test_description='fetch/clone from a shallow clone'
+
+. ./test-lib.sh
+
+commit() {
+	echo "$1" >tracked &&
+	git add tracked &&
+	git commit -m "$1"
+}
+
+test_expect_success 'setup' '
+	commit 1 &&
+	commit 2 &&
+	commit 3 &&
+	commit 4 &&
+	git config --global transfer.fsckObjects true
+'
+
+test_expect_success 'setup shallow clone' '
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'clone from shallow clone' '
+	git clone --no-local shallow shallow2 &&
+	(
+	cd shallow2 &&
+	git fsck &&
+	git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch from shallow clone' '
+	(
+	cd shallow &&
+	commit 5
+	) &&
+	(
+	cd shallow2 &&
+	git fetch &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch --depth from shallow clone' '
+	(
+	cd shallow &&
+	commit 6
+	) &&
+	(
+	cd shallow2 &&
+	git fetch --depth=2 &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+6
+5
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'fetch something upstream has but hidden by clients shallow boundaries' '
+	# the blob "1" is available in .git but hidden by the
+	# shallow2/.git/shallow and it should be resent
+	! git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null &&
+	echo 1 > 1.t &&
+	git add 1.t &&
+	git commit -m add-1-back &&
+	(
+	cd shallow2 &&
+	git fetch ../.git +refs/heads/master:refs/remotes/top/master &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+add-1-back
+4
+3
+EOF
+	test_cmp expect actual
+	) &&
+	git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` >/dev/null
+
+'
+
+test_done
diff --git a/transport.c b/transport.c
index 10a8cb8..f167ca5 100644
--- a/transport.c
+++ b/transport.c
@@ -453,6 +453,7 @@ struct git_transport_data {
 	int fd[2];
 	unsigned got_remote_heads : 1;
 	struct extra_have_objects extra_have;
+	struct extra_have_objects shallow;
 };
 
 static int set_git_option(struct git_transport_options *opts,
@@ -509,7 +510,8 @@ static struct ref *get_refs_via_connect(struct transport *transport, int for_pus
 
 	connect_setup(transport, for_push, 0);
 	get_remote_heads(data->fd[0], NULL, 0, &refs,
-			 for_push ? REF_NORMAL : 0, &data->extra_have, NULL);
+			 for_push ? REF_NORMAL : 0,
+			 &data->extra_have, &data->shallow);
 	data->got_remote_heads = 1;
 
 	return refs;
@@ -540,13 +542,13 @@ static int fetch_refs_via_pack(struct transport *transport,
 	if (!data->got_remote_heads) {
 		connect_setup(transport, 0, 0);
 		get_remote_heads(data->fd[0], NULL, 0, &refs_tmp, 0,
-				 NULL, NULL);
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
 
 	refs = fetch_pack(&args, data->fd, data->conn,
 			  refs_tmp ? refs_tmp : transport->remote_refs,
-			  dest, to_fetch, nr_heads,
+			  dest, to_fetch, nr_heads, &data->shallow,
 			  &transport->pack_lockfile);
 	close(data->fd[0]);
 	close(data->fd[1]);
@@ -803,6 +805,8 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL, NULL);
 		data->got_remote_heads = 1;
 	}
+	if (data->shallow.nr)
+		die("pushing to a shallow repository is not supported");
 
 	memset(&args, 0, sizeof(args));
 	args.send_mirror = !!(flags & TRANSPORT_PUSH_MIRROR);

Documentation/technical/pack-protocol.txt, builtin/receive-pack.c, builtin/send-pack.c, send-pack.c, t/t5537-push-shallow.sh

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index eb8edd1..c73b62f 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -464,7 +464,9 @@ contain all the objects that the server will need to complete the new
 references.
 
 ----
-  update-request    =  command-list [pack-file]
+  update-request    =  *shallow command-list [pack-file]
+
+  shallow           =  PKT-LINE("shallow" SP obj-id)
 
   command-list      =  PKT-LINE(command NUL capability-list LF)
 		       *PKT-LINE(command LF)
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 6ffe526..cbb2025 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -41,6 +41,10 @@ static int auto_gc = 1;
 static const char *head_name;
 static void *head_name_to_free;
 static int sent_capabilities;
+static int shallow_changed;
+static const char* alternate_shallow_file;
+static struct lock_file shallow_lock;
+static struct extra_have_objects shallow;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -751,6 +755,13 @@ static void execute_commands(struct command *commands, const char *unpacker_erro
 	}
 }
 
+static void add_extra_have(struct extra_have_objects *extra, unsigned char *sha1)
+{
+	ALLOC_GROW(extra->array, extra->nr + 1, extra->alloc);
+	hashcpy(&(extra->array[extra->nr][0]), sha1);
+	extra->nr++;
+}
+
 static struct command *read_head_info(void)
 {
 	struct command *commands = NULL;
@@ -765,6 +776,17 @@ static struct command *read_head_info(void)
 		line = packet_read_line(0, &len);
 		if (!line)
 			break;
+
+		if (len == 48 && !prefixcmp(line, "shallow ")) {
+			if (get_sha1_hex(line + 8, old_sha1))
+				die("protocol error: expected shallow sha, got '%s'", line + 8);
+			if (!has_sha1_file(old_sha1)) {
+				add_extra_have(&shallow, old_sha1);
+				shallow_changed = 1;
+			}
+			continue;
+		}
+
 		if (len < 83 ||
 		    line[40] != ' ' ||
 		    line[81] != ' ' ||
@@ -827,6 +849,13 @@ static const char *unpack(int err_fd)
 			    ? transfer_fsck_objects
 			    : 0);
 
