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// Copyright 2018 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package builtins
import (
// Failure is an error emitted by fail(...) and captured by FailureCollector.
type Failure struct {
Message string // the error message, as passed to fail(...)
UserTrace *CapturedStacktrace // value of 'trace' passed to fail or nil
FailTrace *CapturedStacktrace // where 'fail' itself was called
// Error is the short error message, as passed to fail(...).
func (f *Failure) Error() string {
return f.Message
// Backtrace returns a user-friendly error message describing the stack of
// calls that led to this error.
// If fail(...) was called with a custom stack trace, this trace is shown here.
// Otherwise the trace of where fail(...) happened is used.
func (f *Failure) Backtrace() string {
tr := f.UserTrace
if tr == nil {
tr = f.FailTrace
return tr.String() + "Error: " + f.Message
// Fail is fail(*args, sep=" ", trace=None) builtin.
// def fail(*args, sep=" ", trace=None):
// """Aborts the script execution with an error message."
// Args:
// args: values to print in the message.
// sep: separator to use between values from `args`.
// trace: a trace (as returned by stacktrace()) to attach to the error.
// """
// Custom stack traces are recoverable through FailureCollector. This is due
// to Starlark's insistence on stringying all errors. If there's no
// FailureCollector in the thread locals, custom traces are silently ignored.
// Note that the assert.fails(...) check in the default starlark tests library
// doesn't clear the failure collector state when it "catches" an error, so
// tests that use assert.fails(...) should be careful with using the failure
// collector (or just don't use it at all).
var Fail = starlark.NewBuiltin("fail", func(th *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
sep := " "
var trace starlark.Value
err := starlark.UnpackArgs("fail", nil, kwargs,
"sep?", &sep,
"trace?", &trace)
if err != nil {
return nil, err
var userTrace *CapturedStacktrace
if trace != nil && trace != starlark.None {
if userTrace, _ = trace.(*CapturedStacktrace); userTrace == nil {
return nil, fmt.Errorf("fail: bad 'trace' - got %s, expecting stacktrace", trace.Type())
buf := strings.Builder{}
for i, v := range args {
if i > 0 {
if s, ok := starlark.AsString(v); ok {
} else {
msg := buf.String()
if fc := GetFailureCollector(th); fc != nil {
failTrace, _ := CaptureStacktrace(th, 0)
fc.failure = &Failure{
Message: msg,
UserTrace: userTrace,
FailTrace: failTrace,
return nil, errors.New(msg)
// A key in thread.Locals to hold *FailureCollector.
const failSlotKey = ""
// FailureCollector receives structured error messages from fail(...).
// It should be installed into Starlark thread locals (via Install) for
// fail(...) to be able to discover it. If it's not there, fail(...) will not
// return any additional information (like a custom stack trace) besides the
// information contained in *starlark.EvalError.
type FailureCollector struct {
// failure is the error passed to fail(...).
// fail(...) aborts the execution of starlark scripts, so its fine to keep
// only one error. There can't really be more.
failure *Failure
// GetFailureCollector returns a failure collector installed in the thread.
func GetFailureCollector(th *starlark.Thread) *FailureCollector {
fc, _ := th.Local(failSlotKey).(*FailureCollector)
return fc
// Install installs this failure collector into the thread.
func (fc *FailureCollector) Install(t *starlark.Thread) {
t.SetLocal(failSlotKey, fc)
// LatestFailure returns the latest captured failure or nil if there are none.
func (fc *FailureCollector) LatestFailure() *Failure {
return fc.failure
// Clear resets the state.
// Useful if the same FailureCollector is reused between calls to Starlark.
func (fc *FailureCollector) Clear() {
fc.failure = nil