packfile.c: protect against disappearing indexes

In 17c35c8969 (packfile: skip loading index if in multi-pack-index,
2018-07-12) we stopped loading the .idx file for packs that are
contained within a multi-pack index.

This saves us the effort of loading an .idx and doing some lightweight
validity checks by way of 'packfile.c:load_idx()', but introduces a race
between processes that need to load the index (e.g., to generate a
reverse index) and processes that can delete the index.

For example, running the following in your shell:

    $ git init repo && cd repo
    $ git commit --allow-empty -m 'base'
    $ git repack -ad && git multi-pack-index write

followed by:

    $ rm -f .git/objects/pack/pack-*.idx
    $ git rev-parse HEAD | git cat-file --batch-check='%(objectsize:disk)'

will result in a segfault prior to this patch. What's happening here is
that we notice that the pack is in the multi-pack index, and so don't
check that it still has a .idx. When we then try and load that index to
generate a reverse index, we don't have it, so the call to
'find_pack_revindex()' in 'packfile.c:packed_object_info()' returns
NULL, and then dereferencing it causes a segfault.

Of course, we don't ever expect someone to remove the index file by
hand, or to be in a state where we never wrote it to begin with (yet
find that pack in the multi-pack-index). But, this can happen in a
timing race with 'git repack -ad', which removes all existing packs
after writing a new pack containing all of their objects.

Avoid this by reverting the hunk of 17c35c8969 which stops loading the
index when the pack is contained in a MIDX. This makes the latter half
of 17c35c8969 useless, since we'll always have a non-NULL
'p->index_data', in which case that if statement isn't guarding
anything.

These two together effectively revert 17c35c8969, and avoid the race
explained above.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 17 deletions

diff --git a/packfile.c b/packfile.c
index 355066d..eb14f7c 100644
--- a/packfile.c
+++ b/packfile.c
@@ -514,19 +514,8 @@ static int open_packed_git_1(struct packed_git *p)
 	ssize_t read_result;
 	const unsigned hashsz = the_hash_algo->rawsz;
 
-	if (!p->index_data) {
-		struct multi_pack_index *m;
-		const char *pack_name = pack_basename(p);
-
-		for (m = the_repository->objects->multi_pack_index;
-		     m; m = m->next) {
-			if (midx_contains_pack(m, pack_name))
-				break;
-		}
-
-		if (!m && open_pack_index(p))
-			return error("packfile %s index unavailable", p->pack_name);
-	}
+	if (open_pack_index(p))
+		return error("packfile %s index unavailable", p->pack_name);
 
 	if (!pack_max_fds) {
 		unsigned int max_fds = get_max_fd_limit();
@@ -577,10 +566,6 @@ static int open_packed_git_1(struct packed_git *p)
 			" supported (try upgrading GIT to a newer version)",
 			p->pack_name, ntohl(hdr.hdr_version));
 
-	/* Skip index checking if in multi-pack-index */
-	if (!p->index_data)
-		return 0;
-
 	/* Verify the pack matches its index. */
 	if (p->num_objects != ntohl(hdr.hdr_entries))
 		return error("packfile %s claims to have %"PRIu32" objects"