Add Writer and WriterLevel to Entry

This lets you do things like:

    cmd := exec.Command("command")
    stdout := logrus.WithField("fd", "stdout").Writer()
    defer stdout.Close()
    stderr := logrus.WithField("fd", "stderr").Writer()
    defer stderr.Close()
    cmd.Stdout = stdout
    cmd.Stderr = stderr

logrus_test.go

@@ -359,3 +359,28 @@ func TestLogrusInterface(t *testing.T) {
 	e := logger.WithField("another", "value")
 	fn(e)
 }
+
+// Implements io.Writer using channels for synchronization, so we can wait on
+// the Entry.Writer goroutine to write in a non-racey way. This does assume that
+// there is a single call to Logger.Out for each message.
+type channelWriter chan []byte
+
+func (cw channelWriter) Write(p []byte) (int, error) {
+	cw <- p
+	return len(p), nil
+}
+
+func TestEntryWriter(t *testing.T) {
+	cw := channelWriter(make(chan []byte, 1))
+	log := New()
+	log.Out = cw
+	log.Formatter = new(JSONFormatter)
+	log.WithField("foo", "bar").WriterLevel(WarnLevel).Write([]byte("hello\n"))
+
+	bs := <-cw
+	var fields Fields
+	err := json.Unmarshal(bs, &fields)
+	assert.Nil(t, err)
+	assert.Equal(t, fields["foo"], "bar")
+	assert.Equal(t, fields["level"], "warning")
+}

writer.go

@@ -11,39 +11,48 @@ func (logger *Logger) Writer() *io.PipeWriter {
 }
 
 func (logger *Logger) WriterLevel(level Level) *io.PipeWriter {
+	return NewEntry(logger).WriterLevel(level)
+}
+
+func (entry *Entry) Writer() *io.PipeWriter {
+	return entry.WriterLevel(InfoLevel)
+}
+
+func (entry *Entry) WriterLevel(level Level) *io.PipeWriter {
 	reader, writer := io.Pipe()
 
 	var printFunc func(args ...interface{})
+
 	switch level {
 	case DebugLevel:
-		printFunc = logger.Debug
+		printFunc = entry.Debug
 	case InfoLevel:
-		printFunc = logger.Info
+		printFunc = entry.Info
 	case WarnLevel:
-		printFunc = logger.Warn
+		printFunc = entry.Warn
 	case ErrorLevel:
-		printFunc = logger.Error
+		printFunc = entry.Error
 	case FatalLevel:
-		printFunc = logger.Fatal
+		printFunc = entry.Fatal
 	case PanicLevel:
-		printFunc = logger.Panic
+		printFunc = entry.Panic
 	default:
-		printFunc = logger.Print
+		printFunc = entry.Print
 	}
 
-	go logger.writerScanner(reader, printFunc)
+	go entry.writerScanner(reader, printFunc)
 	runtime.SetFinalizer(writer, writerFinalizer)
 
 	return writer
 }
 
-func (logger *Logger) writerScanner(reader *io.PipeReader, printFunc func(args ...interface{})) {
+func (entry *Entry) writerScanner(reader *io.PipeReader, printFunc func(args ...interface{})) {
 	scanner := bufio.NewScanner(reader)
 	for scanner.Scan() {
 		printFunc(scanner.Text())
 	}
 	if err := scanner.Err(); err != nil {
-		logger.Errorf("Error while reading from Writer: %s", err)
+		entry.Errorf("Error while reading from Writer: %s", err)
 	}
 	reader.Close()
 }