+	if (shallow_changed)
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					&shallow,
+					WRITE_SHALLOW_NO_CUT);
+
+
 	hdr_err = parse_pack_header(&hdr);
 	if (hdr_err) {
 		if (err_fd > 0)
@@ -840,7 +869,12 @@ static const char *unpack(int err_fd)
 	if (ntohl(hdr.hdr_entries) < unpack_limit) {
 		int code, i = 0;
 		struct child_process child;
-		const char *unpacker[5];
+		const char *unpacker[7];
+		if (alternate_shallow_file &&
+		    *alternate_shallow_file) {
+			unpacker[i++] = "--shallow-file";
+			unpacker[i++] = alternate_shallow_file;
+		}
 		unpacker[i++] = "unpack-objects";
 		if (quiet)
 			unpacker[i++] = "-q";
@@ -854,15 +888,19 @@ static const char *unpack(int err_fd)
 		child.err = err_fd;
 		child.git_cmd = 1;
 		code = run_command(&child);
-		if (!code)
-			return NULL;
-		return "unpack-objects abnormal exit";
+		if (code)
+			return "unpack-objects abnormal exit";
 	} else {
-		const char *keeper[7];
+		const char *keeper[9];
 		int s, status, i = 0;
 		char keep_arg[256];
 		struct child_process ip;
 
+		if (alternate_shallow_file &&
+		    *alternate_shallow_file) {
+			keeper[i++] = "--shallow-file";
+			keeper[i++] = alternate_shallow_file;
+		}
 		s = sprintf(keep_arg, "--keep=receive-pack %"PRIuMAX" on ", (uintmax_t) getpid());
 		if (gethostname(keep_arg + s, sizeof(keep_arg) - s))
 			strcpy(keep_arg + s, "localhost");
@@ -887,12 +925,20 @@ static const char *unpack(int err_fd)
 		pack_lockfile = index_pack_lockfile(ip.out);
 		close(ip.out);
 		status = finish_command(&ip);
-		if (!status) {
-			reprepare_packed_git();
-			return NULL;
-		}
-		return "index-pack abnormal exit";
+		if (status)
+			return "index-pack abnormal exit";
+		reprepare_packed_git();
+	}
+
+	if (shallow_changed) {
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					&shallow,
+					WRITE_SHALLOW_NO_CUT |
+					WRITE_SHALLOW_REWRITE);
+		commit_lock_file(&shallow_lock);
 	}
+	return NULL;
 }
 
 static const char *unpack_with_sideband(void)
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 4be5931..7dd254f 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,7 +184,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	    (send_all && args.send_mirror))
 		usage(send_pack_usage);
 
-	if (is_repository_shallow())
+	if (is_repository_shallow() && args.stateless_rpc)
 		die("attempt to push from a shallow repository");
 
 	if (remote_name) {
diff --git a/send-pack.c b/send-pack.c
index 7d172ef..dcd3345 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -212,6 +212,9 @@ int send_pack(struct send_pack_args *args,
 		return 0;
 	}
 
+	if (!args->dry_run)
+		advertise_shallow_grafts(out);
+
 	/*
 	 * Finally, tell the other end!
 	 */
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
new file mode 100755
index 0000000..37ad69b
--- /dev/null
+++ b/t/t5537-push-shallow.sh
@@ -0,0 +1,74 @@
+#!/bin/sh
+
+test_description='push from/to a shallow clone'
+
+. ./test-lib.sh
+
+commit() {
+	echo "$1" >tracked &&
+	git add tracked &&
+	git commit -m "$1"
+}
+
+test_expect_success 'setup' '
+	git config --global transfer.fsckObjects true &&
+	commit 1 &&
+	commit 2 &&
+	commit 3 &&
+	commit 4 &&
+	(
+	git init full-abc &&
+	cd full-abc &&
+	commit a &&
+	commit b &&
+	commit c
+	) &&
+	git clone --no-local --depth=2 .git shallow &&
+	git --git-dir=shallow/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+4
+3
+EOF
+	test_cmp expect actual &&
+	git clone --no-local --depth=2 full-abc/.git shallow2 &&
+	git --git-dir=shallow2/.git log --format=%s >actual &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone' '
+	(
+	cd shallow &&
+	commit 5 &&
+	git push ../.git +master:refs/remotes/shallow/master
+	) &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+2
+1
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'push from shallow clone, with grafted roots' '
+	(
+	cd shallow2 &&
+	git push ../.git +master:refs/remotes/shallow2/master
+	) &&
+	git log --format=%s shallow2/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+c
+b
+EOF
+	test_cmp expect actual
+'
+
+test_done

builtin/send-pack.c, send-pack.c, send-pack.h, t/t5537-push-shallow.sh, transport.c

diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 7dd254f..14a73ba 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -94,6 +94,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	int fd[2];
 	struct child_process *conn;
 	struct extra_have_objects extra_have;
+	struct extra_have_objects shallow;
 	struct ref *remote_refs, *local_refs;
 	int ret;
 	int helper_status = 0;
@@ -209,8 +210,10 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	}
 
 	memset(&extra_have, 0, sizeof(extra_have));
+	memset(&shallow, 0, sizeof(shallow));
 
-	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL, &extra_have, NULL);
+	get_remote_heads(fd[0], NULL, 0, &remote_refs, REF_NORMAL,
+			 &extra_have, &shallow);
 
 	transport_verify_remote_names(nr_refspecs, refspecs);
 
@@ -230,7 +233,7 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	set_ref_status_for_push(remote_refs, args.send_mirror,
 		args.force_update);
 
