stderr implementation in webcam

same stderr messages made avaliable in raspivid.go now in webcam, so can see when there are problems with with the webcam, rather then our code

device/webcam/webcam.go

@@ -28,6 +28,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os/exec"
 	"strings"
 
@@ -61,15 +62,19 @@ var (
 // Webcam is an implementation of the AVDevice interface for a Webcam. Webcam
 // uses an ffmpeg process to pipe the video data from the webcam.
 type Webcam struct {
-	out io.ReadCloser
+	out  io.ReadCloser
-	log config.Logger
+	log  config.Logger
-	cfg config.Config
+	cfg  config.Config
-	cmd *exec.Cmd
+	cmd  *exec.Cmd
+	done chan struct{}
 }
 
 // New returns a new Webcam.
 func New(l config.Logger) *Webcam {
-	return &Webcam{log: l}
+	return &Webcam{
+		log:  l,
+		done: make(chan struct{}),
+	}
 }
 
 // Name returns the name of the device.
@@ -151,6 +156,32 @@ func (w *Webcam) Start() error {
 		return fmt.Errorf("failed to create pipe: %w", err)
 	}
 
+	stderr, err := w.cmd.StderrPipe()
+	if err != nil {
+		return fmt.Errorf("could not pipe command error: %w", err)
+	}
+
+	go func() {
+		for {
+			select {
+			case <-w.done:
+				w.cfg.Logger.Log(logger.Info, "raspivid.Stop() called, finished checking stderr")
+				return
+			default:
+				buf, err := ioutil.ReadAll(stderr)
+				if err != nil {
+					w.cfg.Logger.Log(logger.Error, "could not read stderr", "error", err)
+					return
+				}
+
+				if len(buf) != 0 {
+					w.cfg.Logger.Log(logger.Error, "error from raspivid stderr", "error", string(buf))
+					return
+				}
+			}
+		}
+	}()
+
 	err = w.cmd.Start()
 	if err != nil {
 		return fmt.Errorf("failed to start ffmpeg: %w", err)
@@ -161,6 +192,7 @@ func (w *Webcam) Start() error {
 
 // Stop will kill the ffmpeg process and close the output pipe.
 func (w *Webcam) Stop() error {
+	close(w.done)
 	if w.cmd == nil || w.cmd.Process == nil {
 		return errors.New("ffmpeg process was never started")
 	}