Use MultiWriter to simplify the writing implementation. Also make it possible to initialize the log target to variable amounts of writers, and doesn't hardcode the use of StdErr as output

cmd/grumble/grumble.go

@@ -37,7 +37,7 @@ func main() {
 	dataDir.Close()
 
 	// Set up logging
-	logtarget.Default, err = logtarget.OpenFile(Args.LogPath)
+	logtarget.Default, err = logtarget.OpenFile(Args.LogPath, os.Stderr)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Unable to open log file (%v): %v", Args.LogPath, err)
 		return

pkg/logtarget/logtarget.go

@@ -21,57 +21,66 @@ type LogTarget interface {
 	Rotate() error
 }
 
-type fileLogTarget struct {
+// logTarget is the default implementation of a log
+// target. It can write to more than one writer at the same time
+// but only rotate one file
+type logTarget struct {
 	mu    sync.Mutex
 	logfn string
 	file  *os.File
+	w     io.Writer
+	ws    []io.Writer
 }
 
+// Default is the default log target for the application
+// It has to be initialized before used
 var Default LogTarget
 
+// OpenWriters returns a log target that will
+// log to all the given writers at the same time
+func OpenWriters(ws ...io.Writer) LogTarget {
+	target := &logTarget{}
+	target.w = io.MultiWriter(ws...)
+	return target
+}
+
 // OpenFile creates a LogTarget pointing to a log file
 // and returns it.
 // This method will open the file in append-only mode.
-func OpenFile(fileName string) (t LogTarget, err error) {
-	target := &fileLogTarget{}
+// It also takes a variable number of writers that are
+// other log targets
+func OpenFile(fileName string, ws ...io.Writer) (t LogTarget, err error) {
+	target := &logTarget{}
 	target.logfn = fileName
 	target.file, err = os.OpenFile(fileName, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0650)
 	if err != nil {
 		return nil, err
 	}
+	target.ws = ws
+	target.w = io.MultiWriter(append(ws, target.file)...)
 	return target, nil
 }
 
 // Write writes a log message to all registered io.Writers
-func (target *fileLogTarget) Write(in []byte) (int, error) {
+func (target *logTarget) Write(out []byte) (int, error) {
+	target.mu.Lock()
+	defer target.mu.Unlock()
+
+	return target.Write(out)
+}
+
+// Rotate rotates the current log file, if one is opened.
+// This method holds a lock while rotating the log file,
+// and all log writes will be held back until the rotation
+// is complete.
+func (target *logTarget) Rotate() error {
 	target.mu.Lock()
 	defer target.mu.Unlock()
 
 	if target.file == nil {
-		panic("no log file opened")
+		return nil
 	}
 
-	n, err := os.Stderr.Write(in)
-	if err != nil {
-		return n, err
-	}
-
-	n, err = target.file.Write(in)
-	if err != nil {
-		return n, err
-	}
-
-	return len(in), nil
-}
-
-// Rotate rotates the current log file.
-// This method holds a lock while rotating the log file,
-// and all log writes will be held back until the rotation
-// is complete.
-func (target *fileLogTarget) Rotate() error {
-	target.mu.Lock()
-	defer target.mu.Unlock()
-
 	// Close the existing log file
 	err := target.file.Close()
 	if err != nil {
@@ -82,6 +91,7 @@ func (target *fileLogTarget) Rotate() error {
 	if err != nil {
 		return err
 	}
+	target.w = io.MultiWriter(append(target.ws, target.file)...)
 
 	return nil
 }