patchrepack: fix geometric repacking with gitalternates

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 87f73c8923..c6d12fa4bd 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -333,6 +333,12 @@ static void init_pack_geometry(struct pack_geometry **geometry_p,
 	geometry = *geometry_p;
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
+		/*
+		 * We don't want to repack packfiles which are not part of the
+		 * primary object database.
+		 */
+		if (!p->pack_local)
+			continue;
 		if (!pack_kept_objects) {
 			/*
 			 * Any pack that has its pack_keep bit set will appear
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 8821fbd2dd..9f8bc663e4 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -281,4 +281,63 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 	)
 '
 
+packed_objects() {
+	git verify-pack -v "$@" >tmp-object-list &&
+	sed -n -e "s/^\([0-9a-f][0-9a-f]*\).*\(commit\|tree\|blob\|tag\).*/\1/p" <tmp-object-list &&
+	rm -f tmp-object-list
+}
+
+test_expect_success '--geometric with no local objects creates no pack' '
+	git init shared &&
+	test_when_finished "rm -fr shared" &&
+	test_commit -C shared "shared" &&
+	git -C shared repack -Ad &&
+
+	# Set up the member repository so that it does not have
+	# any objects on its own.
+	git clone --shared shared member &&
+	test_when_finished "rm -fr member" &&
+
+	git -C member repack --geometric 2 --write-midx &&
+	find member/.git/objects/pack -type f >actual &&
+	test_must_be_empty actual
+'
+
+test_expect_success '--geometric does not include shared packfiles' '
+	git init shared &&
+	test_when_finished "rm -fr shared" &&
+	test_commit -C shared "shared" &&
+	git -C shared repack -Ad &&
+
+	git clone --shared shared member &&
+	test_when_finished "rm -fr member" &&
+	git -C member commit --allow-empty --message "not-shared" &&
+	git -C member repack --geometric 2 --write-midx &&
+
+	# We expect the created packfile to only contain the new commit.
+	packed_objects member/.git/objects/pack/pack-*.idx >actual &&
+	git -C member rev-parse HEAD >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--geometric with same packfile in shared repository' '
+	git init shared &&
+	test_when_finished "rm -fr shared" &&
+	test_commit -C shared "shared" &&
+	git -C shared repack -Ad &&
+
+	# Prepare the member repository so that it got the exact same packfile
+	# as the shared repository and set up gitalternates.
+	cp -r shared member &&
+	test_when_finished "rm -fr member" &&
+	test-tool path-utils real_path shared/.git/objects >member/.git/objects/info/alternates &&
+	find shared/.git/objects -type f >expect &&
+
+	# After repacking, contents of the member repository should not have
+	# changed.
+	git -C member repack --geometric 2 --write-midx 2>error &&
+	find shared/.git/objects -type f >actual &&
+	test_cmp expect actual
+'
+
 test_done

diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 9f8bc663e4..e3ac5001bd 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -282,9 +282,8 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 '

 packed_objects() {
-	git verify-pack -v "$@" >tmp-object-list &&
-	sed -n -e "s/^\([0-9a-f][0-9a-f]*\).*\(commit\|tree\|blob\|tag\).*/\1/p" <tmp-object-list &&
-	rm -f tmp-object-list
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list
 }

 test_expect_success '--geometric with no local objects creates no pack' '
--- >8 ---

I removed the "&& rm -f tmp-object-list", since we overwrite it anyway
whenever we call packed_objects().

> +test_expect_success '--geometric with no local objects creates no pack' '
> +	git init shared &&
> +	test_when_finished "rm -fr shared" &&
> +	test_commit -C shared "shared" &&
> +	git -C shared repack -Ad &&
> +
> +	# Set up the member repository so that it does not have
> +	# any objects on its own.
> +	git clone --shared shared member &&
> +	test_when_finished "rm -fr member" &&
> +
> +	git -C member repack --geometric 2 --write-midx &&
> +	find member/.git/objects/pack -type f >actual &&
> +	test_must_be_empty actual

Another small nitpick, but I think these last two lines could be
replaced with

    test_dir_is_empty member/.git/objects/pack

or even

    test_dir_is_empty member/$packdir

> +test_expect_success '--geometric does not include shared packfiles' '
> +	git init shared &&
> +	test_when_finished "rm -fr shared" &&
> +	test_commit -C shared "shared" &&
> +	git -C shared repack -Ad &&
> +
> +	git clone --shared shared member &&
> +	test_when_finished "rm -fr member" &&
> +	git -C member commit --allow-empty --message "not-shared" &&

Any reason to make this commit empty? Could we replace this with

    test_commit -C member not-shared

?

> +test_expect_success '--geometric with same packfile in shared repository' '
> +	git init shared &&
> +	test_when_finished "rm -fr shared" &&
> +	test_commit -C shared "shared" &&
> +	git -C shared repack -Ad &&
> +
> +	# Prepare the member repository so that it got the exact same packfile
> +	# as the shared repository and set up gitalternates.
> +	cp -r shared member &&
> +	test_when_finished "rm -fr member" &&
> +	test-tool path-utils real_path shared/.git/objects >member/.git/objects/info/alternates &&
> +	find shared/.git/objects -type f >expect &&

This works, though it may be easier to write something like:

    git clone -s shared member &&
    test_when_finished "rm -fr member" &&
    # ensure that "shared" and "member" have an identical set of packs
    git -C member repack -a &&
    find shared/.git/objects -type f >expect &&

> +	# After repacking, contents of the member repository should not have
> +	# changed.
> +	git -C member repack --geometric 2 --write-midx 2>error &&

I think that "2>error" is a stray change left over from debugging, and
could probably be removed.

> +	find shared/.git/objects -type f >actual &&

This and above could be replaced with shared/$packdir. Whenever I'm
asserting the contents of a directory are unchanged, I typically like to
call the two files under comparison "before" and "after", but "expect"
and "actual" are fine here, too.

Thanks,
Taylor

diff --git a/builtin/repack.c b/builtin/repack.c
index df4d8e0f0b..eab5f58444 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -558,6 +558,10 @@ static void midx_included_packs(struct string_list *include,
 		for (i = geometry->split; i < geometry->pack_nr; i++) {
 			struct packed_git *p = geometry->pack[i];

+			/* MIDXs cannot refer to non-local packs */
+			if (!p->pack_local)
+				continue;
+
 			strbuf_addstr(&buf, pack_basename(p));
 			strbuf_strip_suffix(&buf, ".pack");
 			strbuf_addstr(&buf, ".idx");
--- >8 ---

...and I actually think that might do it, since:

	- existing_nonkept_packs is populated by calling readdir() on the local
		repository's $GIT_DIR/objects/pack directory, so it will never contain
		any non-local packs.

	- existing_kept_packs is also OK for the same reason

	- names (which tracks the packs that we just wrote) will never contain a
		non-local pack, since we never write packs outside of our local pack
		directory

So that would cause you to write a MIDX containing only local packs (as
desired) regardless of whether or not the caller passed --[no]-local or
not.

> > I'd personally be fine to start honoring the `po_args.local` flag so
> > that we skip over any non-local packfiles there while ignoring the
> > larger problem of non-local geometric repacks breaking in certain
> > scenarios. It would at least improve the status quo as users now have a
> > way out in case they ever happen to hit that error. And it allows us to
> > use geometric repacks in alternated repositories. But are we okay with
> > punting on the larger issue for the time being?
>
> I think the real bug is isolated in write_midx_included_packs() in how
> it may specify packs that the multi-pack-index cannot track. It should
> be worth the time exploring if there is an easy fix there, and then the
> po_args.local version can be used as a backup/performance tweak on top
> of that.

Yeah, seeing "unknown preferred pack" makes me suspicious that we tried
to write a MIDX that contains a pack outside of our repository. I tried
to reproduce that case with the following script but couldn't do it (the
pack that the MIDX does track is the local one, as expected):

--- 8< ---
#!/bin/sh

rm -fr shared member

git init shared
git -C shared commit --allow-empty -m "base" && git -C shared repack -d

git clone --shared shared member

git -C member commit --allow-empty -m "base" && git -C member repack -d

{
	basename "$(find shared/.git/objects/pack -type f -name '*.idx')"
	basename "$(find member/.git/objects/pack -type f -name '*.idx')"
} |
git.compile -C member multi-pack-index write --stdin-packs

for repo in shared member
do
	echo "==> $repo"
	find $repo/.git/objects/pack -type f
done

( cd member && ~/src/git/t/helper/test-tool read-midx --show-objects .git/objects )
--- >8 ---

But the MIDX code that is responsible for collecting the packs specified
over --stdin-packs enumerates the possible packs by calling
`for_each_file_in_pack_dir()` with our local `object_dir`, so I don't
think it's possible to have a MIDX that contains a non-local pack.

IOW, I agree with you that the bug is in write_midx_included_packs(),
and I think the fix that I outlined would do the trick.

Thanks,
Taylor

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

diff --git a/midx.c b/midx.c
index 9af3e5de88..22ea7ffb75 100644
--- a/midx.c
+++ b/midx.c
@@ -1307,6 +1307,12 @@ static int write_midx_internal(const char *object_dir,
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);
 
+	if (!ctx.nr) {
+		error(_("no pack files to index."));
+		result = 1;
+		goto cleanup;
+	}
+
 	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
 	    !(packs_to_include || packs_to_drop)) {
 		struct bitmap_index *bitmap_git;
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78..be7f3c1e1f 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -183,6 +183,17 @@ test_expect_success 'write midx with --stdin-packs' '
 
 compare_results_with_midx "mixed mode (one pack + extra)"
 
+test_expect_success 'write with no objects and preferred pack' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	test_must_fail git -C empty multi-pack-index write \
+		--stdin-packs --preferred-pack=does-not-exist </dev/null 2>err &&
+	cat >expect <<-EOF &&
+	error: no pack files to index.
+	EOF
+	test_cmp expect err
+'
+
 test_expect_success 'write progress off for redirected stderr' '
 	git multi-pack-index --object-dir=$objdir write 2>err &&
 	test_line_count = 0 err

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

diff --git a/midx.c b/midx.c
index 22ea7ffb75..dc4821eab8 100644
--- a/midx.c
+++ b/midx.c
@@ -1263,6 +1263,7 @@ static int write_midx_internal(const char *object_dir,
 	ctx.nr = 0;
 	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
 	ctx.info = NULL;
+	ctx.preferred_pack_idx = -1;
 	ALLOC_ARRAY(ctx.info, ctx.alloc);

 	if (ctx.m) {
@@ -1307,10 +1308,10 @@ static int write_midx_internal(const char *object_dir,
 	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
 	stop_progress(&ctx.progress);

-	if (!ctx.nr) {
-		error(_("no pack files to index."));
-		result = 1;
-		goto cleanup;
+	if (!ctx.entries_nr) {
+		if (flags & MIDX_WRITE_BITMAP)
+			warning(_("refusing to write multi-pack .bitmap without any objects"));
+		flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
 	}

 	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
@@ -1488,12 +1489,6 @@ static int write_midx_internal(const char *object_dir,
 		goto cleanup;
 	}