-	ret = send_pack(&args, fd, conn, remote_refs, &extra_have);
+	ret = send_pack(&args, fd, conn, remote_refs, &extra_have, &shallow);
 
 	if (helper_status)
 		print_helper_status(remote_refs);
diff --git a/send-pack.c b/send-pack.c
index dcd3345..1b828b0 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -27,14 +27,19 @@ static int feed_object(const unsigned char *sha1, int fd, int negative)
 /*
  * Make a pack stream and spit it out into file descriptor fd
  */
-static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *extra, struct send_pack_args *args)
+static int pack_objects(int fd, struct ref *refs,
+			struct extra_have_objects *extra,
+			struct extra_have_objects *extra_shallow,
+			struct send_pack_args *args)
 {
 	/*
 	 * The child becomes pack-objects --revs; we feed
 	 * the revision parameters to it via its stdin and
 	 * let its stdout go back to the other end.
 	 */
-	const char *argv[] = {
+	const char *av[] = {
+		"--shallow-file",
+		NULL,
 		"pack-objects",
 		"--all-progress-implied",
 		"--revs",
@@ -45,10 +50,26 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 		NULL,
 		NULL,
 	};
+	const char **argv;
 	struct child_process po;
+	static struct lock_file shallow_lock;
+	const char *alternate_shallow_file = NULL;
 	int i;
 
-	i = 4;
+	if (extra_shallow->nr) {
+		memset(&shallow_lock, 0, sizeof(shallow_lock));
+		/* just to load up .git/shallow if exists */
+		is_repository_shallow();
+		setup_alternate_shallow(&shallow_lock,
+					&alternate_shallow_file,
+					extra_shallow, 0);
+		av[1] = alternate_shallow_file;
+		argv = av;
+		i = 6;
+	} else {
+		argv = &av[2];
+		i = 4;
+	}
 	if (args->use_thin_pack)
 		argv[i++] = "--thin";
 	if (args->use_ofs_delta)
@@ -100,6 +121,10 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 
 	if (finish_command(&po))
 		return -1;
+
+	if (extra_shallow->nr)
+		rollback_lock_file(&shallow_lock);
+
 	return 0;
 }
 
@@ -176,7 +201,8 @@ static int sideband_demux(int in, int out, void *data)
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
-	      struct extra_have_objects *extra_have)
+	      struct extra_have_objects *extra_have,
+	      struct extra_have_objects *extra_shallow)
 {
 	int in = fd[0];
 	int out = fd[1];
@@ -294,7 +320,8 @@ int send_pack(struct send_pack_args *args,
 	}
 
 	if (new_refs && cmds_sent) {
-		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
+		if (pack_objects(out, remote_refs, extra_have, extra_shallow,
+				 args) < 0) {
 			for (ref = remote_refs; ref; ref = ref->next)
 				ref->status = REF_STATUS_NONE;
 			if (args->stateless_rpc)
diff --git a/send-pack.h b/send-pack.h
index 05d7ab1..1c8a343 100644
--- a/send-pack.h
+++ b/send-pack.h
@@ -16,6 +16,8 @@ struct send_pack_args {
 
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
-	      struct ref *remote_refs, struct extra_have_objects *extra_have);
+	      struct ref *remote_refs,
+	      struct extra_have_objects *extra_have,
+	      struct extra_have_objects *extra_shallow);
 
 #endif
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
index 37ad69b..beb85ce 100755
--- a/t/t5537-push-shallow.sh
+++ b/t/t5537-push-shallow.sh
@@ -71,4 +71,40 @@ EOF
 	test_cmp expect actual
 '
 
+test_expect_success 'push from shallow to shallow' '
+	(
+	cd shallow &&
+	git push ../shallow2/.git +master:refs/remotes/shallow/master
+	) &&
+	(
+	cd shallow2 &&
+	git log --format=%s shallow/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+test_expect_success 'push from full to shallow' '
+	! git --git-dir=shallow2/.git cat-file blob `echo 1|git hash-object --stdin` &&
+	commit 1 &&
+	git push shallow2/.git +master:refs/remotes/top/master &&
+	(
+	cd shallow2 &&
+	git log --format=%s top/master >actual &&
+	git fsck &&
+	cat <<EOF >expect &&
+1
+4
+3
+EOF
+	test_cmp expect actual &&
+	git cat-file blob `echo 1|git hash-object --stdin` >/dev/null
+	)
+'
+
 test_done
diff --git a/transport.c b/transport.c
index f167ca5..d592e8d 100644
--- a/transport.c
+++ b/transport.c
@@ -802,11 +802,10 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 		struct ref *tmp_refs;
 		connect_setup(transport, 1, 0);
 
-		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL, NULL, NULL);
+		get_remote_heads(data->fd[0], NULL, 0, &tmp_refs, REF_NORMAL,
+				 NULL, &data->shallow);
 		data->got_remote_heads = 1;
 	}
-	if (data->shallow.nr)
-		die("pushing to a shallow repository is not supported");
 
 	memset(&args, 0, sizeof(args));
 	args.send_mirror = !!(flags & TRANSPORT_PUSH_MIRROR);
@@ -819,7 +818,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 	args.porcelain = !!(flags & TRANSPORT_PUSH_PORCELAIN);
 
 	ret = send_pack(&args, data->fd, data->conn, remote_refs,
-			&data->extra_have);
+			&data->extra_have, &data->shallow);
 
 	close(data->fd[1]);
 	close(data->fd[0]);

t/t5530-upload-pack-error.sh, upload-pack.c

diff --git a/t/t5530-upload-pack-error.sh b/t/t5530-upload-pack-error.sh
index c983d36..3932e79 100755
--- a/t/t5530-upload-pack-error.sh
+++ b/t/t5530-upload-pack-error.sh
@@ -54,9 +54,6 @@ test_expect_success 'upload-pack fails due to error in rev-list' '
 	printf "0032want %s\n0034shallow %s00000009done\n0000" \
 		$(git rev-parse HEAD) $(git rev-parse HEAD^) >input &&
 	test_must_fail git upload-pack . <input >/dev/null 2>output.err &&
-	# pack-objects survived
-	grep "Total.*, reused" output.err &&
-	# but there was an error, which must have been in rev-list
 	grep "bad tree object" output.err
 '
 
