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>

1 files changed, 42 insertions, 2 deletions

diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 7214cab..0a8a1f5 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/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' '