-	if (!ctx.entries_nr) {
-		if (flags & MIDX_WRITE_BITMAP)
-			warning(_("refusing to write multi-pack .bitmap without any objects"));
-		flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
-	}
-
 	cf = init_chunkfile(f);

 	add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index be7f3c1e1f..8a17361272 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -188,10 +188,9 @@ test_expect_success 'write with no objects and preferred pack' '
 	git init empty &&
 	test_must_fail git -C empty multi-pack-index write \
 		--stdin-packs --preferred-pack=does-not-exist </dev/null 2>err &&
-	cat >expect <<-EOF &&
-	error: no pack files to index.
-	EOF
-	test_cmp expect err
+		cat err &&
+	grep "warning: unknown preferred pack: .does-not-exist." err &&
+	grep "error: no pack files to index." err
 '

 test_expect_success 'write progress off for redirected stderr' '
--- >8 ---

Thanks,
Taylor

diff --git a/midx.c b/midx.c
index 47989f7ea7..67eb617591 100644
--- a/midx.c
+++ b/midx.c
@@ -1328,17 +1328,17 @@ static int write_midx_internal(const char *object_dir,
        }

        if (preferred_pack_name) {
-               int found = 0;
+               ctx.preferred_pack_idx = -1;
+
                for (i = 0; i < ctx.nr; i++) {
                        if (!cmp_idx_or_pack_name(preferred_pack_name,
                                                  ctx.info[i].pack_name)) {
                                ctx.preferred_pack_idx = i;
-                               found = 1;
                                break;
                        }
                }

-               if (!found)
+               if (ctx.preferred_pack_idx == -1)
                        warning(_("unknown preferred pack: '%s'"),
                                preferred_pack_name);
        } else if (ctx.nr &&

The other cases already set `preferred_pack_idx = -1`, so this is really
all we need to do to fix the segfault.

Patrick

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index df4d8e0f0b..9d36dc8b84 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -448,8 +448,10 @@ static void split_pack_geometry(struct pack_geometry *geometry, int factor)
 	geometry->split = split;
 }
 
-static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry)
+static struct packed_git *get_preferred_pack(struct pack_geometry *geometry)
 {
+	uint32_t i;
+
 	if (!geometry) {
 		/*
 		 * No geometry means either an all-into-one repack (in which
@@ -464,7 +466,16 @@ static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry
 	}
 	if (geometry->split == geometry->pack_nr)
 		return NULL;
-	return geometry->pack[geometry->pack_nr - 1];
+
+	for (i = geometry->pack_nr; i > 0; i--)
+		/*
+		 * A pack that is not local would never be included in a
+		 * multi-pack index. We thus skip over any non-local packs.
+		 */
+		if (geometry->pack[i - 1]->pack_local)
+			return geometry->pack[i - 1];
+
+	return NULL;
 }
 
 static void clear_pack_geometry(struct pack_geometry *geometry)
@@ -591,7 +602,7 @@ static int write_midx_included_packs(struct string_list *include,
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list_item *item;
-	struct packed_git *largest = get_largest_active_pack(geometry);
+	struct packed_git *preferred = get_preferred_pack(geometry);
 	FILE *in;
 	int ret;
 
@@ -612,9 +623,9 @@ static int write_midx_included_packs(struct string_list *include,
 	if (write_bitmaps)
 		strvec_push(&cmd.args, "--bitmap");
 
-	if (largest)
+	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-			     pack_basename(largest));
+			     pack_basename(preferred));
 
 	if (refs_snapshot)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s", refs_snapshot);
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 8821fbd2dd..92a1aaa754 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -281,4 +281,35 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 	)
 '
 
+test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a shared repository that will serve as the alternate object
+	# database for the member linked to it. It has got some objects on its
+	# own that are packed into a single packfile.
+	git init shared &&
+	test_commit -C shared common-object &&
+	git -C shared repack -ad &&
+
+	# We create member so that its alternates file points to the shared
+	# repository. We then create a commit in it so that git-repack(1) has
+	# something to repack.
+	# of the shared object database.
+	git clone --shared shared member &&
+	test_commit -C member unique-object &&
+	git -C member repack --geometric=2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# We should see that a new packfile was generated.
+	find shared/.git/objects/pack -type f -name "*.pack" | sort >packs &&
+	test $(wc -l <packs) = 1 &&
+
+	# We should also see a multi-pack-index. This multi-pack-index should
+	# never refer to any packfiles in the alternate object database.
+	# Consequentially, it should be valid even with the alternate ODB
+	# deleted.
+	rm -r shared &&
+	git -C member multi-pack-index verify
+'
+
 test_done

diff --git a/builtin/repack.c b/builtin/repack.c
index 9d36dc8b84..ba468ac44e 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -467,13 +467,15 @@ static struct packed_git *get_preferred_pack(struct pack_geometry *geometry)
 	if (geometry->split == geometry->pack_nr)
 		return NULL;

-	for (i = geometry->pack_nr; i > 0; i--)
+	for (i = geometry->pack_nr - 1; i >= 0; i--) {
 		/*
 		 * A pack that is not local would never be included in a
 		 * multi-pack index. We thus skip over any non-local packs.
 		 */
-		if (geometry->pack[i - 1]->pack_local)
-			return geometry->pack[i - 1];
+		struct packed_git *p = geometry->pack[i];
+		if (p->pack_local)
+			return p;
+	}

 	return NULL;
 }
--- >8 ---

but I'm not sure that the loop condition is quite right to begin with.
We don't want to iterate all the way down to the beginning of the pack
list, since some of those packs may be below the "split" line, IOW their
contents are going to be rolled up and the packs destroyed.

So I think the right loop condition would be:

    for (i = geometry->pack_nr - 1; i >= geometry->split; i--)

which I think makes the "if (geometry->split == geometry->pack_nr)"
condition above redundant with this loop, so you could probably drop
that.

I would definitely appreciate a second set of eye here. The pack *at*
the split line is considered frozen (IOW, we create a new pack
consisting of the packs in range [0, geometry->split), and leave the
packs in range [geometry->split, geometry->nr) alone).

So it should be fine to enter that loop when "i == geometry->split",
because it's OK to return that as a potential preferred pack.

> +test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' '
> +	test_when_finished "rm -fr shared member" &&

Looks familiar ;-).

> +	# Create a shared repository that will serve as the alternate object
> +	# database for the member linked to it. It has got some objects on its
> +	# own that are packed into a single packfile.
> +	git init shared &&
> +	test_commit -C shared common-object &&
> +	git -C shared repack -ad &&
> +
> +	# We create member so that its alternates file points to the shared
> +	# repository. We then create a commit in it so that git-repack(1) has
> +	# something to repack.
> +	# of the shared object database.
> +	git clone --shared shared member &&
> +	test_commit -C member unique-object &&
> +	git -C member repack --geometric=2 --write-midx 2>err &&
> +	test_must_be_empty err &&
> +
> +	# We should see that a new packfile was generated.
> +	find shared/.git/objects/pack -type f -name "*.pack" | sort >packs &&

Do we need a sort here? I don't think we rely on the order anywhere, and
it looks like we only care that there is a single entry.

If that's the case, I think we could replace the wc invocation here
with:

    test_line_count = 1 packs

> +	test $(wc -l <packs) = 1 &&
> +
> +	# We should also see a multi-pack-index. This multi-pack-index should
> +	# never refer to any packfiles in the alternate object database.
> +	# Consequentially, it should be valid even with the alternate ODB
> +	# deleted.
> +	rm -r shared &&
> +	git -C member multi-pack-index verify

To test this even more directly, I think that you could ensure that the
set of packs (which should just be the single entry found in "packs"
above) matches what we expect. That should look something like:

    test-tool read-midx member/.git/objects >packs.midx &&
    grep "^pack-.*\.idx$" packs.midx | sort >actual &&
    xargs -n 1 basename <packs | sort >expect
    test_cmp expect actual

Note that the read-midx test helper outputs packs with the "*.idx"
suffix, so you'd probably want to alter your find invocation a few lines
above accordingly, or rewrite it before writing into "actual".

Alternatively, since we expect there to be just a single pack here, I
think we could just as easily write something like:

    test-tool read-midx member/.git/objects >packs.midx &&
    grep "^pack-.*\.idx$" packs.midx >actual &&
    test_line_count = 1 actual &&
    grep $(cat actual) packs

though I slightly prefer the former since it is less brittle to changing
this test. Either way, though.

Thanks,
Taylor

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 9d36dc8b84..80d4e813c8 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -569,6 +569,17 @@ static void midx_included_packs(struct string_list *include,
 		for (i = geometry->split; i < geometry->pack_nr; i++) {
 			struct packed_git *p = geometry->pack[i];
 
+			/*
+			 * The multi-pack index never refers to packfiles part
+			 * of an alternate object database, so we skip these.
+			 * While git-multi-pack-index(1) would silently ignore
+			 * them anyway, this allows us to skip executing the
+			 * command completely when we have only non-local
+			 * packfiles.
+			 */
+			if (!p->pack_local)
+				continue;
+
 			strbuf_addstr(&buf, pack_basename(p));
 			strbuf_strip_suffix(&buf, ".pack");
 			strbuf_addstr(&buf, ".idx");
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 92a1aaa754..9e95350cbf 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -312,4 +312,28 @@ test_expect_success '--geometric --write-midx with packfiles in main and alterna
 	git -C member multi-pack-index verify
 '
 
+test_expect_success '--geometric --with-midx with no local objects' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# Create a second repository linked to the first one and perform a
+	# geometric repack on it.
+	git clone --shared shared member &&
+	git -C member repack --geometric 2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# Assert that we wrote neither a new packfile nor a multi-pack-index.
+	# We should not have a packfile because the single packfile in the
+	# alternate object database does not invalidate the geometric sequence.
+	# And we should not have a multi-pack-index because these only index
+	# local packfiles, and there are none.
+	find member/.git/objects/pack -type f >actual &&
+	test_must_be_empty actual
+'
+
 test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh, t/t7703-repack-geometric.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 77d88f85b0..fdf3f440be 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3359,7 +3359,9 @@ static void read_packs_list_from_stdin(void)
 	}
 
 	string_list_sort(&include_packs);
+	string_list_remove_duplicates(&include_packs, 0);
 	string_list_sort(&exclude_packs);
+	string_list_remove_duplicates(&exclude_packs, 0);
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
new file mode 100755
index 0000000000..ab34cfc729
--- /dev/null
+++ b/t/t5331-pack-objects-stdin.sh
@@ -0,0 +1,29 @@
+#!/bin/sh
+
+test_description='pack-objects --stdin'
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'pack-objects --stdin with duplicate packfile' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		(
+			basename .git/objects/pack/pack-*.pack &&
+			basename .git/objects/pack/pack-*.pack
+		) >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		test_cmp generated-pack-*.pack .git/objects/pack/pack-*.pack
+	)
+'
+
+test_done
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 9e95350cbf..0a2f2bd46c 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -336,4 +336,29 @@ test_expect_success '--geometric --with-midx with no local objects' '
 	test_must_be_empty actual
 '
 