diff --git a/upload-pack.c b/upload-pack.c
index c377a3e..c3e68ae 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -35,7 +35,6 @@ static int no_done;
 static int use_thin_pack, use_ofs_delta, use_include_tag;
 static int no_progress, daemon_mode;
 static int allow_tip_sha1_in_want;
-static int shallow_nr;
 static struct object_array have_obj;
 static struct object_array want_obj;
 static struct object_array extra_edge_obj;
@@ -68,87 +67,32 @@ static ssize_t send_client_data(int fd, const char *data, ssize_t sz)
 	return sz;
 }
 
-static FILE *pack_pipe = NULL;
-static void show_commit(struct commit *commit, void *data)
-{
-	if (commit->object.flags & BOUNDARY)
-		fputc('-', pack_pipe);
-	if (fputs(sha1_to_hex(commit->object.sha1), pack_pipe) < 0)
-		die("broken output pipe");
-	fputc('\n', pack_pipe);
-	fflush(pack_pipe);
-	free(commit->buffer);
-	commit->buffer = NULL;
-}
-
-static void show_object(struct object *obj,
-			const struct name_path *path, const char *component,
-			void *cb_data)
-{
-	show_object_with_name(pack_pipe, obj, path, component);
-}
-
-static void show_edge(struct commit *commit)
-{
-	fprintf(pack_pipe, "-%s\n", sha1_to_hex(commit->object.sha1));
-}
-
-static int do_rev_list(int in, int out, void *user_data)
-{
-	int i;
-	struct rev_info revs;
-
-	pack_pipe = xfdopen(out, "w");
-	init_revisions(&revs, NULL);
-	revs.tag_objects = 1;
-	revs.tree_objects = 1;
-	revs.blob_objects = 1;
-	if (use_thin_pack)
-		revs.edge_hint = 1;
-
-	for (i = 0; i < want_obj.nr; i++) {
-		struct object *o = want_obj.objects[i].item;
-		/* why??? */
-		o->flags &= ~UNINTERESTING;
-		add_pending_object(&revs, o, NULL);
-	}
-	for (i = 0; i < have_obj.nr; i++) {
-		struct object *o = have_obj.objects[i].item;
-		o->flags |= UNINTERESTING;
-		add_pending_object(&revs, o, NULL);
-	}
-	setup_revisions(0, NULL, &revs, NULL);
-	if (prepare_revision_walk(&revs))
-		die("revision walk setup failed");
-	mark_edges_uninteresting(revs.commits, &revs, show_edge);
-	if (use_thin_pack)
-		for (i = 0; i < extra_edge_obj.nr; i++)
-			fprintf(pack_pipe, "-%s\n", sha1_to_hex(
-					extra_edge_obj.objects[i].item->sha1));
-	traverse_commit_list(&revs, show_commit, show_object, NULL);
-	fflush(pack_pipe);
-	fclose(pack_pipe);
-	return 0;
-}
-
 static void create_pack_file(void)
 {
-	struct async rev_list;
 	struct child_process pack_objects;
 	char data[8193], progress[128];
 	char abort_msg[] = "aborting due to possible repository "
 		"corruption on the remote side.";
 	int buffered = -1;
 	ssize_t sz;
-	const char *argv[10];
-	int arg = 0;
+	const char *argv[12];
+	int i, arg = 0;
+	FILE *pipe_fd;
+	static struct lock_file shallow_lock;
+	const char *alternate_shallow_file = NULL;
+
+	setup_alternate_shallow(&shallow_lock,
+				&alternate_shallow_file,
+				NULL, 0);
+	if (*alternate_shallow_file) {
+		argv[arg++] = "--shallow-file";
+		argv[arg++] = alternate_shallow_file;
+	}
 
 	argv[arg++] = "pack-objects";
-	if (!shallow_nr) {
-		argv[arg++] = "--revs";
-		if (use_thin_pack)
-			argv[arg++] = "--thin";
-	}
+	argv[arg++] = "--revs";
+	if (use_thin_pack)
+		argv[arg++] = "--thin";
 
 	argv[arg++] = "--stdout";
 	if (!no_progress)
@@ -169,28 +113,18 @@ static void create_pack_file(void)
 	if (start_command(&pack_objects))
 		die("git upload-pack: unable to fork git-pack-objects");
 
-	if (shallow_nr) {
-		memset(&rev_list, 0, sizeof(rev_list));
-		rev_list.proc = do_rev_list;
-		rev_list.out = pack_objects.in;
-		if (start_async(&rev_list))
-			die("git upload-pack: unable to fork git-rev-list");
-	}
-	else {
-		FILE *pipe_fd = xfdopen(pack_objects.in, "w");
-		int i;
+	pipe_fd = xfdopen(pack_objects.in, "w");
 
-		for (i = 0; i < want_obj.nr; i++)
-			fprintf(pipe_fd, "%s\n",
-				sha1_to_hex(want_obj.objects[i].item->sha1));
-		fprintf(pipe_fd, "--not\n");
-		for (i = 0; i < have_obj.nr; i++)
-			fprintf(pipe_fd, "%s\n",
-				sha1_to_hex(have_obj.objects[i].item->sha1));
-		fprintf(pipe_fd, "\n");
-		fflush(pipe_fd);
-		fclose(pipe_fd);
-	}
+	for (i = 0; i < want_obj.nr; i++)
+		fprintf(pipe_fd, "%s\n",
+			sha1_to_hex(want_obj.objects[i].item->sha1));
+	fprintf(pipe_fd, "--not\n");
+	for (i = 0; i < have_obj.nr; i++)
+		fprintf(pipe_fd, "%s\n",
+			sha1_to_hex(have_obj.objects[i].item->sha1));
+	fprintf(pipe_fd, "\n");
+	fflush(pipe_fd);
+	fclose(pipe_fd);
 
