repack: add --keep-unreachable option

The usual way to do a full repack (and what is done by
git-gc) is to run "repack -Ad --unpack-unreachable=<when>",
which will loosen any unreachable objects newer than
"<when>", and drop any older ones.

This is a safer alternative to "repack -ad", because
"<when>" becomes a grace period during which we will not
drop any new objects that are about to be referenced.
However, it isn't perfectly safe. It's always possible that
a process is about to reference an old object. Even if that
process were to take care to update the timestamp on the
object, there is no atomicity with a simultaneously running
"repack" process.

So while unlikely, there is a small race wherein we may drop
an object that is in the process of being referenced. If you
do automated repacking on a large number of active
repositories, you may hit it eventually, and the result is a
corrupted repository.

It would be nice to fix that race in the long run, but it's
complicated.  In the meantime, there is a much simpler
strategy for automated repository maintenance: do not drop
objects at all. We already have a "--keep-unreachable"
option in pack-objects; we just need to plumb it through
from git-repack.

Note that this _isn't_ plumbed through from git-gc, so at
this point it's strictly a tool for people doing their own
advanced repository maintenance strategy.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/t/t7701-repack-unpack-unreachable.sh b/t/t7701-repack-unpack-unreachable.sh
index b66e383866..f13df43299 100755
--- a/t/t7701-repack-unpack-unreachable.sh
+++ b/t/t7701-repack-unpack-unreachable.sh
@@ -122,4 +122,19 @@ test_expect_success 'keep packed objects found only in index' '
 	git cat-file blob :file
 '
 
+test_expect_success 'repack -k keeps unreachable packed objects' '
+	# create packed-but-unreachable object
+	sha1=$(echo unreachable-packed | git hash-object -w --stdin) &&
+	pack=$(echo $sha1 | git pack-objects .git/objects/pack/pack) &&
+	git prune-packed &&
+
+	# -k should keep it
+	git repack -adk &&
+	git cat-file -p $sha1 &&
+
+	# and double check that without -k it would have been removed
+	git repack -ad &&
+	test_must_fail git cat-file -p $sha1
+'
+
 test_done