reflog-walk: don't segfault on non-commit sha1's in the reflog

git reflog (ab)uses the log machinery to display its list of log
entries. To do so it must fake commit parent information for the log
walker.

For refs in refs/heads this is no problem, as they should only ever
point to commits. Tags and other refs however can point to anything,
thus their reflog may contain non-commit objects.

To avoid segfaulting, we check whether reflog entries are commits before
feeding them to the log walker and skip any non-commits. This means that
git reflog output will be incomplete for such refs, but that's one step
up from segfaulting. A more complete solution would be to decouple git
reflog from the log walker machinery.

Signed-off-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reflog-walk.c

diff --git a/reflog-walk.c b/reflog-walk.c
index f8e743a..d3cc524 100644
--- a/reflog-walk.c
+++ b/reflog-walk.c
@@ -222,6 +222,7 @@
 	struct commit_info *commit_info =
 		get_commit_info(commit, &info->reflogs, 0);
 	struct commit_reflog *commit_reflog;
+	struct object *logobj;
 	struct reflog_info *reflog;
 
 	info->last_commit_reflog = NULL;
@@ -233,15 +234,20 @@
 		commit->parents = NULL;
 		return;
 	}
-
-	reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
 	info->last_commit_reflog = commit_reflog;
-	commit_reflog->recno--;
-	commit_info->commit = (struct commit *)parse_object(reflog->osha1);
-	if (!commit_info->commit) {
+
+	do {
+		reflog = &commit_reflog->reflogs->items[commit_reflog->recno];
+		commit_reflog->recno--;
+		logobj = parse_object(reflog->osha1);
+	} while (commit_reflog->recno && (logobj && logobj->type != OBJ_COMMIT));
+
+	if (!logobj || logobj->type != OBJ_COMMIT) {
+		commit_info->commit = NULL;
 		commit->parents = NULL;
 		return;
 	}
+	commit_info->commit = (struct commit *)logobj;
 
 	commit->parents = xcalloc(1, sizeof(struct commit_list));
 	commit->parents->item = commit_info->commit;

t/t1410-reflog.sh

diff --git a/t/t1410-reflog.sh b/t/t1410-reflog.sh
index b79049f..17a194b 100755
--- a/t/t1410-reflog.sh
+++ b/t/t1410-reflog.sh
@@ -325,4 +325,17 @@
 	test_cmp expect actual
 '
 
+test_expect_success 'no segfaults for reflog containing non-commit sha1s' '
+	git update-ref --create-reflog -m "Creating ref" \
+		refs/tests/tree-in-reflog HEAD &&
+	git update-ref -m "Forcing tree" refs/tests/tree-in-reflog HEAD^{tree} &&
+	git update-ref -m "Restoring to commit" refs/tests/tree-in-reflog HEAD &&
+	git reflog refs/tests/tree-in-reflog
+'
+
+test_expect_failure 'reflog with non-commit entries displays all entries' '
+	git reflog refs/tests/tree-in-reflog >actual &&
+	test_line_count = 3 actual
+'
+
 test_done