+test_expect_success '--geometric with same pack in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# We create the member repository as an exact copy so that it has the
+	# same packfile.
+	cp -r shared member &&
+	test-tool path-utils real_path shared/.git/objects >member/.git/objects/info/alternates &&
+	find shared/.git/objects -type f >expected-files &&
+
+	# Verify that we can repack objects as expected without observing any
+	# error. Having the same packfile in both ODBs used to cause an error
+	# in git-pack-objects(1).
+	git -C member repack --geometric 2 2>err &&
+	test_must_be_empty err &&
+	# Nothing should have changed.
+	find shared/.git/objects -type f >actual-files &&
+	test_cmp expected-files actual-files
+'
+
 test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index fdf3f440be..522eb4dd31 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3366,11 +3366,9 @@ static void read_packs_list_from_stdin(void)
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
 
-		item = string_list_lookup(&include_packs, pack_name);
-		if (!item)
-			item = string_list_lookup(&exclude_packs, pack_name);
-
-		if (item)
+		if ((item = string_list_lookup(&include_packs, pack_name)))
+			item->util = p;
+		if ((item = string_list_lookup(&exclude_packs, pack_name)))
 			item->util = p;
 	}
 
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index ab34cfc729..f389a78b38 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -7,6 +7,12 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
+packed_objects() {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list &&
+	rm tmp-object-list
+ }
+
 test_expect_success 'pack-objects --stdin with duplicate packfile' '
 	test_when_finished "rm -fr repo" &&
 
@@ -26,4 +32,24 @@ test_expect_success 'pack-objects --stdin with duplicate packfile' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with same packfile excluded and included' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		(
+			basename .git/objects/pack/pack-*.pack &&
+			printf "^%s\n" "$(basename .git/objects/pack/pack-*.pack)"
+		) >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects generated-pack-*.idx >packed-objects &&
+		test_must_be_empty packed-objects
+	)
+'
+
 test_done

diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index f389a78b38..8bd1d26caf 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -52,4 +52,52 @@ test_expect_success 'pack-objects --stdin with same packfile excluded and includ
 	)
 '
 
+test_expect_success 'pack-objects --stdin with packfiles from alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+	basename shared/.git/objects/pack/pack-*.pack >packfile &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has no objects on its own, but we still expect to be able
+	# to pack objects from its alternate.
+	git clone --shared shared member &&
+	git -C member pack-objects --stdin-packs generated-pack <packfile &&
+	test_cmp shared/.git/objects/pack/pack-*.pack member/generated-pack-*.pack
+'
+
+test_expect_success 'pack-objects --stdin with packfiles from main and alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-commit" &&
+	git -C shared repack -ad &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has a second packfile so that we can verify that it is
+	# possible to write packs that include packfiles from different object
+	# databases.
+	git clone --shared shared member &&
+	test_commit -C member "local-commit" &&
+	git -C member repack -dl &&
+
+	(
+		basename shared/.git/objects/pack/pack-*.pack  &&
+		basename member/.git/objects/pack/pack-*.pack
+	) >packfiles &&
+
+	(
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	) | sort >expected-objects &&
+
+	git -C member pack-objects --stdin-packs generated-pack <packfiles &&
+	packed_objects member/generated-pack-*.idx | sort >actual-objects &&
+	test_cmp expected-objects actual-objects
+'
+
 test_done

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 80d4e813c8..f57869f14a 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -325,7 +325,8 @@ static int geometry_cmp(const void *va, const void *vb)
 }
 
 static void init_pack_geometry(struct pack_geometry **geometry_p,
-			       struct string_list *existing_kept_packs)
+			       struct string_list *existing_kept_packs,
+			       const struct pack_objects_args *args)
 {
 	struct packed_git *p;
 	struct pack_geometry *geometry;
@@ -335,6 +336,14 @@ static void init_pack_geometry(struct pack_geometry **geometry_p,
 	geometry = *geometry_p;
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
+		if (args->local && !p->pack_local)
+			/*
+			 * When asked to only repack local packfiles we skip
+			 * over any packfiles that are borrowed from alternate
+			 * object directories.
+			 */
+			continue;
+
 		if (!pack_kept_objects) {
 			/*
 			 * Any pack that has its pack_keep bit set will appear
@@ -897,7 +906,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (geometric_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
-		init_pack_geometry(&geometry, &existing_kept_packs);
+		init_pack_geometry(&geometry, &existing_kept_packs, &po_args);
 		split_pack_geometry(geometry, geometric_factor);
 	}
 
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 0a2f2bd46c..96c8d4cdfa 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -10,6 +10,12 @@ objdir=.git/objects
 packdir=$objdir/pack
 midx=$objdir/pack/multi-pack-index
 
+packed_objects() {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list &&
+	rm tmp-object-list
+ }
+
 test_expect_success '--geometric with no packs' '
 	git init geometric &&
 	test_when_finished "rm -fr geometric" &&
@@ -361,4 +367,57 @@ test_expect_success '--geometric with same pack in main and alternate ODB' '
 	test_cmp expected-files actual-files
 '
 
+test_expect_success '--geometric -l with non-intact geometric sequence across ODBs' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# Verify that our assumptions actually hold: both generated packfiles
+	# should have three objects and should be non-equal.
+	packed_objects shared/.git/objects/pack/pack-*.idx >packed-objects &&
+	test_line_count = 3 packed-objects &&
+	packed_objects member/.git/objects/pack/pack-*.idx >packed-objetcs &&
+	test_line_count = 3 packed-objects &&
+	test "$(basename member/.git/objects/pack/pack-*.pack)" != "$(basename shared/.git/objects/pack/pack-*.pack)" &&
+
+	# Perform the geometric repack. With `-l`, we should only see the local
+	# packfile and thus arrive at the conclusion that the geometric
+	# sequence is intact. We thus expect no changes.
+	#
+	# Note that we are using stat(1) to verify idempotence to also verify
+	# that the mtime did not change. This is done in order to detect the
+	# case where we do repack objects, but the resulting packfile is the
+	# same.
+	stat member/.git/objects/pack/* >expected-member-packs &&
+	git -C member repack --geometric=2 -l -d &&
+	stat member/.git/objects/pack/* >actual-member-packs &&
+	test_cmp expected-member-packs actual-member-packs &&
+
+	(
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	) | sort >expected-objects &&
+
+	# On the other hand, when doing a non-local geometric repack we should
+	# see both packfiles and thus repack them. We expect that the shared
+	# object database was not changed.
+	stat shared/.git/objects/pack/* >expected-shared-packs &&
+	git -C member repack --geometric=2 -d &&
+	stat shared/.git/objects/pack/* >actual-shared-packs &&
+	test_cmp expected-shared-packs actual-shared-packs &&
+
+	# Furthermore, we expect that the member repository now has a single
+	# packfile that contains the combined shared and non-shared objects.
+	ls member/.git/objects/pack/pack-*.idx >actual &&
+	test_line_count = 1 actual &&
+	packed_objects member/.git/objects/pack/pack-*.idx >actual-objects &&
+	test_line_count = 6 actual-objects &&
+	sort <actual-objects >actual-objects.sorted &&
+	test_cmp expected-objects actual-objects.sorted
+'
+
 test_done

@@ -2,7 +2,7 @@
   Size: 1156           Blocks: 8          IO Block: 4096   regular file
 Device: 0,25   Inode: 2330515     Links: 1
 Access: (0400/-r--------)  Uid: (1001/   gitster)   Gid: (1001/gitster)
-Access: 2023-04-12 23:48:52.801034210 +0000
+Access: 2023-04-12 23:48:52.829036100 +0000
 Modify: 2023-04-12 23:48:52.773032320 +0000
 Change: 2023-04-12 23:48:52.773032320 +0000
  Birth: 2023-04-12 23:48:52.773032320 +0000
@@ -10,7 +10,7 @@
   Size: 276            Blocks: 8          IO Block: 4096   regular file
 Device: 0,25   Inode: 2330514     Links: 1
 Access: (0400/-r--------)  Uid: (1001/   gitster)   Gid: (1001/gitster)
-Access: 2023-04-12 23:48:52.781032860 +0000
+Access: 2023-04-12 23:48:52.829036100 +0000
 Modify: 2023-04-12 23:48:52.773032320 +0000
 Change: 2023-04-12 23:48:52.773032320 +0000
  Birth: 2023-04-12 23:48:52.769032050 +0000

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index f57869f14a..07d92fdf87 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -881,6 +881,26 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (pack_kept_objects < 0)
 		pack_kept_objects = write_bitmaps > 0 && !write_midx;
 
+	if (write_midx && write_bitmaps && geometric_factor && po_args.local) {
+		struct packed_git *p;
+
+		for (p = get_all_packs(the_repository); p; p = p->next) {
+			if (p->pack_local)
+				continue;
+
+			/*
+			 * When asked to do a local repack, but we have
+			 * packfiles that are inherited from an alternate, then
+			 * we cannot guarantee that the multi-pack-index would
+			 * have full coverage of all objects. We thus disable
+			 * writing bitmaps in that case.
+			 */
+			warning(_("disabling bitmap writing, as some objects are not being packed"));
+			write_bitmaps = 0;
+			break;
+		}
+	}
+
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));
 
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 96c8d4cdfa..0aaec9f853 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -420,4 +420,31 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	test_cmp expected-objects actual-objects.sorted
 '
 
+test_expect_success '--geometric -l disables writing bitmaps with non-local packfiles' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# When performing a geometric repack with `-l` while connecting to an
+	# alternate object database that has a packfile we do not have full
+	# coverage of objects. As a result, we expect that writing the bitmap
+	# will be disabled.
+	git -C member repack -l --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	cat >expect <<-EOF &&
+	warning: disabling bitmap writing, as some objects are not being packed
+	EOF
+	test_cmp expect err &&
+	test ! -f member/.git/objects/pack/multi-pack-index-*.bitmap &&
+
+	# On the other hand, when we repack without `-l`, we should see that
+	# the bitmap gets created.
+	git -C member repack --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	test_must_be_empty err &&
+	test -f member/.git/objects/pack/multi-pack-index-*.bitmap
+'
+
 test_done

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

diff --git a/midx.c b/midx.c
index 47989f7ea7..67eb617591 100644
--- a/midx.c
+++ b/midx.c
@@ -1328,17 +1328,17 @@ static int write_midx_internal(const char *object_dir,
 	}
 
 	if (preferred_pack_name) {
-		int found = 0;
+		ctx.preferred_pack_idx = -1;
+
 		for (i = 0; i < ctx.nr; i++) {
 			if (!cmp_idx_or_pack_name(preferred_pack_name,
 						  ctx.info[i].pack_name)) {
 				ctx.preferred_pack_idx = i;
-				found = 1;
 				break;
 			}
 		}
 
-		if (!found)
+		if (ctx.preferred_pack_idx == -1)
 			warning(_("unknown preferred pack: '%s'"),
 				preferred_pack_name);
 	} else if (ctx.nr &&
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78..0883c7c6bd 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -183,6 +183,18 @@ test_expect_success 'write midx with --stdin-packs' '
 
 compare_results_with_midx "mixed mode (one pack + extra)"
 
+test_expect_success 'write with no objects and preferred pack' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	test_must_fail git -C empty multi-pack-index write \
+		--stdin-packs --preferred-pack=does-not-exist </dev/null 2>err &&
+	cat >expect <<-EOF &&
+	warning: unknown preferred pack: ${SQ}does-not-exist${SQ}
+	error: no pack files to index.
+	EOF
+	test_cmp expect err
+'
+
 test_expect_success 'write progress off for redirected stderr' '
 	git multi-pack-index --object-dir=$objdir write 2>err &&
 	test_line_count = 0 err

