builtin/repack.c: avoid making cruft packs preferred

When doing a `--geometric` repack, we make sure that the preferred pack
(if writing a MIDX) is the largest pack that we *didn't* repack. That
has the effect of keeping the preferred pack in sync with the pack
containing a majority of the repository's reachable objects.

But if the repository happens to double in size, we'll repack
everything. Here we don't specify any `--preferred-pack`, and instead
let the MIDX code choose.

In the past, that worked fine, since there would only be one pack to
choose from: the one we just wrote. But it's no longer necessarily the
case that there is one pack to choose from. It's possible that the
repository also has a cruft pack, too.

If the cruft pack happens to come earlier in lexical order (and has an
earlier mtime than any non-cruft pack), we'll pick that pack as
preferred. This makes it impossible to reuse chunks of the reachable
pack verbatim from pack-objects, so is sub-optimal.

Luckily, this is a somewhat rare circumstance to be in, since we would
have to repack the entire repository during a `--geometric` repack, and
the cruft pack would have to sort ahead of the pack we just created.

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

1 files changed, 39 insertions, 0 deletions

diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index dc86ca8269..be3735dff0 100755
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -372,4 +372,43 @@ test_expect_success '--max-cruft-size ignores non-local packs' '
 	)
 '
 
+test_expect_success 'reachable packs are preferred over cruft ones' '
+	repo="cruft-preferred-packs" &&
+	git init "$repo" &&
+	(
+		cd "$repo" &&
+
+		# This test needs to exercise careful control over when a MIDX
+		# is and is not written. Unset the corresponding TEST variable
+		# accordingly.
+		sane_unset GIT_TEST_MULTI_PACK_INDEX &&
+
+		test_commit base &&
+		test_commit --no-tag cruft &&
+
+		non_cruft="$(echo base | git pack-objects --revs $packdir/pack)" &&
+		# Write a cruft pack which both (a) sorts ahead of the non-cruft
+		# pack in lexical order, and (b) has an older mtime to appease
+		# the MIDX preferred pack selection routine.
+		cruft="$(echo pack-$non_cruft.pack | git pack-objects --cruft $packdir/pack-A)" &&
+		test-tool chmtime -1000 $packdir/pack-A-$cruft.pack &&
+
+		test_commit other &&
+		git repack -d &&
+
+		git repack --geometric 2 -d --write-midx --write-bitmap-index &&
+
+		# After repacking, there are two packs left: one reachable one
+		# (which is the result of combining both of the existing two
+		# non-cruft packs), and one cruft pack.
+		find .git/objects/pack -type f -name "*.pack" >packs &&
+		test_line_count = 2 packs &&
+
+		# Make sure that the pack we just wrote is marked as preferred,
+		# not the cruft one.
+		pack="$(test-tool read-midx --preferred-pack $objdir)" &&
+		test_path_is_missing "$packdir/$(basename "$pack" ".idx").mtimes"
+	)
+'
+
 test_done