midx.c: prevent `expire` from removing the cruft pack

The `expire` sub-command unlinks any packs that are (a) contained in the
MIDX, but (b) have no objects referenced by the MIDX.

This sub-command ignores `.keep` packs, which remain on-disk even if
they have no objects referenced by the MIDX. Cruft packs, however,
aren't given the same treatment: if none of the objects contained in the
cruft pack are selected from the cruft pack by the MIDX, then the cruft
pack is eligible to be expired.

This is less than desireable, since the cruft pack has important
metadata about the individual object mtimes, which is useful to
determine how quickly an object should age out of the repository when
pruning.

Ordinarily, we wouldn't expect the contents of a cruft pack to
duplicated across non-cruft packs (and we'd expect to see the MIDX
select all cruft objects from other sources even less often). But
nonetheless, it is still possible to trick the `expire` sub-command into
removing the `.mtimes` file in this circumstance.

Teach the `expire` sub-command to ignore cruft packs in the same manner
as it does `.keep` packs, in order to keep their metadata around, even
when they are unreferenced by the MIDX.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-multi-pack-index.txt

@@ -72,8 +72,8 @@ verify::
 expire::
 	Delete the pack-files that are tracked by the MIDX file, but
 	have no objects referenced by the MIDX (with the exception of
-	`.keep` packs). Rewrite the MIDX file afterward to remove all
-	references to these pack-files.
+	`.keep` packs and cruft packs). Rewrite the MIDX file afterward
+	to remove all references to these pack-files.
 
 repack::
 	Create a new pack-file containing objects in small pack-files

midx.c

@@ -1839,7 +1839,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 		if (prepare_midx_pack(r, m, i))
 			continue;
 
-		if (m->packs[i]->pack_keep)
+		if (m->packs[i]->pack_keep || m->packs[i]->is_cruft)
 			continue;
 
 		pack_name = xstrdup(m->packs[i]->pack_name);

t/t5319-multi-pack-index.sh

@@ -847,6 +847,36 @@ test_expect_success 'expire respects .keep files' '
 	)
 '
 
+test_expect_success 'expiring unreferenced cruft pack retains pack' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit base &&
+		test_commit --no-tag unreachable &&
+		unreachable=$(git rev-parse HEAD) &&
+
+		git reset --hard base &&
+		git reflog expire --all --expire=all &&
+		git repack --cruft -d &&
+		mtimes="$(ls $objdir/pack/pack-*.mtimes)" &&
+
+		echo "base..$unreachable" >in &&
+		pack="$(git pack-objects --revs --delta-base-offset \
+			$objdir/pack/pack <in)" &&
+
+		# Preferring the contents of "$pack" will leave the
+		# cruft pack unreferenced (ie., none of the objects
+		# contained in the cruft pack will have their MIDX copy
+		# selected from the cruft pack).
+		git multi-pack-index write --preferred-pack="pack-$pack.pack" &&
+		git multi-pack-index expire &&
+
+		test_path_is_file "$mtimes"
+	)
+'
+
 test_expect_success 'repack --batch-size=0 repacks everything' '
 	cp -r dup dup2 &&
 	(