 
 	/* We read from pack_objects.err to capture stderr output for
@@ -290,8 +224,9 @@ static void create_pack_file(void)
 		error("git upload-pack: git-pack-objects died with error.");
 		goto fail;
 	}
-	if (shallow_nr && finish_async(&rev_list))
-		goto fail;	/* error was already reported */
+
+	if (*alternate_shallow_file)
+		rollback_lock_file(&shallow_lock);
 
 	/* flush the data */
 	if (0 <= buffered) {
@@ -575,7 +510,6 @@ static void receive_needs(void)
 	int depth = 0;
 	int has_non_tip = 0;
 
-	shallow_nr = 0;
 	for (;;) {
 		struct object *o;
 		const char *features;
@@ -679,7 +613,6 @@ static void receive_needs(void)
 				packet_write(1, "shallow %s",
 						sha1_to_hex(object->sha1));
 				register_shallow(object->sha1);
-				shallow_nr++;
 			}
 			result = result->next;
 		}
@@ -715,7 +648,6 @@ static void receive_needs(void)
 				register_shallow(shallows.objects[i].item->sha1);
 		}
 
-	shallow_nr += shallows.nr;
 	free(shallows.objects);
 }
 

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index c73b62f..a1672bc 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -551,3 +551,69 @@ An example client/server communication might look like this:
    S: 0018ok refs/heads/debug\n
    S: 002ang refs/heads/master non-fast-forward\n
 ----
+
+Smart HTTP Transport
+--------------------
+
+Smart HTTP protocol is basically "git protocol on top of http". The
+base protocol is modified slightly to fit HTTP processing model: no
+bidirectional full-duplex connections, the program may read the
+request, write a response and must exit.
+
+Reference Discovery
+-------------------
+
+The server end always sends the list of references in both push and
+fetch cases. This ref list is retrieved by the client's sending HTTP
+GET request to a smart http url ending with
+"/info/refs?service=<service>" where <service> could be either
+git-upload-pack or git-receive-pack for fetching or pushing
+respectively. The output is in pkt-line format.
+
+----
+  advertised-refs  =  service
+		      flush-pkt
+		      (no-refs / list-of-refs)
+		      flush-pkt
+
+  service          =  PKT-LINE("# service=" service-name)
+  service-name     =  ("git-upload-pack" / "git-receive-pack")
+
+  no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
+		      NUL capability-list LF)
+
+  list-of-refs     =  first-ref *other-ref
+  first-ref        =  PKT-LINE(obj-id SP refname
+		      NUL capability-list LF)
+
+  other-ref        =  PKT-LINE(other-tip / other-peeled)
+  other-tip        =  obj-id SP refname LF
+  other-peeled     =  obj-id SP refname "^{}" LF
+
+  capability-list  =  capability *(SP capability)
+  capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
+  LC_ALPHA         =  %x61-7A
+----
+
+Packfile Negotiation
+--------------------
+
+For fetching, packet negotiation is via a series of HTTP POST requests
+to an url ending with "/git-upload-pack" with the content in pkt-line
+format. git-upload-pack's response consists of a "service" line like
+in Reference Discovery followed by normal git-upload-pack packet
+lines. Capability multi_ack_detailed is required by Smart HTTP.
+
+Common objects that are discovered are appended onto the request as
+have lines and are sent again on the next request. This allows the
+remote side to reinitialize its in-memory list of common objects
+during the next request and the remote does not need to maintain the
+negotiation state.
+
+Reference Update Request
+------------------------
+
+For pushing, a HTTP POST request is sent to an url ending with
+"/git-receive-pack" with the content in pkt-line format.
+git-receive-pack's response consists of a "service" line like in
+Reference Discovery followed by normal git-receive-pack packet lines.

Documentation/git-fetch-pack.txt, Documentation/git-receive-pack.txt, Documentation/git-send-pack.txt, Documentation/git-upload-pack.txt

diff --git a/Documentation/git-fetch-pack.txt b/Documentation/git-fetch-pack.txt
index 1e71754..85a9437 100644
--- a/Documentation/git-fetch-pack.txt
+++ b/Documentation/git-fetch-pack.txt
@@ -9,10 +9,7 @@ git-fetch-pack - Receive missing objects from another repository
 SYNOPSIS
 --------
 [verse]
-'git fetch-pack' [--all] [--quiet|-q] [--keep|-k] [--thin] [--include-tag]
-	[--upload-pack=<git-upload-pack>]
-	[--depth=<n>] [--no-progress]
-	[-v] [<host>:]<directory> [<refs>...]
+'git fetch-pack' [options] [<host>:]<directory> [<refs>...]
 
 DESCRIPTION
 -----------
@@ -90,6 +87,12 @@ be in a separate packet, and the list must end with a flush packet.
 --no-progress::
 	Do not show the progress.
 
+--stateless-rpc::
+	Smart HTTP mode.
+
+--lock-pack::
+	Issue "lock" command to the remote helper via stdout.
+
 -v::
 	Run verbosely.
 
diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index b1f7dc6..b56d2eb 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -9,7 +9,7 @@ git-receive-pack - Receive what is pushed into the repository
 SYNOPSIS
 --------
 [verse]
-'git-receive-pack' <directory>
+'git-receive-pack' [options] <directory>
 
 DESCRIPTION
 -----------
@@ -35,6 +35,20 @@ are not fast-forwards.
 
