[PATCH 7/7] test-read-midx: check midx_fill_entry() result

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The `--show-objects` mode of `read_midx_file()` uses the output of
`midx_fill_entry()` without checking whether the lookup succeeded. A
failed lookup or unavailable pack can leave that output unusable,
allowing malformed or concurrently changed MIDX data to make this test
helper crash instead of reporting a controlled error.

Reject the entry unless `midx_fill_entry()` returns `MIDX_FILL_HIT`. The
unchecked call was introduced by 86d174b7246b
(t/helper/test-read-midx.c: add '--show-objects', 2021-03-30); later
incremental-MIDX changes expanded the possible failure modes, but this
remains a test-helper robustness issue, not a production Git attack
surface or an arbitrary-code-execution vulnerability.

It is unclear why Coverity reports this issue in Git for Windows only
after merging v2.56.0-rc0; The issue was not reported before.

Assisted-by: GPT-5.6 Luna
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/helper/test-read-midx.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/t/helper/test-read-midx.c b/t/helper/test-read-midx.c
index 83b07c6236..412089563f 100644
--- a/t/helper/test-read-midx.c
+++ b/t/helper/test-read-midx.c
@@ -90,7 +90,11 @@ static int read_midx_file(const char *object_dir, const char *checksum,
 		for (i = 0; i < m->num_objects; i++) {
 			nth_midxed_object_oid(&oid, m,
 					      i + m->num_objects_in_base);
-			midx_fill_entry(m, &oid, &e, NULL);
+			if (midx_fill_entry(m, &oid, &e, NULL) !=
+			    MIDX_FILL_HIT) {
+				ret = error(_("failed to load pack entry"));
+				goto out;
+			}
 
 			printf("%s %"PRIu64"\t%s\n",
 			       oid_to_hex(&oid), e.offset, e.p->pack_name);
-- 
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