tests: object: test looking up corrupted objects

We currently have no tests which check whether we fail reading corrupted
objects. Add one which modifies contents of an object stored on disk and
then tries to read the object.

tests/object/lookup.c

@@ -62,3 +62,33 @@ void test_object_lookup__lookup_wrong_type_eventually_returns_enotfound(void)
 		GIT_ENOTFOUND, git_object_lookup(&object, g_repo, &oid, GIT_OBJ_TAG));
 }
 
+void test_object_lookup__lookup_corrupt_object_returns_error(void)
+{
+	const char *commit = "8e73b769e97678d684b809b163bebdae2911720f",
+	      *file = "objects/8e/73b769e97678d684b809b163bebdae2911720f";
+	git_buf path = GIT_BUF_INIT, contents = GIT_BUF_INIT;
+	git_oid oid;
+	git_object *object;
+	size_t i;
+
+	cl_git_pass(git_oid_fromstr(&oid, commit));
+	cl_git_pass(git_buf_joinpath(&path, git_repository_path(g_repo), file));
+	cl_git_pass(git_futils_readbuffer(&contents, path.ptr));
+
+	/* Corrupt and try to read the object */
+	for (i = 0; i < contents.size; i++) {
+		contents.ptr[i] ^= 0x1;
+		cl_git_pass(git_futils_writebuffer(&contents, path.ptr, O_RDWR, 0644));
+		cl_git_fail(git_object_lookup(&object, g_repo, &oid, GIT_OBJ_COMMIT));
+		contents.ptr[i] ^= 0x1;
+	}
+
+	/* Restore original content and assert we can read the object */
+	cl_git_pass(git_futils_writebuffer(&contents, path.ptr, O_RDWR, 0644));
+	cl_git_pass(git_object_lookup(&object, g_repo, &oid, GIT_OBJ_COMMIT));
+
+	git_object_free(object);
+	git_buf_free(&path);
+	git_buf_free(&contents);
+}
+