 OPTIONS
 -------
+--stateless-rpc::
+	git-receive-pack performs only a single read-write cycle with
+	stdin and stdout to fit with the HTTP POST request processing
+	model where a program may read the request, write a response,
+	and must exit.
+
+--advertise-refs::
+	Only the initial ref advertisement is output then exits
+	immediately to fit with the HTTP GET request model, where no
+	request content is received but a response must be produced.
+
+--quiet::
+	Make unpack-objects at the receive-pack end quiet.
+
 <directory>::
 	The repository to sync into.
 
diff --git a/Documentation/git-send-pack.txt b/Documentation/git-send-pack.txt
index dc3a568..a88e7e0 100644
--- a/Documentation/git-send-pack.txt
+++ b/Documentation/git-send-pack.txt
@@ -9,7 +9,7 @@ git-send-pack - Push objects over Git protocol to another repository
 SYNOPSIS
 --------
 [verse]
-'git send-pack' [--all] [--dry-run] [--force] [--receive-pack=<git-receive-pack>] [--verbose] [--thin] [<host>:]<directory> [<ref>...]
+'git send-pack' [options] [<host>:]<directory> [<ref>...]
 
 DESCRIPTION
 -----------
@@ -52,6 +52,13 @@ OPTIONS
 	Send a "thin" pack, which records objects in deltified form based
 	on objects not included in the pack to reduce network traffic.
 
+--stateless-rpc::
+	Smart HTTP mode.
+
+--helper-status:
+	Issue status commands (e.g. "ok" or "error") to the remote
+	help via stdout.
+
 <host>::
 	A remote host to house the repository.  When this
 	part is specified, 'git-receive-pack' is invoked via
diff --git a/Documentation/git-upload-pack.txt b/Documentation/git-upload-pack.txt
index 0abc806..98d73cc 100644
--- a/Documentation/git-upload-pack.txt
+++ b/Documentation/git-upload-pack.txt
@@ -9,7 +9,7 @@ git-upload-pack - Send objects packed back to git-fetch-pack
 SYNOPSIS
 --------
 [verse]
-'git-upload-pack' [--strict] [--timeout=<n>] <directory>
+'git-upload-pack' [options] <directory>
 
 DESCRIPTION
 -----------
@@ -31,6 +31,17 @@ OPTIONS
 --timeout=<n>::
 	Interrupt transfer after <n> seconds of inactivity.
 
+--stateless-rpc::
+	git-upload-pack performs only a single read-write cycle with
+	stdin and stdout to fit with the HTTP POST request processing
+	model where a program may read the request, write a response,
+	and must exit.
+
+--advertise-refs::
+	Only the initial ref advertisement is output then exits
+	immediately to fit with the HTTP GET request model, where no
+	request content is received but a response must be produced.
+
 <directory>::
 	The repository to sync from.
 

Documentation/technical/pack-protocol.txt, builtin/fetch-pack.c, remote-curl.c, t/t5536-fetch-shallow.sh, upload-pack.c

diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index a1672bc..5013652 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -574,6 +574,7 @@ respectively. The output is in pkt-line format.
   advertised-refs  =  service
 		      flush-pkt
 		      (no-refs / list-of-refs)
+		      *shallow
 		      flush-pkt
 
   service          =  PKT-LINE("# service=" service-name)
@@ -590,6 +591,8 @@ respectively. The output is in pkt-line format.
   other-tip        =  obj-id SP refname LF
   other-peeled     =  obj-id SP refname "^{}" LF
 
+  shallow           =  PKT-LINE("shallow" SP obj-id)
+
   capability-list  =  capability *(SP capability)
   capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
   LC_ALPHA         =  %x61-7A
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index f6a6d76..b89d753 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -146,8 +146,7 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 	}
 
 	memset(&shallow, 0, sizeof(shallow));
-	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL,
-			 args.stateless_rpc ? NULL : &shallow);
+	get_remote_heads(fd[0], NULL, 0, &ref, 0, NULL, &shallow);
 
 	ref = fetch_pack(&args, fd, conn, ref, dest,
 			 sought, nr_sought, &shallow, pack_lockfile_ptr);
diff --git a/remote-curl.c b/remote-curl.c
index c329bd3..de2cc8a 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -78,6 +78,7 @@ struct discovery {
 	char *buf;
 	size_t len;
 	struct ref *refs;
+	struct extra_have_objects shallow;
 	unsigned proto_git : 1;
 };
 static struct discovery *last_discovery;
@@ -86,7 +87,7 @@ static struct ref *parse_git_refs(struct discovery *heads, int for_push)
 {
 	struct ref *list = NULL;
 	get_remote_heads(-1, heads->buf, heads->len, &list,
-			 for_push ? REF_NORMAL : 0, NULL, NULL);
+			 for_push ? REF_NORMAL : 0, NULL, &heads->shallow);
 	return list;
 }
 
@@ -146,6 +147,7 @@ static void free_discovery(struct discovery *d)
 	if (d) {
 		if (d == last_discovery)
 			last_discovery = NULL;
+		free(d->shallow.array);
 		free(d->buf_alloc);
 		free_refs(d->refs);
 		free(d);
diff --git a/t/t5536-fetch-shallow.sh b/t/t5536-fetch-shallow.sh
index 15a8208..6ea6347 100755
--- a/t/t5536-fetch-shallow.sh
+++ b/t/t5536-fetch-shallow.sh
@@ -102,4 +102,31 @@ EOF
 
 '
 
