Merge branch 'jk/pull-null-merge-head'

'git pull' has been taught to fail more gracefully instead of
segfaulting when lookup_commit_reference() fails on unparseable
objects.

* jk/pull-null-merge-head:
  pull: avoid segfault when commit lookup fails

builtin/pull.c

@@ -800,8 +800,12 @@ static int get_can_ff(struct object_id *orig_head,
 
 	orig_merge_head = &merge_heads->oid[0];
 	head = lookup_commit_reference(the_repository, orig_head);
-	commit_list_insert(head, &list);
+	if (!head)
+		return 0;
 	merge_head = lookup_commit_reference(the_repository, orig_merge_head);
+	if (!merge_head)
+		return 0;
+	commit_list_insert(head, &list);
 	ret = repo_is_descendant_of(the_repository, merge_head, list);
 	commit_list_free(list);
 	if (ret < 0)
@@ -820,12 +824,16 @@ static int already_up_to_date(struct object_id *orig_head,
 	struct commit *ours;
 
 	ours = lookup_commit_reference(the_repository, orig_head);
+	if (!ours)
+		return 0;
 	for (size_t i = 0; i < merge_heads->nr; i++) {
 		struct commit_list *list = NULL;
 		struct commit *theirs;
 		int ok;
 
 		theirs = lookup_commit_reference(the_repository, &merge_heads->oid[i]);
+		if (!theirs)
+			return 0;
 		commit_list_insert(theirs, &list);
 		ok = repo_is_descendant_of(the_repository, ours, list);
 		commit_list_free(list);

t/t5520-pull.sh

@@ -888,4 +888,31 @@ test_expect_success 'git pull --rebase against local branch' '
 	test_cmp expect file2
 '
 
+test_expect_success 'pull does not crash when a merge head does not resolve' '
+	test_when_finished "rm -rf up dn" &&
+	git init up &&
+	(
+		cd up &&
+		test_commit base &&
+		git switch -c sideA &&
+		test_commit a &&
+		git switch -c sideB base &&
+		test_commit b
+	) &&
+	git clone up dn &&
+	(
+		cd dn &&
+		git commit-graph write --reachable &&
+		oid=$(git rev-parse refs/remotes/origin/sideA) &&
+		obj=.git/objects/$(test_oid_to_path "$oid") &&
+
+		# Corrupt the object instead of removing it: a missing
+		# object that is still in the commit-graph is caught by
+		# fetch before pull ever reaches the fast-forward check.
+		rm -f "$obj" &&
+		echo garbage >"$obj" &&
+		test_must_fail git pull --no-rebase origin sideA sideB
+	)
+'
+
 test_done