go/tb/error.go

// Copyright 2012, Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tb

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"runtime"
)

var (
	dunno     = []byte("???")
	centerDot = []byte("·")
	dot       = []byte(".")
)

// This package exposes some handy traceback functionality buried in the runtime.
//
// It can also be used to provide context to errors reducing the temptation to
// panic carelessly, just to get stack information.
//
// The theory is that most errors that are created with the fmt.Errorf
// style are likely to be rare, but require more context to debug
// properly. The additional cost of computing a stack trace is
// therefore negligible.
type StackError interface {
	Error() string
	StackTrace() string
}

type stackError struct {
	err        error
	stackTrace string
}

func (e stackError) Error() string {
	return fmt.Sprintf("%v\n%v", e.err, e.stackTrace)
}

func (e stackError) StackTrace() string {
	return e.stackTrace
}

func Errorf(msg string, args ...interface{}) error {
	stack := ""
	// See if any arg is already embedding a stack - no need to
	// recompute something expensive and make the message unreadable.
	for _, arg := range args {
		if stackErr, ok := arg.(stackError); ok {
			stack = stackErr.stackTrace
			break
		}
	}

	if stack == "" {
		// magic 5 trims off just enough stack data to be clear
		stack = string(Stack(5))
	}

	return stackError{fmt.Errorf(msg, args...), stack}
}

// Taken from runtime/debug.go
func Stack(calldepth int) []byte {
	return stack(calldepth)
}

func stack(calldepth int) []byte {
	buf := new(bytes.Buffer) // the returned data
	// As we loop, we open files and read them. These variables record the currently
	// loaded file.
	var lines [][]byte
	var lastFile string
	for i := calldepth; ; i++ { // Caller we care about is the user, 2 frames up
		pc, file, line, ok := runtime.Caller(i)
		if !ok {
			break
		}
		// Print this much at least.  If we can't find the source, it won't show.
		fmt.Fprintf(buf, "%s:%d (0x%x)\n", file, line, pc)
		if file != lastFile {
			data, err := ioutil.ReadFile(file)
			if err != nil {
				continue
			}
			lines = bytes.Split(data, []byte{'\n'})
			lastFile = file
		}
		line-- // in stack trace, lines are 1-indexed but our array is 0-indexed
		fmt.Fprintf(buf, "\t%s: %s\n", function(pc), source(lines, line))
	}
	return buf.Bytes()
}

// source returns a space-trimmed slice of the n'th line.
func source(lines [][]byte, n int) []byte {
	if n < 0 || n >= len(lines) {
		return dunno
	}
	return bytes.Trim(lines[n], " \t")
}

// function returns, if possible, the name of the function containing the PC.
func function(pc uintptr) []byte {
	fn := runtime.FuncForPC(pc)
	if fn == nil {
		return dunno
	}
	name := []byte(fn.Name())
	// The name includes the path name to the package, which is unnecessary
	// since the file name is already included.  Plus, it has center dots.
	// That is, we see
	//	runtime/debug.*T·ptrmethod
	// and want
	//	*T.ptrmethod
	if period := bytes.Index(name, dot); period >= 0 {
		name = name[period+1:]
	}
	name = bytes.Replace(name, centerDot, dot, -1)
	return name
}