Merge branch 'tb/midx-with-changing-preferred-pack-fix'

Multi-pack index got corrupted when preferred pack changed from one
pack to another in a certain way, which has been corrected.

* tb/midx-with-changing-preferred-pack-fix:
  midx.c: avoid adding preferred objects twice
  midx.c: include preferred pack correctly with existing MIDX
  midx.c: extract `midx_fanout_add_pack_fanout()`
  midx.c: extract `midx_fanout_add_midx_fanout()`
  midx.c: extract `struct midx_fanout`
  t/lib-bitmap.sh: avoid silencing stderr
  t5326: demonstrate potential bitmap corruption

2 files changed, 45 insertions, 1 deletions

diff --git a/t/lib-bitmap.sh b/t/lib-bitmap.sh
index a95537e759..f595937094 100644
--- a/t/lib-bitmap.sh
+++ b/t/lib-bitmap.sh
@@ -440,7 +440,7 @@ midx_bitmap_partial_tests () {
 		test_commit packed &&
 		git repack &&
 		test_commit loose &&
-		git multi-pack-index write --bitmap 2>err &&
+		git multi-pack-index write --bitmap &&
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap
 	'
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index 4fe57414c1..89ecd1062c 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -307,4 +307,48 @@ test_expect_success 'graceful fallback when missing reverse index' '
 	)
 '
 
+test_expect_success 'preferred pack change with existing MIDX bitmap' '
+	git init preferred-pack-with-existing &&
+	(
+		cd preferred-pack-with-existing &&
+
+		test_commit base &&
+		test_commit other &&
+
+		git rev-list --objects --no-object-names base >p1.objects &&
+		git rev-list --objects --no-object-names other >p2.objects &&
+
+		p1="$(git pack-objects "$objdir/pack/pack" \
+			--delta-base-offset <p1.objects)" &&
+		p2="$(git pack-objects "$objdir/pack/pack" \
+			--delta-base-offset <p2.objects)" &&
+
+		# Generate a MIDX containing the first two packs,
+		# marking p1 as preferred, and ensure that it can be
+		# successfully cloned.
+		git multi-pack-index write --bitmap \
+			--preferred-pack="pack-$p1.pack" &&
+		test_path_is_file $midx &&
+		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
+		git clone --no-local . clone1 &&
+
+		# Then generate a new pack which sorts ahead of any
+		# existing pack (by tweaking the pack prefix).
+		test_commit foo &&
+		git pack-objects --all --unpacked $objdir/pack/pack0 &&
+
+		# Generate a new MIDX which changes the preferred pack
+		# to a pack contained in the existing MIDX.
+		git multi-pack-index write --bitmap \
+			--preferred-pack="pack-$p2.pack" &&
+		test_path_is_file $midx &&
+		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
+
+		# When the above circumstances are met, the preferred
+		# pack should change appropriately and clones should
+		# (still) succeed.
+		git clone --no-local . clone2
+	)
+'
+
 test_done