log: ensure writes are atomic.

This means that any Writer can be used safely
even when Output is called concurrently.
Fixes #1302.

R=r, nigeltao_gnome
CC=golang-dev
https://golang.org/cl/3300042

src/pkg/log/log.go

@@ -19,6 +19,7 @@ import (
 	"runtime"
 	"os"
 	"time"
+	"sync"
 )
 
 // These flags define which text to prefix to each log entry generated by the Logger.
@@ -34,8 +35,12 @@ const (
 	Lshortfile                // final file name element and line number: d.go:23. overrides Llongfile
 )
 
-// Logger represents an active logging object.
+// A Logger represents an active logging object that generates lines of
+// output to an io.Writer.  Each logging operation makes a single call to
+// the Writer's Write method.  A Logger can be used simultaneously from
+// multiple goroutines; it guarantees to serialize access to the Writer.
 type Logger struct {
+	mu     sync.Mutex // ensures atomic writes
 	out    io.Writer  // destination for output
 	prefix string     // prefix to write at beginning of each line
 	flag   int        // properties
@@ -46,7 +51,7 @@ type Logger struct {
 // The prefix appears at the beginning of each generated log line.
 // The flag argument defines the logging properties.
 func New(out io.Writer, prefix string, flag int) *Logger {
-	return &Logger{out, prefix, flag}
+	return &Logger{out: out, prefix: prefix, flag: flag}
 }
 
 var std = New(os.Stderr, "", Ldate|Ltime)
@@ -139,6 +144,8 @@ func (l *Logger) Output(calldepth int, s string) os.Error {
 	if len(s) > 0 && s[len(s)-1] != '\n' {
 		buf.WriteByte('\n')
 	}
+	l.mu.Lock()
+	defer l.mu.Unlock()
 	_, err := l.out.Write(buf.Bytes())
 	return err
 }