Subject: [PATCH 0/2] pack-objects: freshen objects with multi-cruft packs

From: Taylor Blau <[email-masked]>
Date: Thu, 27 Feb 2025 13:29:24 -0500

This short series contains a fix for a bug I noticed while rolling out
multi-cruft packs (via 'git repack --max-cruft-size') within GitHub's
infrastructure.

The series is structured as follows:

 - The first patch simplifies how 'repack' aggregates cruft packs
   together when their size is below the '--max-cruft-size' or
   '--max-pack-size' threshold. This simplification changes behavior
   slightly, but not in a meaningful way. It occurred to me while
   writing the second patch.

 - The second patch describes and fixes the main bug. The gist here is
   that objects which are (a) unreachable, (b) exist in a cruft pack
   being retained, and (c) were freshened to have a more recent mtime
   than any existing cruft copy are unable to be freshened.

The fix pursued in the second patch changes the rules around when we
want to retain an object via builtin/pack-objects.c::want_found_object()
when at least one cruft pack will survive the repack.

Previously the rule was to discard any object which appears in any
surviving pack, regardless of mtime. The rule now is to only discard an
object if it appears in either (a) a non-cruft pack which will survive
the repack, or (b) a cruft pack whose mtime for that object is older
than the one we are trying to pack.

I think that this is the right behavior, but admittedly putting this
series together hurt my brain trying to think through all of the cases.
I'm fairly confident in the testing here as I remember it being fairly
exhaustive of all interesting cases. But I'd appreciate a sanity check
from others that they too are convinced this is the right approach.

Thanks in advance for your review!

Taylor Blau (2):
  builtin/repack.c: simplify cruft pack aggregation
  builtin/pack-objects.c: freshen objects from existing cruft packs

 builtin/pack-objects.c  | 118 ++++++++++++++++++++++++++++++++++------
 builtin/repack.c        |  38 +------------
 packfile.c              |   3 +-
 packfile.h              |   2 +
 t/t7704-repack-cruft.sh | 106 ++++++++++++++++++++++--------------
 5 files changed, 171 insertions(+), 96 deletions(-)

base-commit: 08bdfd453584e489d5a551aecbdcb77584e1b958
-- 
2.49.0.rc0.2.gc0c926adde2