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 87f73c8923..687419776d 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -446,8 +446,10 @@ static void split_pack_geometry(struct pack_geometry *geometry, int factor)
 	geometry->split = split;
 }
 
-static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry)
+static struct packed_git *get_preferred_pack(struct pack_geometry *geometry)
 {
+	uint32_t i;
+
 	if (!geometry) {
 		/*
 		 * No geometry means either an all-into-one repack (in which
@@ -462,7 +464,21 @@ static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry
 	}
 	if (geometry->split == geometry->pack_nr)
 		return NULL;
-	return geometry->pack[geometry->pack_nr - 1];
+
+	/*
+	 * The preferred pack is the largest pack above the split line. In
+	 * other words, it is the largest pack that does not get rolled up in
+	 * the geometric repack.
+	 */
+	for (i = geometry->pack_nr; i > geometry->split; i--)
+		/*
+		 * A pack that is not local would never be included in a
+		 * multi-pack index. We thus skip over any non-local packs.
+		 */
+		if (geometry->pack[i - 1]->pack_local)
+			return geometry->pack[i - 1];
+
+	return NULL;
 }
 
 static void clear_pack_geometry(struct pack_geometry *geometry)
@@ -589,7 +605,7 @@ static int write_midx_included_packs(struct string_list *include,
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list_item *item;
-	struct packed_git *largest = get_largest_active_pack(geometry);
+	struct packed_git *preferred = get_preferred_pack(geometry);
 	FILE *in;
 	int ret;
 
@@ -610,9 +626,9 @@ static int write_midx_included_packs(struct string_list *include,
 	if (write_bitmaps)
 		strvec_push(&cmd.args, "--bitmap");
 
-	if (largest)
+	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-			     pack_basename(largest));
+			     pack_basename(preferred));
 
 	if (refs_snapshot)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s", refs_snapshot);
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 8821fbd2dd..d6db362541 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -281,4 +281,36 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 	)
 '
 
+test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a shared repository that will serve as the alternate object
+	# database for the member linked to it. It has got some objects on its
+	# own that are packed into a single packfile.
+	git init shared &&
+	test_commit -C shared common-object &&
+	git -C shared repack -ad &&
+
+	# We create member so that its alternates file points to the shared
+	# repository. We then create a commit in it so that git-repack(1) has
+	# something to repack.
+	# of the shared object database.
+	git clone --shared shared member &&
+	test_commit -C member unique-object &&
+	git -C member repack --geometric=2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# We should see that a new packfile was generated.
+	find shared/.git/objects/pack -type f -name "*.pack" >packs &&
+	test_line_count = 1 packs &&
+
+	# We should also see a multi-pack-index. This multi-pack-index should
+	# never refer to any packfiles in the alternate object database.
+	test -f member/.git/objects/pack/multi-pack-index &&
+	test-tool read-midx member/.git/objects >packs.midx &&
+	grep "^pack-.*\.idx$" packs.midx | sort >actual &&
+	basename member/.git/objects/pack/pack-*.idx >expect &&
+	test_cmp expect actual
+'
+
 test_done

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 687419776d..80fc860613 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -572,6 +572,17 @@ static void midx_included_packs(struct string_list *include,
 		for (i = geometry->split; i < geometry->pack_nr; i++) {
 			struct packed_git *p = geometry->pack[i];
 
+			/*
+			 * The multi-pack index never refers to packfiles part
+			 * of an alternate object database, so we skip these.
+			 * While git-multi-pack-index(1) would silently ignore
+			 * them anyway, this allows us to skip executing the
+			 * command completely when we have only non-local
+			 * packfiles.
+			 */
+			if (!p->pack_local)
+				continue;
+
 			strbuf_addstr(&buf, pack_basename(p));
 			strbuf_strip_suffix(&buf, ".pack");
 			strbuf_addstr(&buf, ".idx");
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index d6db362541..7027676977 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -313,4 +313,27 @@ test_expect_success '--geometric --write-midx with packfiles in main and alterna
 	test_cmp expect actual
 '
 
+test_expect_success '--geometric --with-midx with no local objects' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# Create a second repository linked to the first one and perform a
+	# geometric repack on it.
+	git clone --shared shared member &&
+	git -C member repack --geometric 2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# Assert that we wrote neither a new packfile nor a multi-pack-index.
+	# We should not have a packfile because the single packfile in the
+	# alternate object database does not invalidate the geometric sequence.
+	# And we should not have a multi-pack-index because these only index
+	# local packfiles, and there are none.
+	test_dir_is_empty member/$packdir
+'
+
 test_done

t/t5300-pack-object.sh, t/t5331-pack-objects-stdin.sh

diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index f8a0f309e2..d2ce236d61 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -589,141 +589,6 @@ test_expect_success 'prefetch objects' '
 	test_line_count = 1 donelines
 '
 
-test_expect_success 'setup for --stdin-packs tests' '
-	git init stdin-packs &&
-	(
-		cd stdin-packs &&
-
-		test_commit A &&
-		test_commit B &&
-		test_commit C &&
-
-		for id in A B C
-		do
-			git pack-objects .git/objects/pack/pack-$id \
-				--incremental --revs <<-EOF || exit 1
-			refs/tags/$id
-			EOF
-		done &&
-
-		ls -la .git/objects/pack
-	)
-'
-
-test_expect_success '--stdin-packs with excluded packs' '
-	(
-		cd stdin-packs &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-
-		git pack-objects test --stdin-packs <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx)
-		) >expect.raw &&
-		git show-index <$(ls test-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
-test_expect_success '--stdin-packs is incompatible with --filter' '
-	(
-		cd stdin-packs &&
-		test_must_fail git pack-objects --stdin-packs --stdout \
-			--filter=blob:none </dev/null 2>err &&
-		test_i18ngrep "cannot use --filter with --stdin-packs" err
-	)
-'
-
-test_expect_success '--stdin-packs is incompatible with --revs' '
-	(
-		cd stdin-packs &&
-		test_must_fail git pack-objects --stdin-packs --revs out \
-			</dev/null 2>err &&
-		test_i18ngrep "cannot use internal rev list with --stdin-packs" err
-	)
-'
-
-test_expect_success '--stdin-packs with loose objects' '
-	(
-		cd stdin-packs &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-
-		test_commit D && # loose
-
-		git pack-objects test2 --stdin-packs --unpacked <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
-			git rev-list --objects --no-object-names \
-				refs/tags/C..refs/tags/D
-
-		) >expect.raw &&
-		ls -la . &&
-		git show-index <$(ls test2-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
-test_expect_success '--stdin-packs with broken links' '
-	(
-		cd stdin-packs &&
-
-		# make an unreachable object with a bogus parent
-		git cat-file -p HEAD >commit &&
-		sed "s/$(git rev-parse HEAD^)/$(test_oid zero)/" <commit |
-		git hash-object -w -t commit --stdin >in &&
-
-		git pack-objects .git/objects/pack/pack-D <in &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-		PACK_D="$(basename .git/objects/pack/pack-D-*.pack)" &&
-
-		git pack-objects test3 --stdin-packs --unpacked <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		$PACK_D
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-D-*.idx) &&
-			git rev-list --objects --no-object-names \
-				refs/tags/C..refs/tags/D
-		) >expect.raw &&
-		git show-index <$(ls test3-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
 test_expect_success 'negative window clamps to 0' '
 	git pack-objects --progress --window=-1 neg-window <obj-list 2>stderr &&
 	check_deltas stderr = 0
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
new file mode 100755
index 0000000000..d5eece5899
--- /dev/null
+++ b/t/t5331-pack-objects-stdin.sh
@@ -0,0 +1,145 @@
+#!/bin/sh
+
+test_description='pack-objects --stdin'
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup for --stdin-packs tests' '
+	git init stdin-packs &&
+	(
+		cd stdin-packs &&
+
+		test_commit A &&
+		test_commit B &&
+		test_commit C &&
+
+		for id in A B C
+		do
+			git pack-objects .git/objects/pack/pack-$id \
+				--incremental --revs <<-EOF || exit 1
+			refs/tags/$id
+			EOF
+		done &&
+
+		ls -la .git/objects/pack
+	)
+'
+
+test_expect_success '--stdin-packs with excluded packs' '
+	(
+		cd stdin-packs &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+
+		git pack-objects test --stdin-packs <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx)
+		) >expect.raw &&
+		git show-index <$(ls test-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--stdin-packs is incompatible with --filter' '
+	(
+		cd stdin-packs &&
+		test_must_fail git pack-objects --stdin-packs --stdout \
+			--filter=blob:none </dev/null 2>err &&
+		test_i18ngrep "cannot use --filter with --stdin-packs" err
+	)
+'
+
+test_expect_success '--stdin-packs is incompatible with --revs' '
+	(
+		cd stdin-packs &&
+		test_must_fail git pack-objects --stdin-packs --revs out \
+			</dev/null 2>err &&
+		test_i18ngrep "cannot use internal rev list with --stdin-packs" err
+	)
+'
+
+test_expect_success '--stdin-packs with loose objects' '
+	(
+		cd stdin-packs &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+
+		test_commit D && # loose
+
+		git pack-objects test2 --stdin-packs --unpacked <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
+			git rev-list --objects --no-object-names \
+				refs/tags/C..refs/tags/D
+
+		) >expect.raw &&
+		ls -la . &&
+		git show-index <$(ls test2-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--stdin-packs with broken links' '
+	(
+		cd stdin-packs &&
+
+		# make an unreachable object with a bogus parent
+		git cat-file -p HEAD >commit &&
+		sed "s/$(git rev-parse HEAD^)/$(test_oid zero)/" <commit |
+		git hash-object -w -t commit --stdin >in &&
+
+		git pack-objects .git/objects/pack/pack-D <in &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+		PACK_D="$(basename .git/objects/pack/pack-D-*.pack)" &&
+
+		git pack-objects test3 --stdin-packs --unpacked <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		$PACK_D
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-D-*.idx) &&
+			git rev-list --objects --no-object-names \
+				refs/tags/C..refs/tags/D
+		) >expect.raw &&
+		git show-index <$(ls test3-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh, t/t7703-repack-geometric.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7e9e20172a..7d0e864c35 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3348,7 +3348,9 @@ static void read_packs_list_from_stdin(void)
 	}
 
 	string_list_sort(&include_packs);
+	string_list_remove_duplicates(&include_packs, 0);
 	string_list_sort(&exclude_packs);
+	string_list_remove_duplicates(&exclude_packs, 0);
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index d5eece5899..def7640c91 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -7,6 +7,12 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
+packed_objects() {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list | sort &&
+	rm tmp-object-list
+ }
+
 test_expect_success 'setup for --stdin-packs tests' '
 	git init stdin-packs &&
 	(
@@ -142,4 +148,25 @@ test_expect_success '--stdin-packs with broken links' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with duplicate packfile' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		{
+			basename .git/objects/pack/pack-*.pack &&
+			basename .git/objects/pack/pack-*.pack
+		} >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects .git/objects/pack/pack-*.idx >expect &&
+		packed_objects generated-pack-*.idx >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 7027676977..d0823f2eb2 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -336,4 +336,29 @@ test_expect_success '--geometric --with-midx with no local objects' '
 	test_dir_is_empty member/$packdir
 '
 
