@@ -2,6 +2,7 @@
 #include "repack.h"
 #include "hash.h"
 #include "hex.h"
+#include "midx.h"
 #include "odb.h"
 #include "oidset.h"
 #include "pack-bitmap.h"

@@ -283,6 +284,92 @@ static void remove_redundant_bitmaps(struct string_list *include,
 	strbuf_release(&path);
 }
 
+static bool midx_needs_update(struct repack_write_midx_opts *opts,
+			      struct string_list *include,
+			      struct packed_git *new_preferred)
+{
+	struct multi_pack_index *midx;
+	uint32_t preferred_pack_id;
+	bool needed = true;
+
+	/*
+	 * If there is no multi-pack index we obviously need to generate a new
+	 * one.  Note that we cannot use `get_multi_pack_index()` here, as we
+	 * might be operating with a closed object database.
+	 */
+	midx = load_multi_pack_index(opts->existing->repo->objects->sources);
+	if (!midx)
+		goto out;
+
+	/*
+	 * If we're instructed to write a bitmap we need to verify whether we
+	 * already got one.
+	 */
+	if (opts->write_bitmaps) {
+		struct bitmap_index *bitmap = prepare_midx_bitmap_git(midx);
+		bool bitmap_exists = bitmap && bitmap_is_midx(bitmap);
+		free_bitmap_index(bitmap);
+		if (!bitmap_exists)
+			goto out;
+	}
+
+	/*
+	 * If we're asked to generate the MIDX with a preferred pack we need to
+	 * verify that the current prepared pack matches the desired one. We
+	 * can only determine this in the case where we write bitmaps though,
+	 * so we ignore this setting otherwise.
+	 */
+	if (new_preferred && opts->write_bitmaps) {
+		struct packed_git *old_preferred = NULL;
+
+		if (!midx_preferred_pack(midx, &preferred_pack_id))
+			old_preferred = nth_midxed_pack(midx, preferred_pack_id);
+
+		if (!old_preferred)
+			goto out;
+
+		if (strcmp(pack_basename(new_preferred),
+			   pack_basename(old_preferred)))
+			goto out;
+	 }
+
+	/*
+	 * Otherwise, we need to verify that the packs that are about to be
+	 * included in the MIDX matches the currently covered packs.
+	 */
+	if (include->nr != opts->existing->midx_packs.nr)
+		goto out;
+
+	string_list_sort(include);
+	string_list_sort(&opts->existing->midx_packs);
+	for (size_t i = 0; i < include->nr; i++) {
+		const char *include_name = include->items[i].string;
+		const char *existing_name = opts->existing->midx_packs.items[i].string;
+		size_t include_len = strlen(include_name);
+		size_t existing_len = strlen(existing_name);
+
+		/*
+		 * We track pack indices for the include list, but the
+		 * packfiles themselves in the existing list. We thus need to
+		 * strip these suffixes.
+		 */
+		if (ends_with(include_name, ".idx"))
+			include_len -= strlen(".idx");
+		if (ends_with(existing_name, ".pack"))
+			existing_len -= strlen(".pack");
+		if (include_len != existing_len)
+			goto out;
+		if (memcmp(include_name, existing_name, include_len))
+			goto out;
+	}
+
+	needed = false;
+
+out:
+	close_midx(midx);
+	return needed;
+}
+
 int write_midx_included_packs(struct repack_write_midx_opts *opts)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;

@@ -296,6 +383,9 @@ int write_midx_included_packs(struct repack_write_midx_opts *opts)
 	if (!include.nr)
 		goto done;
 
+	if (!midx_needs_update(opts, &include, preferred))
+		goto done;
+
 	cmd.in = -1;
 	cmd.git_cmd = 1;
 

@@ -366,6 +366,48 @@ test_expect_success 'preferred pack cannot be determined without bitmap' '
 	)
 '
 
