builtin/gc.c: ignore cruft packs with `--keep-largest-pack`

When cruft packs were implemented, we never adjusted the code for `git
gc`'s `--keep-largest-pack` and `gc.bigPackThreshold` to ignore cruft
packs. This option and configuration option share a common
implementation, but including cruft packs is wrong in both cases:

  - Running `git gc --keep-largest-pack` in a repository where the
    largest pack is the cruft pack itself will make it impossible for
    `git gc` to prune objects, since the cruft pack itself is kept.

  - The same is true for `gc.bigPackThreshold`, if the size of the cruft
    pack exceeds the limit set by the caller.

In the future, it is possible that `gc.bigPackThreshold` could be used
to write a separate cruft pack containing any new unreachable objects
that entered the repository since the last time a cruft pack was
written.

There are some complexities to doing so, mainly around handling
pruning objects that are in an existing cruft pack that is above the
threshold (which would either need to be rewritten, or else delay
pruning). Rewriting a substantially similar cruft pack isn't ideal, but
it is significantly better than the status-quo.

If users have large cruft packs that they don't want to rewrite, they
can mark them as `*.keep` packs. But in general, if a repository has a
cruft pack that is so large it is slowing down GC's, it should probably
be pruned anyway.

In the meantime, ignore cruft packs in the common implementation for
both of these options, and add a pair of tests to prevent any future
regressions here.

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

1 files changed, 43 insertions, 0 deletions

diff --git a/t/t6500-gc.sh b/t/t6500-gc.sh
index d9acb63951..df6f2e6e52 100755
--- a/t/t6500-gc.sh
+++ b/t/t6500-gc.sh
@@ -298,6 +298,49 @@ test_expect_success 'feature.experimental=false avoids cruft packs by default' '
 	)
 '
 
+test_expect_success '--keep-largest-pack ignores cruft packs' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# Generate a pack for reachable objects (of which there
+		# are 3), and one for unreachable objects (of which
+		# there are 6).
+		prepare_cruft_history &&
+		git gc --cruft &&
+
+		mtimes="$(find .git/objects/pack -type f -name "pack-*.mtimes")" &&
+		sz="$(test_file_size "${mtimes%.mtimes}.pack")" &&
+
+		# Ensure that the cruft pack gets removed (due to
+		# `--prune=now`) despite it being the largest pack.
+		git -c gc.bigPackThreshold=$sz gc --cruft --prune=now &&
+
+		assert_no_cruft_packs
+	)
+'
+
+test_expect_success 'gc.bigPackThreshold ignores cruft packs' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# Generate a pack for reachable objects (of which there
+		# are 3), and one for unreachable objects (of which
+		# there are 6).
+		prepare_cruft_history &&
+		git gc --cruft &&
+
+		# Ensure that the cruft pack gets removed (due to
+		# `--prune=now`) despite it being the largest pack.
+		git gc --cruft --prune=now --keep-largest-pack &&
+
+		assert_no_cruft_packs
+	)
+'
+
 run_and_wait_for_auto_gc () {
 	# We read stdout from gc for the side effect of waiting until the
 	# background gc process exits, closing its fd 9.  Furthermore, the