rebase: correctly finish rebasing detached heads

When rebasing with IDs, we do not return to the `branch`,
we remain in a detached HEAD state.

src/rebase.c

@@ -1262,42 +1262,33 @@ done:
 	return error;
 }
 
-int git_rebase_finish(
-	git_rebase *rebase,
-	const git_signature *signature)
+static int return_to_orig_head(git_rebase *rebase)
 {
 	git_reference *terminal_ref = NULL, *branch_ref = NULL, *head_ref = NULL;
 	git_commit *terminal_commit = NULL;
 	git_buf branch_msg = GIT_BUF_INIT, head_msg = GIT_BUF_INIT;
 	char onto[GIT_OID_HEXSZ];
-	int error;
-
-	assert(rebase);
-
-	if (rebase->inmemory)
-		return 0;
+	int error = 0;
 
 	git_oid_fmt(onto, &rebase->onto_id);
 
-	if ((error = git_buf_printf(&branch_msg, "rebase finished: %s onto %.*s",
-			rebase->orig_head_name, GIT_OID_HEXSZ, onto)) < 0 ||
-		(error = git_buf_printf(&head_msg, "rebase finished: returning to %s",
-			rebase->orig_head_name)) < 0 ||
-		(error = git_repository_head(&terminal_ref, rebase->repo)) < 0 ||
+	if ((error = git_buf_printf(&branch_msg,
+			"rebase finished: %s onto %.*s",
+			rebase->orig_head_name, GIT_OID_HEXSZ, onto)) == 0 &&
+		(error = git_buf_printf(&head_msg,
+			"rebase finished: returning to %s",
+			rebase->orig_head_name)) == 0 &&
+		(error = git_repository_head(&terminal_ref, rebase->repo)) == 0 &&
 		(error = git_reference_peel((git_object **)&terminal_commit,
-			terminal_ref, GIT_OBJ_COMMIT)) < 0 ||
+			terminal_ref, GIT_OBJ_COMMIT)) == 0 &&
 		(error = git_reference_create_matching(&branch_ref,
-			rebase->repo, rebase->orig_head_name, git_commit_id(terminal_commit), 1,
-			&rebase->orig_head_id, branch_msg.ptr)) < 0 ||
-		(error = git_reference_symbolic_create(&head_ref,
+			rebase->repo, rebase->orig_head_name,
+			git_commit_id(terminal_commit), 1,
+			&rebase->orig_head_id, branch_msg.ptr)) == 0)
+		error = git_reference_symbolic_create(&head_ref,
 			rebase->repo, GIT_HEAD_FILE, rebase->orig_head_name, 1,
-			head_msg.ptr)) < 0 ||
-		(error = rebase_copy_notes(rebase, signature)) < 0)
-		goto done;
-
-	error = rebase_cleanup(rebase);
+			head_msg.ptr);
 
-done:
 	git_buf_free(&head_msg);
 	git_buf_free(&branch_msg);
 	git_commit_free(terminal_commit);
@@ -1308,6 +1299,26 @@ done:
 	return error;
 }
 
+int git_rebase_finish(
+	git_rebase *rebase,
+	const git_signature *signature)
+{
+	int error = 0;
+
+	assert(rebase);
+
+	if (rebase->inmemory)
+		return 0;
+
+	if (!rebase->head_detached)
+		error = return_to_orig_head(rebase);
+
+	if (error == 0 && (error = rebase_copy_notes(rebase, signature)) == 0)
+		error = rebase_cleanup(rebase);
+
+	return error;
+}
+
 size_t git_rebase_operation_entrycount(git_rebase *rebase)
 {
 	assert(rebase);

tests/rebase/merge.c

@@ -475,6 +475,56 @@ void test_rebase_merge__finish(void)
 	git_rebase_free(rebase);
 }
 
+void test_rebase_merge__finish_with_ids(void)
+{
+	git_rebase *rebase;
+	git_reference *head_ref;
+	git_oid branch_id, upstream_id;
+	git_annotated_commit *branch_head, *upstream_head;
+	git_rebase_operation *rebase_operation;
+	git_oid commit_id;
+	git_reflog *reflog;
+	const git_reflog_entry *reflog_entry;
+	int error;
+
+	cl_git_pass(git_oid_fromstr(&branch_id, "d616d97082eb7bb2dc6f180a7cca940993b7a56f"));
+	cl_git_pass(git_oid_fromstr(&upstream_id, "f87d14a4a236582a0278a916340a793714256864"));
+
+	cl_git_pass(git_annotated_commit_lookup(&branch_head, repo, &branch_id));
+	cl_git_pass(git_annotated_commit_lookup(&upstream_head, repo, &upstream_id));
+
+	cl_git_pass(git_rebase_init(&rebase, repo, branch_head, upstream_head, NULL, NULL));
+
+	cl_git_pass(git_rebase_next(&rebase_operation, rebase));
+	cl_git_pass(git_rebase_commit(&commit_id, rebase, NULL, signature,
+		NULL, NULL));
+
+	cl_git_fail(error = git_rebase_next(&rebase_operation, rebase));
+	cl_assert_equal_i(GIT_ITEROVER, error);
+
+	cl_git_pass(git_rebase_finish(rebase, signature));
+
+	cl_assert_equal_i(GIT_REPOSITORY_STATE_NONE, git_repository_state(repo));
+
+	cl_git_pass(git_reference_lookup(&head_ref, repo, "HEAD"));
+	cl_assert_equal_i(GIT_REF_OID, git_reference_type(head_ref));
+	cl_assert_equal_oid(&commit_id, git_reference_target(head_ref));
+
+	/* reflogs are not updated as if we were operating on proper
+	 * branches.  check that the last reflog entry is the rebase.
+	 */
+	cl_git_pass(git_reflog_read(&reflog, repo, "HEAD"));
+	cl_assert(reflog_entry = git_reflog_entry_byindex(reflog, 0));
+	cl_assert_equal_oid(&commit_id, git_reflog_entry_id_new(reflog_entry));
+	cl_assert_equal_s("rebase: Modification 3 to gravy", git_reflog_entry_message(reflog_entry));
+	git_reflog_free(reflog);
+
+	git_annotated_commit_free(branch_head);
+	git_annotated_commit_free(upstream_head);
+	git_reference_free(head_ref);
+	git_rebase_free(rebase);
+}
+
 static void test_copy_note(
 	const git_rebase_options *opts,
 	bool should_exist)