builtin/repack.c: invalidate MIDX only when necessary

In 525e18c04b (midx: clear midx on repack, 2018-07-12), 'git repack'
learned to remove a multi-pack-index file if it added or removed a pack
from the object store.

This mechanism is a little over-eager, since it is only necessary to
drop a MIDX if 'git repack' removes a pack that the MIDX references.
Adding a pack outside of the MIDX does not require invalidating the
MIDX, and likewise for removing a pack the MIDX does not know about.

Teach 'git repack' to check for this by loading the MIDX, and checking
whether the to-be-removed pack is known to the MIDX. This requires a
slightly odd alternation to a test in t5319, which is explained with a
comment. A new test is added to show that the MIDX is left alone when
both packs known to it are marked as .keep, but two packs unknown to it
are removed and combined into one new pack.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -133,7 +133,11 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	struct multi_pack_index *m = get_multi_pack_index(the_repository);
+	strbuf_addf(&buf, "%s.pack", base_name);
+	if (m && midx_contains_pack(m, buf.buf))
+		clear_midx_file(the_repository);
+	strbuf_insertf(&buf, 0, "%s/", dir_name);
 	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }
@@ -286,7 +290,6 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	int keep_unreachable = 0;
 	struct string_list keep_pack_list = STRING_LIST_INIT_NODUP;
 	int no_update_server_info = 0;
-	int midx_cleared = 0;
 	struct pack_objects_args po_args = {NULL};
 
 	struct option builtin_repack_options[] = {
@@ -439,11 +442,6 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		for (ext = 0; ext < ARRAY_SIZE(exts); ext++) {
 			char *fname, *fname_old;
 
-			if (!midx_cleared) {
-				clear_midx_file(the_repository);
-				midx_cleared = 1;
-			}
-
 			fname = mkpathdup("%s/pack-%s%s", packdir,
 						item->string, exts[ext].name);
 			if (!file_exists(fname)) {

t/t5319-multi-pack-index.sh

@@ -348,12 +348,52 @@ test_expect_success 'repack with the --no-progress option' '
 	test_line_count = 0 err
 '
 
-test_expect_success 'repack removes multi-pack-index' '
+test_expect_success 'repack removes multi-pack-index when deleting packs' '
 	test_path_is_file $objdir/pack/multi-pack-index &&
-	GIT_TEST_MULTI_PACK_INDEX=0 git repack -adf &&
+	# Set GIT_TEST_MULTI_PACK_INDEX to 0 to avoid writing a new
+	# multi-pack-index after repacking, but set "core.multiPackIndex" to
+	# true so that "git repack" can read the existing MIDX.
+	GIT_TEST_MULTI_PACK_INDEX=0 git -c core.multiPackIndex repack -adf &&
 	test_path_is_missing $objdir/pack/multi-pack-index
 '
 
+test_expect_success 'repack preserves multi-pack-index when creating packs' '
+	git init preserve &&
+	test_when_finished "rm -fr preserve" &&
+	(
+		cd preserve &&
+		packdir=.git/objects/pack &&
+		midx=$packdir/multi-pack-index &&
+
+		test_commit 1 &&
+		pack1=$(git pack-objects --all $packdir/pack) &&
+		touch $packdir/pack-$pack1.keep &&
+		test_commit 2 &&
+		pack2=$(git pack-objects --revs $packdir/pack) &&
+		touch $packdir/pack-$pack2.keep &&
+
+		git multi-pack-index write &&
+		cp $midx $midx.bak &&
+
+		cat >pack-input <<-EOF &&
+		HEAD
+		^HEAD~1
+		EOF
+		test_commit 3 &&
+		pack3=$(git pack-objects --revs $packdir/pack <pack-input) &&
+		test_commit 4 &&
+		pack4=$(git pack-objects --revs $packdir/pack <pack-input) &&
+
+		GIT_TEST_MULTI_PACK_INDEX=0 git -c core.multiPackIndex repack -ad &&
+		ls -la $packdir &&
+		test_path_is_file $packdir/pack-$pack1.pack &&
+		test_path_is_file $packdir/pack-$pack2.pack &&
+		test_path_is_missing $packdir/pack-$pack3.pack &&
+		test_path_is_missing $packdir/pack-$pack4.pack &&
+		test_cmp_bin $midx.bak $midx
+	)
+'
+
 compare_results_with_midx "after repack"
 
 test_expect_success 'multi-pack-index and pack-bitmap' '