[PATCH 3/4] fsck: check rev-index position values

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

When checking a rev-index file, it may be helpful to identify exactly
which positions are incorrect. Compare the rev-index to a
freshly-computed in-memory rev-index and report the comparison failures.

This additional check (on top of the checksum validation) can help find
files that were corrupt by a single bit flip on-disk or perhaps were
written incorrectly due to a bug in Git.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
 pack-revindex.c          | 25 +++++++++++++++++++++----
 t/t5325-reverse-index.sh |  5 +++++
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/pack-revindex.c b/pack-revindex.c
index 007a806994f..62a9846470c 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -310,16 +310,33 @@ int load_pack_revindex(struct repository *r, struct packed_git *p)
  */
 int verify_pack_revindex(struct packed_git *p)
 {
+	int res = 0;
+
 	/* Do not bother checking if not initialized. */
-	if (!p->revindex_map)
-		return 0;
+	if (!p->revindex_map || !p->revindex_data)
+		return res;
 
 	if (!hashfile_checksum_valid((const unsigned char *)p->revindex_map, p->revindex_size)) {
 		error(_("invalid checksum"));
-		return -1;
+		res = -1;
 	}
 
-	return 0;
+	/* This may fail due to a broken .idx. */
+	if (create_pack_revindex_in_memory(p))
+		return res;
+
+	for (size_t i = 0; i < p->num_objects; i++) {
+		uint32_t nr = p->revindex[i].nr;
+		uint32_t rev_val = get_be32(p->revindex_data + i);
+
+		if (nr != rev_val) {
+			error(_("invalid rev-index position at %"PRIu64": %"PRIu32" != %"PRIu32""),
+			      (uint64_t)i, nr, rev_val);
+			res = -1;
+		}
+	}
+
+	return res;
 }
 
 int load_midx_revindex(struct multi_pack_index *m)
diff --git a/t/t5325-reverse-index.sh b/t/t5325-reverse-index.sh
index 6b7c709a1f6..5c3c80f88f0 100755
--- a/t/t5325-reverse-index.sh
+++ b/t/t5325-reverse-index.sh
@@ -185,4 +185,9 @@ test_expect_success 'fsck catches invalid checksum' '
 		"invalid checksum"
 '
 
+test_expect_success 'fsck catches invalid row position' '
+	corrupt_rev_and_verify 14 "\07" \
+		"invalid rev-index position"
+'
+
 test_done
-- 
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