+test_expect_success '--geometric with same pack in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# We create the member repository as an exact copy so that it has the
+	# same packfile.
+	cp -r shared member &&
+	test-tool path-utils real_path shared/.git/objects >member/.git/objects/info/alternates &&
+	find shared/.git/objects -type f >expected-files &&
+
+	# Verify that we can repack objects as expected without observing any
+	# error. Having the same packfile in both ODBs used to cause an error
+	# in git-pack-objects(1).
+	git -C member repack --geometric 2 2>err &&
+	test_must_be_empty err &&
+	# Nothing should have changed.
+	find shared/.git/objects -type f >actual-files &&
+	test_cmp expected-files actual-files
+'
+
 test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7d0e864c35..c5147d392f 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3355,11 +3355,9 @@ static void read_packs_list_from_stdin(void)
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
 
-		item = string_list_lookup(&include_packs, pack_name);
-		if (!item)
-			item = string_list_lookup(&exclude_packs, pack_name);
-
-		if (item)
+		if ((item = string_list_lookup(&include_packs, pack_name)))
+			item->util = p;
+		if ((item = string_list_lookup(&exclude_packs, pack_name)))
 			item->util = p;
 	}
 
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index def7640c91..1e23cc2e61 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -169,4 +169,24 @@ test_expect_success 'pack-objects --stdin with duplicate packfile' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with same packfile excluded and included' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		{
+			basename .git/objects/pack/pack-*.pack &&
+			printf "^%s\n" "$(basename .git/objects/pack/pack-*.pack)"
+		} >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects generated-pack-*.idx >packed-objects &&
+		test_must_be_empty packed-objects
+	)
+'
+
 test_done

diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index 1e23cc2e61..45e24fa94a 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -189,4 +189,52 @@ test_expect_success 'pack-objects --stdin with same packfile excluded and includ
 	)
 '
 
+test_expect_success 'pack-objects --stdin with packfiles from alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+	basename shared/.git/objects/pack/pack-*.pack >packfile &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has no objects on its own, but we still expect to be able
+	# to pack objects from its alternate.
+	git clone --shared shared member &&
+	git -C member pack-objects --stdin-packs generated-pack <packfile &&
+	test_cmp shared/.git/objects/pack/pack-*.pack member/generated-pack-*.pack
+'
+
+test_expect_success 'pack-objects --stdin with packfiles from main and alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-commit" &&
+	git -C shared repack -ad &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has a second packfile so that we can verify that it is
+	# possible to write packs that include packfiles from different object
+	# databases.
+	git clone --shared shared member &&
+	test_commit -C member "local-commit" &&
+	git -C member repack -dl &&
+
+	{
+		basename shared/.git/objects/pack/pack-*.pack &&
+		basename member/.git/objects/pack/pack-*.pack
+	} >packfiles &&
+
+	{
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	} | sort >expected-objects &&
+
+	git -C member pack-objects --stdin-packs generated-pack <packfiles &&
+	packed_objects member/generated-pack-*.idx >actual-objects &&
+	test_cmp expected-objects actual-objects
+'
+
 test_done

diff --git a/t/helper/test-chmtime.c b/t/helper/test-chmtime.c
index dc28890a18..0e5538833a 100644
--- a/t/helper/test-chmtime.c
+++ b/t/helper/test-chmtime.c
@@ -94,7 +94,7 @@ int cmd__chmtime(int argc, const char **argv)
 	if (timespec_arg(argv[i], &set_time, &set_eq)) {
 		++i;
 	} else {
-		if (get == 0) {
+		if (get == 0 && verbose == 0) {
 			fprintf(stderr, "Not a base-10 integer: %s\n", argv[i] + 1);
 			goto usage;
 		}

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 80fc860613..5768aeb1f3 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -323,7 +323,8 @@ static int geometry_cmp(const void *va, const void *vb)
 }
 
 static void init_pack_geometry(struct pack_geometry **geometry_p,
-			       struct string_list *existing_kept_packs)
+			       struct string_list *existing_kept_packs,
+			       const struct pack_objects_args *args)
 {
 	struct packed_git *p;
 	struct pack_geometry *geometry;
@@ -333,6 +334,14 @@ static void init_pack_geometry(struct pack_geometry **geometry_p,
 	geometry = *geometry_p;
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
+		if (args->local && !p->pack_local)
+			/*
+			 * When asked to only repack local packfiles we skip
+			 * over any packfiles that are borrowed from alternate
+			 * object directories.
+			 */
+			continue;
+
 		if (!pack_kept_objects) {
 			/*
 			 * Any pack that has its pack_keep bit set will appear
@@ -900,7 +909,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (geometric_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
-		init_pack_geometry(&geometry, &existing_kept_packs);
+		init_pack_geometry(&geometry, &existing_kept_packs, &po_args);
 		split_pack_geometry(geometry, geometric_factor);
 	}
 
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index d0823f2eb2..c440956ad5 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -10,6 +10,12 @@ objdir=.git/objects
 packdir=$objdir/pack
 midx=$objdir/pack/multi-pack-index
 
+packed_objects() {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list &&
+	rm tmp-object-list
+ }
+
 test_expect_success '--geometric with no packs' '
 	git init geometric &&
 	test_when_finished "rm -fr geometric" &&
@@ -361,4 +367,56 @@ test_expect_success '--geometric with same pack in main and alternate ODB' '
 	test_cmp expected-files actual-files
 '
 
+test_expect_success '--geometric -l with non-intact geometric sequence across ODBs' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# Verify that our assumptions actually hold: both generated packfiles
+	# should have three objects and should be non-equal.
+	packed_objects shared/.git/objects/pack/pack-*.idx >packed-objects &&
+	test_line_count = 3 packed-objects &&
+	packed_objects member/.git/objects/pack/pack-*.idx >packed-objetcs &&
+	test_line_count = 3 packed-objects &&
+	test "$(basename member/.git/objects/pack/pack-*.pack)" != "$(basename shared/.git/objects/pack/pack-*.pack)" &&
+
+	# Perform the geometric repack. With `-l`, we should only see the local
+	# packfile and thus arrive at the conclusion that the geometric
+	# sequence is intact. We thus expect no changes.
+	#
+	# Note that we are tweaking mtimes of the packfiles so that we can
+	# verify they did not change. This is done in order to detect the case
+	# where we do repack objects, but the resulting packfile is the same.
+	test-tool chmtime --verbose =0 member/.git/objects/pack/* >expected-member-packs &&
+	git -C member repack --geometric=2 -l -d &&
+	test-tool chmtime --verbose member/.git/objects/pack/* >actual-member-packs &&
+	test_cmp expected-member-packs actual-member-packs &&
+
+	{
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	} | sort >expected-objects &&
+
+	# On the other hand, when doing a non-local geometric repack we should
+	# see both packfiles and thus repack them. We expect that the shared
+	# object database was not changed.
+	test-tool chmtime --verbose =0 shared/.git/objects/pack/* >expected-shared-packs &&
+	git -C member repack --geometric=2 -d &&
+	test-tool chmtime --verbose shared/.git/objects/pack/* >actual-shared-packs &&
+	test_cmp expected-shared-packs actual-shared-packs &&
+
+	# Furthermore, we expect that the member repository now has a single
+	# packfile that contains the combined shared and non-shared objects.
+	ls member/.git/objects/pack/pack-*.idx >actual &&
+	test_line_count = 1 actual &&
+	packed_objects member/.git/objects/pack/pack-*.idx >actual-objects &&
+	test_line_count = 6 actual-objects &&
+	sort <actual-objects >actual-objects.sorted &&
+	test_cmp expected-objects actual-objects.sorted
+'
+
 test_done

diff --git c/t/t5331-pack-objects-stdin.sh w/t/t5331-pack-objects-stdin.sh
index 45e24fa94a..acab31667a 100755
--- c/t/t5331-pack-objects-stdin.sh
+++ w/t/t5331-pack-objects-stdin.sh
@@ -7,7 +7,7 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
-packed_objects() {
+packed_objects () {
 	git show-index <"$1" >tmp-object-list &&
 	cut -d' ' -f2 tmp-object-list | sort &&
 	rm tmp-object-list
diff --git c/t/t7703-repack-geometric.sh w/t/t7703-repack-geometric.sh
index 33d7977fca..57796f3437 100755
--- c/t/t7703-repack-geometric.sh
+++ w/t/t7703-repack-geometric.sh
@@ -10,7 +10,7 @@ objdir=.git/objects
 packdir=$objdir/pack
 midx=$objdir/pack/multi-pack-index
 
-packed_objects() {
+packed_objects () {
 	git show-index <"$1" >tmp-object-list &&
 	cut -d' ' -f2 tmp-object-list &&
 	rm tmp-object-list
@@ -380,7 +380,7 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	# should have three objects and should be non-equal.
 	packed_objects shared/.git/objects/pack/pack-*.idx >packed-objects &&
 	test_line_count = 3 packed-objects &&
-	packed_objects member/.git/objects/pack/pack-*.idx >packed-objetcs &&
+	packed_objects member/.git/objects/pack/pack-*.idx >packed-objects &&
 	test_line_count = 3 packed-objects &&
 	test "$(basename member/.git/objects/pack/pack-*.pack)" != "$(basename shared/.git/objects/pack/pack-*.pack)" &&
 

builtin/repack.c, object-file.c, object-store.h, t/t7700-repack.sh, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 5768aeb1f3..e2abcea2ee 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -887,6 +887,18 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));
 
+	if (write_bitmaps && po_args.local && has_alt_odb(the_repository)) {
+		/*
+		 * When asked to do a local repack, but we have
+		 * packfiles that are inherited from an alternate, then
+		 * we cannot guarantee that the multi-pack-index would
+		 * have full coverage of all objects. We thus disable
+		 * writing bitmaps in that case.
+		 */
+		warning(_("disabling bitmap writing, as some objects are not being packed"));
+		write_bitmaps = 0;
+	}
+
 	if (write_midx && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
diff --git a/object-file.c b/object-file.c
index 8fab8dbe80..3ad11c683b 100644
--- a/object-file.c
+++ b/object-file.c
@@ -946,6 +946,12 @@ void prepare_alt_odb(struct repository *r)
 	r->objects->loaded_alternates = 1;
 }
 
+int has_alt_odb(struct repository *r)
+{
+	prepare_alt_odb(r);
+	return !!r->objects->odb->next;
+}
+
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */
 static int freshen_file(const char *fn)
 {
diff --git a/object-store.h b/object-store.h
index 82201ec3e7..574f16140f 100644
--- a/object-store.h
+++ b/object-store.h
@@ -56,6 +56,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct object_directory *, 1, fspathhash, fspatheq)
 
 void prepare_alt_odb(struct repository *r);
+int has_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index 4aabe98139..93c5763e7d 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -106,6 +106,23 @@ test_expect_success SYMLINKS '--local keeps packs when alternate is objectdir '
 	test_cmp expect actual
 '
 
