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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 (
// CapturedStacktrace represents a stack trace returned by stacktrace(...).
// At the present time it can only be stringified (via str(...) in Starlark or
// via .String() in Go).
type CapturedStacktrace struct {
// Note that *starlark.Frame is mutated after it is "captured". So we either
// need to do a deep copy of the linked list of frames, or just render it to
// the string right away. We opt for the later, since it is simpler and the
// public interface supports only stringification anyway.
backtrace string
// CaptureStacktrace captures thread's stack trace, skipping some number of
// innermost frames.
// Returns an error if the stack is not deep enough to skip the requested number
// of frames.
func CaptureStacktrace(th *starlark.Thread, skip int) (*CapturedStacktrace, error) {
f := th.Caller()
skipped := 0
for skipped < skip && f != nil {
f = f.Parent()
if f == nil {
return nil, fmt.Errorf("stacktrace: the stack is not deep enough to skip %d levels, has only %d frames", skip, skipped)
buf := strings.Builder{}
return &CapturedStacktrace{buf.String()}, nil
// Type is part of starlark.Value interface.
func (*CapturedStacktrace) Type() string { return "stacktrace" }
// Freeze is part of starlark.Value interface.
func (*CapturedStacktrace) Freeze() {}
// Truth is part of starlark.Value interface.
func (*CapturedStacktrace) Truth() starlark.Bool { return starlark.True }
// Hash is part of starlark.Value interface.
func (*CapturedStacktrace) Hash() (uint32, error) { return 0, errors.New("stacktrace is unhashable") }
// String is part of starlark.Value interface.
// Renders the stack trace as string.
func (s *CapturedStacktrace) String() string { return s.backtrace }
// Stacktrace is stacktrace(...) builtin.
// def stacktrace(skip=0):
// """Capture and returns a stack trace of the caller.
// A captured stacktrace is an opaque object that can be stringified to get a
// nice looking trace (e.g. for error messages).
// Args:
// skip: how many inner most frames to skip.
// """
var Stacktrace = starlark.NewBuiltin("stacktrace", func(th *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
skip := starlark.MakeInt(0)
if err := starlark.UnpackArgs("stacktrace", args, kwargs, "skip?", &skip); err != nil {
return nil, err
switch lvl, err := starlark.AsInt32(skip); {
case err != nil:
return nil, fmt.Errorf("stacktrace: bad 'skip' value %s - %s", skip, err)
case lvl < 0:
return nil, fmt.Errorf("stacktrace: bad 'skip' value %d - must be non-negative", lvl)
return CaptureStacktrace(th, lvl)