teelogger: make writes to /dev/console non-blocking

fixes https://github.com/rtr7/router7/issues/68

internal/teelogger/teelogger.go

@@ -23,12 +23,36 @@ import (
 	"os"
 )
 
-// NewConsole returns a logger which returns to /dev/console and os.Stderr.
+type nonBlockingWriter struct {
+	W chan<- string
+}
+
+func (w *nonBlockingWriter) Write(p []byte) (n int, _ error) {
+	select {
+	// Intentionally convert from byte slice ([]byte) to string because sending
+	// a byte slice over a channel is not safe: it may point to new contents,
+	// resulting in duplicate log lines showing up.
+	case w.W <- string(p):
+	default:
+		// channel unavailable, ignore
+	}
+	return len(p), nil
+}
+
+// NewConsole returns a logger which returns to /dev/console and
+// os.Stderr. Writes to /dev/console are non-blocking, i.e. messages will be
+// discarded if /dev/console stalls (e.g. when enabling Scroll Lock on a HDMI
+// console).
 func NewConsole() *log.Logger {
-	var w io.Writer
-	w, err := os.OpenFile("/dev/console", os.O_RDWR, 0600)
-	if err != nil {
-		w = ioutil.Discard
+	w := ioutil.Discard
+	if console, err := os.OpenFile("/dev/console", os.O_RDWR, 0600); err == nil {
+		ch := make(chan string, 1)
+		go func() {
+			for buf := range ch {
+				console.Write([]byte(buf))
+			}
+		}()
+		w = &nonBlockingWriter{W: ch}
 	}
 	return log.New(io.MultiWriter(os.Stderr, w), "", log.LstdFlags|log.Lshortfile)
 }