+test_expect_success '--local disables writing bitmaps when connected to alternate ODB' '
+	test_when_finished "rm -rf shared member" &&
+
+	git init shared &&
+	git clone --shared shared member &&
+	(
+		cd member &&
+		test_commit "object" &&
+		git repack -Adl --write-bitmap-index 2>err &&
+		cat >expect <<-EOF &&
+		warning: disabling bitmap writing, as some objects are not being packed
+		EOF
+		test_cmp expect err &&
+		test ! -f .git/objects/pack-*.bitmap
+	)
+'
+
 test_expect_success 'packed obs in alt ODB are repacked even when local repo is packless' '
 	mkdir alt_objects/pack &&
 	mv .git/objects/pack/* alt_objects/pack &&
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index c440956ad5..33d7977fca 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -419,4 +419,31 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	test_cmp expected-objects actual-objects.sorted
 '
 
+test_expect_success '--geometric -l disables writing bitmaps with non-local packfiles' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# When performing a geometric repack with `-l` while connecting to an
+	# alternate object database that has a packfile we do not have full
+	# coverage of objects. As a result, we expect that writing the bitmap
+	# will be disabled.
+	git -C member repack -l --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	cat >expect <<-EOF &&
+	warning: disabling bitmap writing, as some objects are not being packed
+	EOF
+	test_cmp expect err &&
+	test ! -f member/.git/objects/pack/multi-pack-index-*.bitmap &&
+
+	# On the other hand, when we repack without `-l`, we should see that
+	# the bitmap gets created.
+	git -C member repack --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	test_must_be_empty err &&
+	test -f member/.git/objects/pack/multi-pack-index-*.bitmap
+'
+
 test_done

t/t5331-pack-objects-stdin.sh, t/t7700-repack.sh, t/t7703-repack-geometric.sh

diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index 45e24fa94a..acab31667a 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -7,7 +7,7 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
-packed_objects() {
+packed_objects () {
 	git show-index <"$1" >tmp-object-list &&
 	cut -d' ' -f2 tmp-object-list | sort &&
 	rm tmp-object-list
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index 93c5763e7d..faa739eeb9 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -114,12 +114,12 @@ test_expect_success '--local disables writing bitmaps when connected to alternat
 	(
 		cd member &&
 		test_commit "object" &&
-		git repack -Adl --write-bitmap-index 2>err &&
+		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adl --write-bitmap-index 2>err &&
 		cat >expect <<-EOF &&
 		warning: disabling bitmap writing, as some objects are not being packed
 		EOF
 		test_cmp expect err &&
-		test ! -f .git/objects/pack-*.bitmap
+		test_path_is_missing .git/objects/pack-*.bitmap
 	)
 '
 
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 33d7977fca..00f28fb558 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -10,9 +10,9 @@ objdir=.git/objects
 packdir=$objdir/pack
 midx=$objdir/pack/multi-pack-index
 
-packed_objects() {
+packed_objects () {
 	git show-index <"$1" >tmp-object-list &&
-	cut -d' ' -f2 tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list | sort &&
 	rm tmp-object-list
  }
 
@@ -312,7 +312,7 @@ test_expect_success '--geometric --write-midx with packfiles in main and alterna
 
 	# We should also see a multi-pack-index. This multi-pack-index should
 	# never refer to any packfiles in the alternate object database.
-	test -f member/.git/objects/pack/multi-pack-index &&
+	test_path_is_file member/.git/objects/pack/multi-pack-index &&
 	test-tool read-midx member/.git/objects >packs.midx &&
 	grep "^pack-.*\.idx$" packs.midx | sort >actual &&
 	basename member/.git/objects/pack/pack-*.idx >expect &&
@@ -378,11 +378,11 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 
 	# Verify that our assumptions actually hold: both generated packfiles
 	# should have three objects and should be non-equal.
-	packed_objects shared/.git/objects/pack/pack-*.idx >packed-objects &&
-	test_line_count = 3 packed-objects &&
-	packed_objects member/.git/objects/pack/pack-*.idx >packed-objetcs &&
-	test_line_count = 3 packed-objects &&
-	test "$(basename member/.git/objects/pack/pack-*.pack)" != "$(basename shared/.git/objects/pack/pack-*.pack)" &&
+	packed_objects shared/.git/objects/pack/pack-*.idx >shared-objects &&
+	packed_objects member/.git/objects/pack/pack-*.idx >member-objects &&
+	test_line_count = 3 shared-objects &&
+	test_line_count = 3 member-objects &&
+	! test_cmp shared-objects member-objects &&
 
 	# Perform the geometric repack. With `-l`, we should only see the local
 	# packfile and thus arrive at the conclusion that the geometric
@@ -415,8 +415,7 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	test_line_count = 1 actual &&
 	packed_objects member/.git/objects/pack/pack-*.idx >actual-objects &&
 	test_line_count = 6 actual-objects &&
-	sort <actual-objects >actual-objects.sorted &&
-	test_cmp expected-objects actual-objects.sorted
+	test_cmp expected-objects actual-objects
 '
 
 test_expect_success '--geometric -l disables writing bitmaps with non-local packfiles' '
@@ -428,7 +427,7 @@ test_expect_success '--geometric -l disables writing bitmaps with non-local pack
 	git clone --shared shared member &&
 	test_commit_bulk -C member --start=2 1 &&
 
-	# When performing a geometric repack with `-l` while connecting to an
+	# When performing a geometric repack with `-l` while connected to an
 	# alternate object database that has a packfile we do not have full
 	# coverage of objects. As a result, we expect that writing the bitmap
 	# will be disabled.
@@ -437,13 +436,13 @@ test_expect_success '--geometric -l disables writing bitmaps with non-local pack
 	warning: disabling bitmap writing, as some objects are not being packed
 	EOF
 	test_cmp expect err &&
-	test ! -f member/.git/objects/pack/multi-pack-index-*.bitmap &&
+	test_path_is_missing member/.git/objects/pack/multi-pack-index-*.bitmap &&
 
 	# On the other hand, when we repack without `-l`, we should see that
 	# the bitmap gets created.
 	git -C member repack --geometric=2 --write-midx --write-bitmap-index 2>err &&
 	test_must_be_empty err &&
-	test -f member/.git/objects/pack/multi-pack-index-*.bitmap
+	test_path_is_file member/.git/objects/pack/multi-pack-index-*.bitmap
 '
 
 test_done

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

diff --git a/midx.c b/midx.c
index 47989f7ea7..67eb617591 100644
--- a/midx.c
+++ b/midx.c
@@ -1328,17 +1328,17 @@ static int write_midx_internal(const char *object_dir,
 	}
 
 	if (preferred_pack_name) {
-		int found = 0;
+		ctx.preferred_pack_idx = -1;
+
 		for (i = 0; i < ctx.nr; i++) {
 			if (!cmp_idx_or_pack_name(preferred_pack_name,
 						  ctx.info[i].pack_name)) {
 				ctx.preferred_pack_idx = i;
-				found = 1;
 				break;
 			}
 		}
 
-		if (!found)
+		if (ctx.preferred_pack_idx == -1)
 			warning(_("unknown preferred pack: '%s'"),
 				preferred_pack_name);
 	} else if (ctx.nr &&
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 499d5d4c78..0883c7c6bd 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -183,6 +183,18 @@ test_expect_success 'write midx with --stdin-packs' '
 
 compare_results_with_midx "mixed mode (one pack + extra)"
 
+test_expect_success 'write with no objects and preferred pack' '
+	test_when_finished "rm -rf empty" &&
+	git init empty &&
+	test_must_fail git -C empty multi-pack-index write \
+		--stdin-packs --preferred-pack=does-not-exist </dev/null 2>err &&
+	cat >expect <<-EOF &&
+	warning: unknown preferred pack: ${SQ}does-not-exist${SQ}
+	error: no pack files to index.
+	EOF
+	test_cmp expect err
+'
+
 test_expect_success 'write progress off for redirected stderr' '
 	git multi-pack-index --object-dir=$objdir write 2>err &&
 	test_line_count = 0 err

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 687419776d..80fc860613 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -572,6 +572,17 @@ static void midx_included_packs(struct string_list *include,
 		for (i = geometry->split; i < geometry->pack_nr; i++) {
 			struct packed_git *p = geometry->pack[i];
 
+			/*
+			 * The multi-pack index never refers to packfiles part
+			 * of an alternate object database, so we skip these.
+			 * While git-multi-pack-index(1) would silently ignore
+			 * them anyway, this allows us to skip executing the
+			 * command completely when we have only non-local
+			 * packfiles.
+			 */
+			if (!p->pack_local)
+				continue;
+
 			strbuf_addstr(&buf, pack_basename(p));
 			strbuf_strip_suffix(&buf, ".pack");
 			strbuf_addstr(&buf, ".idx");
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 4abc7d4c55..9dd002437f 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -313,4 +313,27 @@ test_expect_success '--geometric --write-midx with packfiles in main and alterna
 	test_cmp expect actual
 '
 
+test_expect_success '--geometric --with-midx with no local objects' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# Create a second repository linked to the first one and perform a
+	# geometric repack on it.
+	git clone --shared shared member &&
+	git -C member repack --geometric 2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# Assert that we wrote neither a new packfile nor a multi-pack-index.
+	# We should not have a packfile because the single packfile in the
+	# alternate object database does not invalidate the geometric sequence.
+	# And we should not have a multi-pack-index because these only index
+	# local packfiles, and there are none.
+	test_dir_is_empty member/$packdir
+'
+
 test_done

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 87f73c8923..687419776d 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -446,8 +446,10 @@ static void split_pack_geometry(struct pack_geometry *geometry, int factor)
 	geometry->split = split;
 }
 
-static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry)
+static struct packed_git *get_preferred_pack(struct pack_geometry *geometry)
 {
+	uint32_t i;
+
 	if (!geometry) {
 		/*
 		 * No geometry means either an all-into-one repack (in which
@@ -462,7 +464,21 @@ static struct packed_git *get_largest_active_pack(struct pack_geometry *geometry
 	}
 	if (geometry->split == geometry->pack_nr)
 		return NULL;
-	return geometry->pack[geometry->pack_nr - 1];
+
+	/*
+	 * The preferred pack is the largest pack above the split line. In
+	 * other words, it is the largest pack that does not get rolled up in
+	 * the geometric repack.
+	 */
+	for (i = geometry->pack_nr; i > geometry->split; i--)
+		/*
+		 * A pack that is not local would never be included in a
+		 * multi-pack index. We thus skip over any non-local packs.
+		 */
+		if (geometry->pack[i - 1]->pack_local)
+			return geometry->pack[i - 1];
+
+	return NULL;
 }
 
 static void clear_pack_geometry(struct pack_geometry *geometry)
@@ -589,7 +605,7 @@ static int write_midx_included_packs(struct string_list *include,
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct string_list_item *item;
-	struct packed_git *largest = get_largest_active_pack(geometry);
+	struct packed_git *preferred = get_preferred_pack(geometry);
 	FILE *in;
 	int ret;
 
@@ -610,9 +626,9 @@ static int write_midx_included_packs(struct string_list *include,
 	if (write_bitmaps)
 		strvec_push(&cmd.args, "--bitmap");
 