+if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
+	say 'skipping remaining tests, git built without http support'
+	test_done
+fi
+
+LIB_HTTPD_PORT=${LIB_HTTPD_PORT-'5536'}
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+test_expect_success 'clone http repository' '
+	git clone --bare --no-local shallow "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git clone --quiet $HTTPD_URL/smart/repo.git clone &&
+	(
+	cd clone &&
+	git fsck &&
+	git log --format=%s origin/master >actual &&
+	cat <<EOF >expect &&
+6
+5
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+stop_httpd
 test_done
diff --git a/upload-pack.c b/upload-pack.c
index c3e68ae..263ae08 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -770,8 +770,6 @@ int main(int argc, char **argv)
 
 	if (!enter_repo(dir, strict))
 		die("'%s' does not appear to be a git repository", dir);
-	if (is_repository_shallow() && stateless_rpc)
-		die("attempt to push into a shallow repository");
 
 	git_config(upload_pack_config, NULL);
 	upload_pack();

builtin/receive-pack.c, t/t5537-push-shallow.sh

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index cbb2025..54bf6b2 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1038,9 +1038,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 	if (!enter_repo(dir, 0))
 		die("'%s' does not appear to be a git repository", dir);
 
-	if (is_repository_shallow() && stateless_rpc)
-		die("attempt to push into a shallow repository");
-
 	git_config(receive_pack_config, NULL);
 
 	if (0 <= transfer_unpack_limit)
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
index beb85ce..02d9198 100755
--- a/t/t5537-push-shallow.sh
+++ b/t/t5537-push-shallow.sh
@@ -16,6 +16,7 @@ test_expect_success 'setup' '
 	commit 2 &&
 	commit 3 &&
 	commit 4 &&
+	git clone . full &&
 	(
 	git init full-abc &&
 	cd full-abc &&
@@ -107,4 +108,38 @@ EOF
 	)
 '
 
+if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
+	say 'skipping remaining tests, git built without http support'
+	test_done
+fi
+
+LIB_HTTPD_PORT=${LIB_HTTPD_PORT-'5537'}
+. "$TEST_DIRECTORY"/lib-httpd.sh
+start_httpd
+
+test_expect_success 'push to shallow repo via http' '
+	git clone --bare --no-local shallow "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git config http.receivepack true
+	) &&
+	(
+	cd full &&
+	commit 9 &&
+	git push $HTTPD_URL/smart/repo.git +master:refs/remotes/top/master
+	) &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+9
+4
+3
+EOF
+	test_cmp expect actual
+	)
+'
+
+stop_httpd
 test_done

builtin/send-pack.c, send-pack.c, t/t5537-push-shallow.sh

diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 14a73ba..b177120 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -185,9 +185,6 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 	    (send_all && args.send_mirror))
 		usage(send_pack_usage);
 
-	if (is_repository_shallow() && args.stateless_rpc)
-		die("attempt to push from a shallow repository");
-
 	if (remote_name) {
 		remote = remote_get(remote_name);
 		if (!remote_has_url(remote, dest)) {
diff --git a/send-pack.c b/send-pack.c
index 1b828b0..520c838 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -198,6 +198,21 @@ static int sideband_demux(int in, int out, void *data)
 	return ret;
 }
 
+static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *cb)
+{
+	struct strbuf *sb = cb;
+	if (graft->nr_parent == -1)
+		packet_buf_write(sb, "shallow %s\n", sha1_to_hex(graft->sha1));
+	return 0;
+}
+
+void advertise_shallow_grafts_buf(struct strbuf *sb)
+{
+	if (!is_repository_shallow())
+		return;
+	for_each_commit_graft(advertise_shallow_grafts_cb, sb);
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
@@ -239,7 +254,7 @@ int send_pack(struct send_pack_args *args,
 	}
 
 	if (!args->dry_run)
-		advertise_shallow_grafts(out);
+		advertise_shallow_grafts_buf(&req_buf);
 
 	/*
 	 * Finally, tell the other end!
@@ -299,7 +314,7 @@ int send_pack(struct send_pack_args *args,
 	}
 
 	if (args->stateless_rpc) {
-		if (!args->dry_run && cmds_sent) {
+		if (!args->dry_run && (cmds_sent || is_repository_shallow())) {
 			packet_buf_flush(&req_buf);
 			send_sideband(out, -1, req_buf.buf, req_buf.len, LARGE_PACKET_MAX);
 		}
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
index 02d9198..8bea496 100755
--- a/t/t5537-push-shallow.sh
+++ b/t/t5537-push-shallow.sh
@@ -141,5 +141,30 @@ EOF
 	)
 '
 
+test_expect_success 'push from shallow repo via http' '
+	mv "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" shallow-upstream.git &&
+	git clone --bare --no-local full "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git config http.receivepack true
+	) &&
+	commit 10 &&
+	git push $HTTPD_URL/smart/repo.git +master:refs/remotes/top/master &&
+	(
+	cd "$HTTPD_DOCUMENT_ROOT_PATH/repo.git" &&
+	git fsck &&
+	git log --format=%s top/master >actual &&
+	cat <<EOF >expect &&
+10
+1
+4
+3
+2
+1
+EOF
+	test_cmp expect actual
+	)
+'
+
 stop_httpd
 test_done

diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 450f158..4987857 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -181,12 +181,7 @@ objects from the source repository into a pack in the cloned repository.
 
 --depth <depth>::
 	Create a 'shallow' clone with a history truncated to the
-	specified number of revisions.  A shallow repository has a
-	number of limitations (you cannot clone or fetch from
-	it, nor push from nor into it), but is adequate if you
-	are only interested in the recent history of a large project
-	with a long history, and would want to send in fixes
-	as patches.
+	specified number of revisions.
 
