merge drivers: handle configured but not found driver

src/merge.c

@@ -891,7 +891,8 @@ static int merge_conflict_resolve_contents(
 	git_index_entry *merge_result;
 	git_odb *odb = NULL;
 	const char *name;
-	int error = 0;
+	bool fallback = false;
+	int error;
 
 	assert(resolved && diff_list && conflict);
 
@@ -924,12 +925,20 @@ static int merge_conflict_resolve_contents(
 		/* find the merge driver for this file */
 		if ((error = git_merge_driver_for_source(&name, &driver, &source)) < 0)
 			goto done;
+
+		if (driver == NULL)
+			fallback = true;
 	}
 
-	error = merge_conflict_invoke_driver(&merge_result, name, driver,
-		diff_list, &source);
+	if (driver) {
+		error = merge_conflict_invoke_driver(&merge_result, name, driver,
+			diff_list, &source);
+
+		if (error == GIT_PASSTHROUGH)
+			fallback = true;
+	}
 
-	if (error == GIT_PASSTHROUGH) {
+	if (fallback) {
 		error = merge_conflict_invoke_driver(&merge_result, "text",
 			&git_merge_driver__text.base, diff_list, &source);
 	}

tests/merge/driver.c

@@ -368,3 +368,21 @@ void test_merge_driver__unset_forces_binary(void)
 	cl_git_pass(git_index_conflict_get(&ancestor, &ours, &theirs,
 		repo_index, "automergeable.txt"));
 }
+
+void test_merge_driver__not_configured_driver_falls_back(void)
+{
+	const git_index_entry *idx;
+
+	test_drivers_unregister();
+
+	/* `merge` without specifying a driver indicates `text` */
+	set_gitattributes_to("notfound");
+
+	merge_branch();
+
+	cl_assert((idx = git_index_get_bypath(repo_index, "automergeable.txt", 0)));
+	cl_assert_equal_oid(&automergeable_id, &idx->id);
+
+	test_drivers_register();
+}
+