-	if (largest)
+	if (preferred)
 		strvec_pushf(&cmd.args, "--preferred-pack=%s",
-			     pack_basename(largest));
+			     pack_basename(preferred));
 
 	if (refs_snapshot)
 		strvec_pushf(&cmd.args, "--refs-snapshot=%s", refs_snapshot);
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 8821fbd2dd..4abc7d4c55 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -281,4 +281,36 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 	)
 '
 
+test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a shared repository that will serve as the alternate object
+	# database for the member linked to it. It has got some objects on its
+	# own that are packed into a single packfile.
+	git init shared &&
+	test_commit -C shared common-object &&
+	git -C shared repack -ad &&
+
+	# We create member so that its alternates file points to the shared
+	# repository. We then create a commit in it so that git-repack(1) has
+	# something to repack.
+	# of the shared object database.
+	git clone --shared shared member &&
+	test_commit -C member unique-object &&
+	git -C member repack --geometric=2 --write-midx 2>err &&
+	test_must_be_empty err &&
+
+	# We should see that a new packfile was generated.
+	find shared/.git/objects/pack -type f -name "*.pack" >packs &&
+	test_line_count = 1 packs &&
+
+	# We should also see a multi-pack-index. This multi-pack-index should
+	# never refer to any packfiles in the alternate object database.
+	test_path_is_file member/.git/objects/pack/multi-pack-index &&
+	test-tool read-midx member/.git/objects >packs.midx &&
+	grep "^pack-.*\.idx$" packs.midx | sort >actual &&
+	basename member/.git/objects/pack/pack-*.idx >expect &&
+	test_cmp expect actual
+'
+
 test_done

t/t5300-pack-object.sh, t/t5331-pack-objects-stdin.sh

diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index f8a0f309e2..d2ce236d61 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -589,141 +589,6 @@ test_expect_success 'prefetch objects' '
 	test_line_count = 1 donelines
 '
 
-test_expect_success 'setup for --stdin-packs tests' '
-	git init stdin-packs &&
-	(
-		cd stdin-packs &&
-
-		test_commit A &&
-		test_commit B &&
-		test_commit C &&
-
-		for id in A B C
-		do
-			git pack-objects .git/objects/pack/pack-$id \
-				--incremental --revs <<-EOF || exit 1
-			refs/tags/$id
-			EOF
-		done &&
-
-		ls -la .git/objects/pack
-	)
-'
-
-test_expect_success '--stdin-packs with excluded packs' '
-	(
-		cd stdin-packs &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-
-		git pack-objects test --stdin-packs <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx)
-		) >expect.raw &&
-		git show-index <$(ls test-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
-test_expect_success '--stdin-packs is incompatible with --filter' '
-	(
-		cd stdin-packs &&
-		test_must_fail git pack-objects --stdin-packs --stdout \
-			--filter=blob:none </dev/null 2>err &&
-		test_i18ngrep "cannot use --filter with --stdin-packs" err
-	)
-'
-
-test_expect_success '--stdin-packs is incompatible with --revs' '
-	(
-		cd stdin-packs &&
-		test_must_fail git pack-objects --stdin-packs --revs out \
-			</dev/null 2>err &&
-		test_i18ngrep "cannot use internal rev list with --stdin-packs" err
-	)
-'
-
-test_expect_success '--stdin-packs with loose objects' '
-	(
-		cd stdin-packs &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-
-		test_commit D && # loose
-
-		git pack-objects test2 --stdin-packs --unpacked <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
-			git rev-list --objects --no-object-names \
-				refs/tags/C..refs/tags/D
-
-		) >expect.raw &&
-		ls -la . &&
-		git show-index <$(ls test2-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
-test_expect_success '--stdin-packs with broken links' '
-	(
-		cd stdin-packs &&
-
-		# make an unreachable object with a bogus parent
-		git cat-file -p HEAD >commit &&
-		sed "s/$(git rev-parse HEAD^)/$(test_oid zero)/" <commit |
-		git hash-object -w -t commit --stdin >in &&
-
-		git pack-objects .git/objects/pack/pack-D <in &&
-
-		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
-		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
-		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
-		PACK_D="$(basename .git/objects/pack/pack-D-*.pack)" &&
-
-		git pack-objects test3 --stdin-packs --unpacked <<-EOF &&
-		$PACK_A
-		^$PACK_B
-		$PACK_C
-		$PACK_D
-		EOF
-
-		(
-			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
-			git show-index <$(ls .git/objects/pack/pack-D-*.idx) &&
-			git rev-list --objects --no-object-names \
-				refs/tags/C..refs/tags/D
-		) >expect.raw &&
-		git show-index <$(ls test3-*.idx) >actual.raw &&
-
-		cut -d" " -f2 <expect.raw | sort >expect &&
-		cut -d" " -f2 <actual.raw | sort >actual &&
-		test_cmp expect actual
-	)
-'
-
 test_expect_success 'negative window clamps to 0' '
 	git pack-objects --progress --window=-1 neg-window <obj-list 2>stderr &&
 	check_deltas stderr = 0
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
new file mode 100755
index 0000000000..d5eece5899
--- /dev/null
+++ b/t/t5331-pack-objects-stdin.sh
@@ -0,0 +1,145 @@
+#!/bin/sh
+
+test_description='pack-objects --stdin'
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup for --stdin-packs tests' '
+	git init stdin-packs &&
+	(
+		cd stdin-packs &&
+
+		test_commit A &&
+		test_commit B &&
+		test_commit C &&
+
+		for id in A B C
+		do
+			git pack-objects .git/objects/pack/pack-$id \
+				--incremental --revs <<-EOF || exit 1
+			refs/tags/$id
+			EOF
+		done &&
+
+		ls -la .git/objects/pack
+	)
+'
+
+test_expect_success '--stdin-packs with excluded packs' '
+	(
+		cd stdin-packs &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+
+		git pack-objects test --stdin-packs <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx)
+		) >expect.raw &&
+		git show-index <$(ls test-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--stdin-packs is incompatible with --filter' '
+	(
+		cd stdin-packs &&
+		test_must_fail git pack-objects --stdin-packs --stdout \
+			--filter=blob:none </dev/null 2>err &&
+		test_i18ngrep "cannot use --filter with --stdin-packs" err
+	)
+'
+
+test_expect_success '--stdin-packs is incompatible with --revs' '
+	(
+		cd stdin-packs &&
+		test_must_fail git pack-objects --stdin-packs --revs out \
+			</dev/null 2>err &&
+		test_i18ngrep "cannot use internal rev list with --stdin-packs" err
+	)
+'
+
+test_expect_success '--stdin-packs with loose objects' '
+	(
+		cd stdin-packs &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+
+		test_commit D && # loose
+
+		git pack-objects test2 --stdin-packs --unpacked <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
+			git rev-list --objects --no-object-names \
+				refs/tags/C..refs/tags/D
+
+		) >expect.raw &&
+		ls -la . &&
+		git show-index <$(ls test2-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--stdin-packs with broken links' '
+	(
+		cd stdin-packs &&
+
+		# make an unreachable object with a bogus parent
+		git cat-file -p HEAD >commit &&
+		sed "s/$(git rev-parse HEAD^)/$(test_oid zero)/" <commit |
+		git hash-object -w -t commit --stdin >in &&
+
+		git pack-objects .git/objects/pack/pack-D <in &&
+
+		PACK_A="$(basename .git/objects/pack/pack-A-*.pack)" &&
+		PACK_B="$(basename .git/objects/pack/pack-B-*.pack)" &&
+		PACK_C="$(basename .git/objects/pack/pack-C-*.pack)" &&
+		PACK_D="$(basename .git/objects/pack/pack-D-*.pack)" &&
+
+		git pack-objects test3 --stdin-packs --unpacked <<-EOF &&
+		$PACK_A
+		^$PACK_B
+		$PACK_C
+		$PACK_D
+		EOF
+
+		(
+			git show-index <$(ls .git/objects/pack/pack-A-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-C-*.idx) &&
+			git show-index <$(ls .git/objects/pack/pack-D-*.idx) &&
+			git rev-list --objects --no-object-names \
+				refs/tags/C..refs/tags/D
+		) >expect.raw &&
+		git show-index <$(ls test3-*.idx) >actual.raw &&
+
+		cut -d" " -f2 <expect.raw | sort >expect &&
+		cut -d" " -f2 <actual.raw | sort >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh, t/t7703-repack-geometric.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7e9e20172a..7d0e864c35 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3348,7 +3348,9 @@ static void read_packs_list_from_stdin(void)
 	}
 
 	string_list_sort(&include_packs);
+	string_list_remove_duplicates(&include_packs, 0);
 	string_list_sort(&exclude_packs);
+	string_list_remove_duplicates(&exclude_packs, 0);
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index d5eece5899..71c8a4a635 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -7,6 +7,12 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
+packed_objects () {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list | sort &&
+	rm tmp-object-list
+ }
+
 test_expect_success 'setup for --stdin-packs tests' '
 	git init stdin-packs &&
 	(
@@ -142,4 +148,25 @@ test_expect_success '--stdin-packs with broken links' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with duplicate packfile' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		{
+			basename .git/objects/pack/pack-*.pack &&
+			basename .git/objects/pack/pack-*.pack
+		} >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects .git/objects/pack/pack-*.idx >expect &&
+		packed_objects generated-pack-*.idx >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index 9dd002437f..a358dfb7bd 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -336,4 +336,29 @@ test_expect_success '--geometric --with-midx with no local objects' '
 	test_dir_is_empty member/$packdir
 '
 
+test_expect_success '--geometric with same pack in main and alternate ODB' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Create a repository with a single packfile that acts as alternate
+	# object database.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+
+	# We create the member repository as an exact copy so that it has the
+	# same packfile.
+	cp -r shared member &&
+	test-tool path-utils real_path shared/.git/objects >member/.git/objects/info/alternates &&
+	find shared/.git/objects -type f >expected-files &&
+
+	# Verify that we can repack objects as expected without observing any
+	# error. Having the same packfile in both ODBs used to cause an error
+	# in git-pack-objects(1).
+	git -C member repack --geometric 2 2>err &&
+	test_must_be_empty err &&
+	# Nothing should have changed.
+	find shared/.git/objects -type f >actual-files &&
+	test_cmp expected-files actual-files
+'
+
 test_done

builtin/pack-objects.c, t/t5331-pack-objects-stdin.sh

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 7d0e864c35..c5147d392f 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -3355,11 +3355,9 @@ static void read_packs_list_from_stdin(void)
 	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *pack_name = pack_basename(p);
 
-		item = string_list_lookup(&include_packs, pack_name);
-		if (!item)
-			item = string_list_lookup(&exclude_packs, pack_name);
-
-		if (item)
+		if ((item = string_list_lookup(&include_packs, pack_name)))
+			item->util = p;
+		if ((item = string_list_lookup(&exclude_packs, pack_name)))
 			item->util = p;
 	}
 
diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index 71c8a4a635..3ef736ec05 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -169,4 +169,24 @@ test_expect_success 'pack-objects --stdin with duplicate packfile' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with same packfile excluded and included' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		{
+			basename .git/objects/pack/pack-*.pack &&
+			printf "^%s\n" "$(basename .git/objects/pack/pack-*.pack)"
+		} >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects generated-pack-*.idx >packed-objects &&
+		test_must_be_empty packed-objects
+	)
+'
+
 test_done

diff --git a/t/t5331-pack-objects-stdin.sh b/t/t5331-pack-objects-stdin.sh
index 3ef736ec05..acab31667a 100755
--- a/t/t5331-pack-objects-stdin.sh
+++ b/t/t5331-pack-objects-stdin.sh
@@ -189,4 +189,52 @@ test_expect_success 'pack-objects --stdin with same packfile excluded and includ
 	)
 '
 