 --[no-]single-branch::
 	Clone only the history leading to the tip of a single branch,

Documentation/config.txt, builtin/receive-pack.c, cache.h, config.c, environment.c, fetch-pack.c, t/t5536-fetch-shallow.sh, t/t5537-push-shallow.sh

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 81856dd..e811180 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -655,6 +655,11 @@ core.abbrev::
 	for abbreviated object names to stay unique for sufficiently long
 	time.
 
+core.noshallow::
+	If true, reject any pushes or fetches that may turn the
+	repository into a shallow one. This setting is ignored if the
+	repository is already shallow.
+
 add.ignore-errors::
 add.ignoreErrors::
 	Tells 'git add' to continue adding files when some files cannot be
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 54bf6b2..95ea481 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -936,7 +936,14 @@ static const char *unpack(int err_fd)
 					&shallow,
 					WRITE_SHALLOW_NO_CUT |
 					WRITE_SHALLOW_REWRITE);
-		commit_lock_file(&shallow_lock);
+		if (*alternate_shallow_file == '\0') {
+			unlink_or_warn(git_path("shallow"));
+			rollback_lock_file(&shallow_lock);
+		} else {
+			if (!is_repository_shallow() && cannot_be_shallow)
+				die("not allowed to turn this repository into a shallow one");
+			commit_lock_file(&shallow_lock);
+		}
 	}
 	return NULL;
 }
diff --git a/cache.h b/cache.h
index 7f17228..3a52b08 100644
--- a/cache.h
+++ b/cache.h
@@ -592,6 +592,7 @@ extern int fsync_object_files;
 extern int core_preload_index;
 extern int core_apply_sparse_checkout;
 extern int precomposed_unicode;
+extern int cannot_be_shallow;
 
 /*
  * The character that begins a commented line in user-editable file
diff --git a/config.c b/config.c
index d04e815..31f5a57 100644
--- a/config.c
+++ b/config.c
@@ -784,6 +784,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.noshallow")) {
+		cannot_be_shallow = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }
diff --git a/environment.c b/environment.c
index 0cb67b2..14c8005 100644
--- a/environment.c
+++ b/environment.c
@@ -61,6 +61,7 @@ int merge_log_config = -1;
 int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 struct startup_info *startup_info;
 unsigned long pack_size_limit_cfg;
+int cannot_be_shallow;
 
 /*
  * The character that begins a commented line in user-editable file
diff --git a/fetch-pack.c b/fetch-pack.c
index f337526..40e7aa2 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -960,7 +960,14 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 							WRITE_SHALLOW_NO_CUT |
 							WRITE_SHALLOW_REWRITE);
 			}
-			commit_lock_file(&shallow_lock);
+			if (*alternate_shallow_file == '\0') {
+				unlink_or_warn(git_path("shallow"));
+				rollback_lock_file(&shallow_lock);
+			} else {
+				if (!is_repository_shallow() && cannot_be_shallow)
+					die("not allowed to turn this repository into a shallow one");
+				commit_lock_file(&shallow_lock);
+			}
 		}
 	}
 
diff --git a/t/t5536-fetch-shallow.sh b/t/t5536-fetch-shallow.sh
index 6ea6347..b7f89b1 100755
--- a/t/t5536-fetch-shallow.sh
+++ b/t/t5536-fetch-shallow.sh
@@ -102,6 +102,15 @@ EOF
 
 '
 
+test_expect_success 'core.noshallow' '
+	git init clean &&
+	(
+	cd clean &&
+	git config core.noshallow true &&
+	test_must_fail git fetch ../shallow/.git
+	)
+'
+
 if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
 	say 'skipping remaining tests, git built without http support'
 	test_done
diff --git a/t/t5537-push-shallow.sh b/t/t5537-push-shallow.sh
index 8bea496..0edd51f 100755
--- a/t/t5537-push-shallow.sh
+++ b/t/t5537-push-shallow.sh
@@ -108,6 +108,12 @@ EOF
 	)
 '
 
+test_expect_success 'core.noshallow' '
+	git init clean &&
+	git --git-dir=clean/.git config core.noshallow true &&
+	test_must_fail git --git-dir=shallow/.git push clean master:refs/remotes/shallow/master
+'
+
 if test -n "$NO_CURL" -o -z "$GIT_TEST_HTTPD"; then
 	say 'skipping remaining tests, git built without http support'
 	test_done

builtin/clone.c, t/t5601-clone.sh

diff --git a/builtin/clone.c b/builtin/clone.c
index 17f57cd..09e383e 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -789,8 +789,18 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	else
 		repo = repo_name;
 	is_local = option_local != 0 && path && !is_bundle;
-	if (is_local && option_depth)
-		warning(_("--depth is ignored in local clones; use file:// instead."));
+	if (is_local) {
+		struct strbuf sb = STRBUF_INIT;
+		if (option_depth)
+			warning(_("--depth is ignored in local clones; use file:// instead."));
+		strbuf_addf(&sb, "%s/shallow", path);
+		if (!access(sb.buf, F_OK)) {
+			if (option_local > 0)
+				warning(_("source repository is shallow, ignoring --local"));
+			is_local = 0;
+		}
+		strbuf_release(&sb);
+	}
 	if (option_local > 0 && !is_local)
 		warning(_("--local is ignored"));
 
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index 0629149..35a2a3d 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -285,4 +285,11 @@ test_expect_success NOT_MINGW,NOT_CYGWIN 'clone local path foo:bar' '
 	git clone "./foo:bar" foobar
 '
 
+test_expect_success 'shallow clone locally' '
+	git clone --depth=1 --no-local src ssrrcc &&
+	git clone ssrrcc ddsstt &&
+	test_cmp ssrrcc/.git/shallow ddsstt/.git/shallow &&
+	( cd ddsstt && git fsck )
+'
+
 test_done