+test_midx_is_retained () {
+	test-tool chmtime =0 .git/objects/pack/multi-pack-index &&
+	ls -l .git/objects/pack/multi-pack-index >expect &&
+	git repack "$@" &&
+	ls -l .git/objects/pack/multi-pack-index >actual &&
+	test_cmp expect actual
+}
+
+test_midx_is_rewritten () {
+	test-tool chmtime =0 .git/objects/pack/multi-pack-index &&
+	ls -l .git/objects/pack/multi-pack-index >expect &&
+	git repack --write-midx "$@" &&
+	ls -l .git/objects/pack/multi-pack-index >actual &&
+	! test_cmp expect actual
+}
+
+test_expect_success 'up-to-date multi-pack-index is retained' '
+	test_when_finished "rm -fr midx-up-to-date" &&
+	git init midx-up-to-date &&
+	(
+		cd midx-up-to-date &&
+
+		# Write the initial pack that contains the most objects. This
+		# will be the preferred pack.
+		test_commit first &&
+		test_commit second &&
+		git repack -Ad --write-midx &&
+		test_midx_is_retained -Ad &&
+
+		# Writing a new bitmap index should cause us to regenerate the MIDX.
+		test_midx_is_rewritten -Ad --write-bitmap-index &&
+		test_midx_is_retained -Ad --write-bitmap-index &&
+
+		# Ensure that writing a new packfile causes us to rewrite the index.
+		test_commit incremental &&
+		test_midx_is_rewritten -d &&
+		test_midx_is_retained -d
+	)
+'
+
+test_done
+
 test_expect_success 'verify multi-pack-index success' '
 	git multi-pack-index verify --object-dir=$objdir
 '

@@ -287,6 +287,86 @@ test_expect_success '--geometric with pack.packSizeLimit' '
 	)
 '
 
+test_expect_success '--geometric --write-midx retains up-to-date MIDX without bitmap index' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+
+		test_path_is_missing .git/objects/pack/multi-pack-index &&
+		git repack --geometric=2 --write-midx --no-write-bitmap-index &&
+		test_path_is_file .git/objects/pack/multi-pack-index &&
+		test-tool chmtime =0 .git/objects/pack/multi-pack-index &&
+
+		ls -l .git/objects/pack/ >expect &&
+		git repack --geometric=2 --write-midx --no-write-bitmap-index &&
+		ls -l .git/objects/pack/ >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--geometric --write-midx regenerates MIDX when preferred pack changes' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+		test_commit third &&
+		git repack --geometric=2 --write-midx --write-bitmap-index &&
+		test_commit fourth &&
+		git repack --geometric=2 --write-midx --write-bitmap-index &&
+
+		ls .git/objects/pack/*.pack >packs &&
+		test_line_count = 2 packs &&
+		preferred_pack=$(test-tool read-midx --preferred-pack .git/objects) &&
+		other_pack=$(ls .git/objects/pack/*.idx | grep -v "$preferred_pack") &&
+		echo "$preferred_pack" &&
+		echo "$other_pack" &&
+
+		# Rewrite the multi-pack index with the current preferred pack.
+		# git-repack(1) should decide to _not_ repack the MIDX in that
+		# case. This is mostly a sanity check to verify that the reason
+		# for the repack really only is the changed preferred pack.
+		rm -f .git/objects/pack/multi-pack-index* &&
+		git multi-pack-index write --bitmap --preferred-pack="$preferred_pack" &&
+		test-tool chmtime =0 .git/objects/pack/multi-pack-index &&
+		ls -l .git/objects/pack/ >expect &&
+		git repack --geometric=2 --write-midx --write-bitmap-index &&
+		ls -l .git/objects/pack/ >actual &&
+		test_cmp expect actual &&
+
+		# Rewrite the multi-pack index with a different preferred pack.
+		# This time around, git-repack(1) should decide to repack the
+		# MIDX to rectify the preferred pack.
+		rm -f .git/objects/pack/multi-pack-index* &&
+		git multi-pack-index write --bitmap --preferred-pack="$(basename "$other_pack")" &&
+		test-tool chmtime =0 .git/objects/pack/multi-pack-index &&
+		ls -l .git/objects/pack/ >expect &&
+		git repack --geometric=2 --write-midx --write-bitmap-index &&
+		ls -l .git/objects/pack/ >actual &&
+		! test_cmp expect actual
+	)
+'
+
+test_expect_success '--geometric --write-midx retains up-to-date MIDX with bitmap index' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	test_commit -C repo initial &&
+
+	test_path_is_missing repo/.git/objects/pack/multi-pack-index &&
+	git -C repo repack --geometric=2 --write-midx --write-bitmap-index &&
+	test_path_is_file repo/.git/objects/pack/multi-pack-index &&
+	test-tool chmtime =0 repo/.git/objects/pack/multi-pack-index &&
+
+	ls -l repo/.git/objects/pack/ >expect &&
+	git -C repo repack --geometric=2 --write-midx --write-bitmap-index &&
+	ls -l repo/.git/objects/pack/ >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '--geometric --write-midx with packfiles in main and alternate ODB' '
 	test_when_finished "rm -fr shared member" &&