+test_expect_success 'pack-objects --stdin with packfiles from alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-objects" &&
+	git -C shared repack -ad &&
+	basename shared/.git/objects/pack/pack-*.pack >packfile &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has no objects on its own, but we still expect to be able
+	# to pack objects from its alternate.
+	git clone --shared shared member &&
+	git -C member pack-objects --stdin-packs generated-pack <packfile &&
+	test_cmp shared/.git/objects/pack/pack-*.pack member/generated-pack-*.pack
+'
+
+test_expect_success 'pack-objects --stdin with packfiles from main and alternate object database' '
+	test_when_finished "rm -fr shared member" &&
+
+	# Set up a shared repository with a single packfile.
+	git init shared &&
+	test_commit -C shared "shared-commit" &&
+	git -C shared repack -ad &&
+
+	# Set up a repository that is connected to the shared repository. This
+	# repository has a second packfile so that we can verify that it is
+	# possible to write packs that include packfiles from different object
+	# databases.
+	git clone --shared shared member &&
+	test_commit -C member "local-commit" &&
+	git -C member repack -dl &&
+
+	{
+		basename shared/.git/objects/pack/pack-*.pack &&
+		basename member/.git/objects/pack/pack-*.pack
+	} >packfiles &&
+
+	{
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	} | sort >expected-objects &&
+
+	git -C member pack-objects --stdin-packs generated-pack <packfiles &&
+	packed_objects member/generated-pack-*.idx >actual-objects &&
+	test_cmp expected-objects actual-objects
+'
+
 test_done

diff --git a/t/helper/test-chmtime.c b/t/helper/test-chmtime.c
index dc28890a18..0e5538833a 100644
--- a/t/helper/test-chmtime.c
+++ b/t/helper/test-chmtime.c
@@ -94,7 +94,7 @@ int cmd__chmtime(int argc, const char **argv)
 	if (timespec_arg(argv[i], &set_time, &set_eq)) {
 		++i;
 	} else {
-		if (get == 0) {
+		if (get == 0 && verbose == 0) {
 			fprintf(stderr, "Not a base-10 integer: %s\n", argv[i] + 1);
 			goto usage;
 		}

builtin/repack.c, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 80fc860613..5768aeb1f3 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -323,7 +323,8 @@ static int geometry_cmp(const void *va, const void *vb)
 }
 
 static void init_pack_geometry(struct pack_geometry **geometry_p,
-			       struct string_list *existing_kept_packs)
+			       struct string_list *existing_kept_packs,
+			       const struct pack_objects_args *args)
 {
 	struct packed_git *p;
 	struct pack_geometry *geometry;
@@ -333,6 +334,14 @@ static void init_pack_geometry(struct pack_geometry **geometry_p,
 	geometry = *geometry_p;
 
 	for (p = get_all_packs(the_repository); p; p = p->next) {
+		if (args->local && !p->pack_local)
+			/*
+			 * When asked to only repack local packfiles we skip
+			 * over any packfiles that are borrowed from alternate
+			 * object directories.
+			 */
+			continue;
+
 		if (!pack_kept_objects) {
 			/*
 			 * Any pack that has its pack_keep bit set will appear
@@ -900,7 +909,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (geometric_factor) {
 		if (pack_everything)
 			die(_("options '%s' and '%s' cannot be used together"), "--geometric", "-A/-a");
-		init_pack_geometry(&geometry, &existing_kept_packs);
+		init_pack_geometry(&geometry, &existing_kept_packs, &po_args);
 		split_pack_geometry(geometry, geometric_factor);
 	}
 
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index a358dfb7bd..be8fe78448 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -10,6 +10,12 @@ objdir=.git/objects
 packdir=$objdir/pack
 midx=$objdir/pack/multi-pack-index
 
+packed_objects () {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list | sort &&
+	rm tmp-object-list
+ }
+
 test_expect_success '--geometric with no packs' '
 	git init geometric &&
 	test_when_finished "rm -fr geometric" &&
@@ -361,4 +367,55 @@ test_expect_success '--geometric with same pack in main and alternate ODB' '
 	test_cmp expected-files actual-files
 '
 
+test_expect_success '--geometric -l with non-intact geometric sequence across ODBs' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# Verify that our assumptions actually hold: both generated packfiles
+	# should have three objects and should be non-equal.
+	packed_objects shared/.git/objects/pack/pack-*.idx >shared-objects &&
+	packed_objects member/.git/objects/pack/pack-*.idx >member-objects &&
+	test_line_count = 3 shared-objects &&
+	test_line_count = 3 member-objects &&
+	! test_cmp shared-objects member-objects &&
+
+	# Perform the geometric repack. With `-l`, we should only see the local
+	# packfile and thus arrive at the conclusion that the geometric
+	# sequence is intact. We thus expect no changes.
+	#
+	# Note that we are tweaking mtimes of the packfiles so that we can
+	# verify they did not change. This is done in order to detect the case
+	# where we do repack objects, but the resulting packfile is the same.
+	test-tool chmtime --verbose =0 member/.git/objects/pack/* >expected-member-packs &&
+	git -C member repack --geometric=2 -l -d &&
+	test-tool chmtime --verbose member/.git/objects/pack/* >actual-member-packs &&
+	test_cmp expected-member-packs actual-member-packs &&
+
+	{
+		packed_objects shared/.git/objects/pack/pack-*.idx &&
+		packed_objects member/.git/objects/pack/pack-*.idx
+	} | sort >expected-objects &&
+
+	# On the other hand, when doing a non-local geometric repack we should
+	# see both packfiles and thus repack them. We expect that the shared
+	# object database was not changed.
+	test-tool chmtime --verbose =0 shared/.git/objects/pack/* >expected-shared-packs &&
+	git -C member repack --geometric=2 -d &&
+	test-tool chmtime --verbose shared/.git/objects/pack/* >actual-shared-packs &&
+	test_cmp expected-shared-packs actual-shared-packs &&
+
+	# Furthermore, we expect that the member repository now has a single
+	# packfile that contains the combined shared and non-shared objects.
+	ls member/.git/objects/pack/pack-*.idx >actual &&
+	test_line_count = 1 actual &&
+	packed_objects member/.git/objects/pack/pack-*.idx >actual-objects &&
+	test_line_count = 6 actual-objects &&
+	test_cmp expected-objects actual-objects
+'
+
 test_done

builtin/repack.c, object-file.c, object-store.h, t/t7700-repack.sh, t/t7703-repack-geometric.sh

diff --git a/builtin/repack.c b/builtin/repack.c
index 5768aeb1f3..e2abcea2ee 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -887,6 +887,18 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	if (write_bitmaps && !(pack_everything & ALL_INTO_ONE) && !write_midx)
 		die(_(incremental_bitmap_conflict_error));
 
+	if (write_bitmaps && po_args.local && has_alt_odb(the_repository)) {
+		/*
+		 * When asked to do a local repack, but we have
+		 * packfiles that are inherited from an alternate, then
+		 * we cannot guarantee that the multi-pack-index would
+		 * have full coverage of all objects. We thus disable
+		 * writing bitmaps in that case.
+		 */
+		warning(_("disabling bitmap writing, as some objects are not being packed"));
+		write_bitmaps = 0;
+	}
+
 	if (write_midx && write_bitmaps) {
 		struct strbuf path = STRBUF_INIT;
 
diff --git a/object-file.c b/object-file.c
index 8fab8dbe80..3ad11c683b 100644
--- a/object-file.c
+++ b/object-file.c
@@ -946,6 +946,12 @@ void prepare_alt_odb(struct repository *r)
 	r->objects->loaded_alternates = 1;
 }
 
+int has_alt_odb(struct repository *r)
+{
+	prepare_alt_odb(r);
+	return !!r->objects->odb->next;
+}
+
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */
 static int freshen_file(const char *fn)
 {
diff --git a/object-store.h b/object-store.h
index 82201ec3e7..574f16140f 100644
--- a/object-store.h
+++ b/object-store.h
@@ -56,6 +56,7 @@ KHASH_INIT(odb_path_map, const char * /* key: odb_path */,
 	struct object_directory *, 1, fspathhash, fspatheq)
 
 void prepare_alt_odb(struct repository *r);
+int has_alt_odb(struct repository *r);
 char *compute_alternate_path(const char *path, struct strbuf *err);
 struct object_directory *find_odb(struct repository *r, const char *obj_dir);
 typedef int alt_odb_fn(struct object_directory *, void *);
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index 4aabe98139..faa739eeb9 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -106,6 +106,23 @@ test_expect_success SYMLINKS '--local keeps packs when alternate is objectdir '
 	test_cmp expect actual
 '
 
+test_expect_success '--local disables writing bitmaps when connected to alternate ODB' '
+	test_when_finished "rm -rf shared member" &&
+
+	git init shared &&
+	git clone --shared shared member &&
+	(
+		cd member &&
+		test_commit "object" &&
+		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adl --write-bitmap-index 2>err &&
+		cat >expect <<-EOF &&
+		warning: disabling bitmap writing, as some objects are not being packed
+		EOF
+		test_cmp expect err &&
+		test_path_is_missing .git/objects/pack-*.bitmap
+	)
+'
+
 test_expect_success 'packed obs in alt ODB are repacked even when local repo is packless' '
 	mkdir alt_objects/pack &&
 	mv .git/objects/pack/* alt_objects/pack &&
diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
index be8fe78448..00f28fb558 100755
--- a/t/t7703-repack-geometric.sh
+++ b/t/t7703-repack-geometric.sh
@@ -418,4 +418,31 @@ test_expect_success '--geometric -l with non-intact geometric sequence across OD
 	test_cmp expected-objects actual-objects
 '
 
+test_expect_success '--geometric -l disables writing bitmaps with non-local packfiles' '
+	test_when_finished "rm -fr shared member" &&
+
+	git init shared &&
+	test_commit_bulk -C shared --start=1 1 &&
+
+	git clone --shared shared member &&
+	test_commit_bulk -C member --start=2 1 &&
+
+	# When performing a geometric repack with `-l` while connected to an
+	# alternate object database that has a packfile we do not have full
+	# coverage of objects. As a result, we expect that writing the bitmap
+	# will be disabled.
+	git -C member repack -l --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	cat >expect <<-EOF &&
+	warning: disabling bitmap writing, as some objects are not being packed
+	EOF
+	test_cmp expect err &&
+	test_path_is_missing member/.git/objects/pack/multi-pack-index-*.bitmap &&
+
+	# On the other hand, when we repack without `-l`, we should see that
+	# the bitmap gets created.
+	git -C member repack --geometric=2 --write-midx --write-bitmap-index 2>err &&
+	test_must_be_empty err &&
+	test_path_is_file member/.git/objects/pack/multi-pack-index-*.bitmap
+'
+
 test_done