Subject: [PATCH v2 0/6] midx-write: fix segfault and do several cleanups

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Sat, 30 Aug 2025 21:23:21 +0000

I was motivated to start looking closely at midx-write.c due to multiple
users reporting Git crashes in their background maintenance, specifically
during git multi-pack-index repack calls. I was eventually able to reproduce
it in git multi-pack-index expire as well.

Patch 1 is the only change we need to fix this bug. It includes a test case
that will fail under --stress with SANITIZE=address. It requires creating
many packfiles (50 was not enough, but 100 is enough). As far as I can tell,
this bug has existed since Git 2.47.0 in October 2024, but I started hearing
reports of this from users in July 2025 (and took a while to get a
dump/repro).

The remaining patches are cleanups based on my careful rereading of
midx-write.c. There are some issues about error handling that needed some
cleanup as well as a removal of the DISABLE_SIGN_COMPARE_WARNINGS macro.

Updates in V2
=============

 * A stale comment to an unsubmitted version of the test is removed.
 * More cases needing open_pack_index() are patched.
 * Typos fixed.
 * A new patch assumes error and sets result to zero only on the few
   successful paths.

Thanks, -Stolee

Derrick Stolee (6):
  midx-write: only load initialized packs
  midx-write: put failing response value back
  midx-write: use cleanup when incremental midx fails
  midx-write: use uint32_t for preferred_pack_idx
  midx-write: reenable signed comparison errors
  midx-write: simplify error cases

 midx-write.c                | 134 +++++++++++++++++-------------------
 t/t5319-multi-pack-index.sh |  22 +++++-
 2 files changed, 86 insertions(+), 70 deletions(-)

base-commit: c44beea485f0f2feaf460e2ac87fdd5608d63cf0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1965%2Fderrickstolee%2Fmidx-write-cleanup-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1965/derrickstolee/midx-write-cleanup-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1965

Range-diff vs v1:

 1:  4a4b35c694 ! 1:  e02a444315 midx-write: only load initialized packs
     @@ Commit message
          Add a new test that breaks under --stress when compiled with
          SANITIZE=address. The chosen number of 100 packfiles was selected to get
          the --stress output to fail about 50% of the time, while 50 packfiles
     -    could not get a failure in most --stress runs. This test has a very
     -    minor check at the end confirming only one packfile remaining. The
     -    failing nature of this test actually relies on auto-GC cleaning up some
     -    packfiles during the creation of the commits, as tests setting gc.auto
     -    to zero make the packfile count match the number of added commits but
     -    also avoids hitting the memory issue.
     +    could not get a failure in most --stress runs.
      
          The test case is marked as EXPENSIVE not only because of the number of
          packfiles it creates, but because some CI environments were reporting
     @@ Commit message
              #4 0x562d5d46fff6 in cmd_multi_pack_index builtin/multi-pack-index.c:305
              ...
      
     -    This failure stack trace is disconnected from the real fix because it
     -    the bad pointers are accessed later when closing the packfiles from the
     -    context.
     +    This failure stack trace is disconnected from the real fix because the bad
     +    pointers are accessed later when closing the packfiles from the context.
      
          There are a few different aspects to this fix that are worth noting:
      
     @@ midx-write.c: static int fill_packs_from_midx(struct write_midx_context *ctx,
      -				if (open_pack_index(m->packs[i]))
      -					die(_("could not open index for %s"),
      -					    m->packs[i]->pack_name);
     +-			}
      +			if (prepare_midx_pack(ctx->repo, m,
     -+					      m->num_packs_in_base + i)) {
     -+				error(_("could not load pack"));
     -+				return 1;
     - 			}
     ++					      m->num_packs_in_base + i))
     ++				return error(_("could not load pack"));
       
      +			ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
       			fill_pack_info(&ctx->info[ctx->nr++], m->packs[i],
     @@ midx-write.c: static int write_midx_internal(struct repository *r, const char *o
       		goto cleanup;
       	}
       
     +@@ midx-write.c: static int write_midx_internal(struct repository *r, const char *object_dir,
     + 		struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
     + 		ctx.preferred_pack_idx = 0;
     + 
     ++		/*
     ++		 * Attempt opening the pack index to populate num_objects.
     ++		 * Ignore failiures as they can be expected and are not
     ++		 * fatal during this selection time.
     ++		 */
     ++		open_pack_index(oldest);
     ++
     + 		if (packs_to_drop && packs_to_drop->nr)
     + 			BUG("cannot write a MIDX bitmap during expiration");
     + 
     +@@ midx-write.c: static int write_midx_internal(struct repository *r, const char *object_dir,
     + 
     + 			if (!oldest->num_objects || p->mtime < oldest->mtime) {
     + 				oldest = p;
     ++				open_pack_index(oldest);
     + 				ctx.preferred_pack_idx = i;
     + 			}
     + 		}
      @@ midx-write.c: static int write_midx_internal(struct repository *r, const char *object_dir,
       
       	if (ctx.preferred_pack_idx > -1) {
 2:  709555c531 ! 2:  a1dd3ed874 midx-write: put failing response value back
     @@ Commit message
      
          This instance of setting the result to 1 before going to cleanup was
          accidentally removed in fcb2205b77 (midx: implement support for writing
     -    incremental MIDX chains, 2024-08-06).
     +    incremental MIDX chains, 2024-08-06). Build upon a test that already deletes
     +    a packfile to verify that this error propagates to full command failure.
      
          Signed-off-by: Derrick Stolee <[email-masked]>
      
     @@ midx-write.c: static int write_midx_internal(struct repository *r, const char *o
       		goto cleanup;
       	}
       
     +
     + ## t/t5319-multi-pack-index.sh ##
     +@@ t/t5319-multi-pack-index.sh: test_expect_success 'load reverse index when missing .idx, .pack' '
     + 		mv $idx.bak $idx &&
     + 
     + 		mv $pack $pack.bak &&
     +-		git cat-file --batch-check="%(objectsize:disk)" <tip
     ++		git cat-file --batch-check="%(objectsize:disk)" <tip &&
     ++
     ++		test_must_fail git multi-pack-index write 2>err &&
     ++		grep "could not load pack" err
     + 	)
     + '
     + 
 3:  a5bee03601 = 3:  c4f75cca09 midx-write: use cleanup when incremental midx fails
 4:  bd97db26f7 ! 4:  2290e27ded midx-write: use uint32_t for preferred_pack_idx
     @@ midx-write.c: static int write_midx_internal(struct repository *r, const char *o
      +		struct packed_git *oldest = ctx.info[0].p;
       		ctx.preferred_pack_idx = 0;
       
     - 		if (packs_to_drop && packs_to_drop->nr)
     + 		/*
      @@ midx-write.c: static int write_midx_internal(struct repository *r, const char *object_dir,
       			 * objects to resolve, so the preferred value doesn't
       			 * matter.
 5:  eb1abdca32 = 5:  35302f5228 midx-write: reenable signed comparison errors
 -:  ---------- > 6:  7be25cf534 midx-write: simplify error cases

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