[PATCH v2 06/12] bisect: handle NULL commit in `bisect_successful()`

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

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

When `lookup_commit_reference_by_name()` is called to find the first bad
commit, the result is passed to `repo_format_commit_message()`
immediately, which dereferences commit without checking for NULL.

However, the commit could be NULL, even though in practice this is
unlikely because `bisect_successful()` is only called after a successful
bisect run has identified the bad commit, but the ref could still become
dangling due to a concurrent gc or repository corruption.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/bisect.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/builtin/bisect.c b/builtin/bisect.c
index e7c2d2f3bb..408e0f414e 100644
--- a/builtin/bisect.c
+++ b/builtin/bisect.c
@@ -663,6 +663,11 @@ static int bisect_successful(struct bisect_terms *terms)
 
 	refs_read_ref(get_main_ref_store(the_repository), bad_ref, &oid);
 	commit = lookup_commit_reference_by_name(bad_ref);
+	if (!commit) {
+		error(_("could not find commit for '%s'"), bad_ref);
+		free(bad_ref);
+		return BISECT_FAILED;
+	}
 	repo_format_commit_message(the_repository, commit, "%s", &commit_name,
 				   &pp);
 
-- 
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