add tb package

tb/error.go

@@ -0,0 +1,124 @@
+// 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
+}