Handle errors at download better

Related to #173

cmd/goutubedl/main.go

@@ -25,7 +25,11 @@ func main() {
 	result, err := goutubedl.New(
 		context.Background(),
 		flag.Arg(0),
-		goutubedl.Options{Type: optType, DebugLog: log.Default(), StderrFn: func(cmd *exec.Cmd) io.Writer { return os.Stderr }},
+		goutubedl.Options{
+			Type:     optType,
+			DebugLog: log.Default(),
+			StderrFn: func(cmd *exec.Cmd) io.Writer { return os.Stderr },
+		},
 	)
 	if err != nil {
 		log.Fatal(err)

goutubedl.go

@@ -623,15 +623,17 @@ func (result Result) DownloadWithOptions(
 	}
 
 	cmd.Dir = tempPath
-	var w io.WriteCloser
-	dr.reader, w = io.Pipe()
-
-	stderrWriter := io.Discard
+	var stdoutW io.WriteCloser
+	var stderrW io.WriteCloser
+	var stderrR io.Reader
+	dr.reader, stdoutW = io.Pipe()
+	stderrR, stderrW = io.Pipe()
+	optStderrWriter := io.Discard
 	if result.Options.StderrFn != nil {
-		stderrWriter = result.Options.StderrFn(cmd)
+		optStderrWriter = result.Options.StderrFn(cmd)
 	}
-	cmd.Stdout = w
-	cmd.Stderr = stderrWriter
+	cmd.Stdout = stdoutW
+	cmd.Stderr = io.MultiWriter(optStderrWriter, stderrW)
 
 	debugLog.Print("cmd", " ", cmd.Args)
 	if err := cmd.Start(); err != nil {
@@ -641,12 +643,32 @@ func (result Result) DownloadWithOptions(
 
 	go func() {
 		_ = cmd.Wait()
-		w.Close()
+		stdoutW.Close()
+		stderrW.Close()
 		os.RemoveAll(tempPath)
 		close(dr.waitCh)
 	}()
 
-	return dr, nil
+	// blocks return until yt-dlp is downloading or has errored
+	ytErrCh := make(chan error)
+	go func() {
+		stderrLineScanner := bufio.NewScanner(stderrR)
+		for stderrLineScanner.Scan() {
+			const downloadPrefix = "[download]"
+			const errorPrefix = "ERROR: "
+			line := stderrLineScanner.Text()
+			if strings.HasPrefix(line, downloadPrefix) {
+				break
+			} else if strings.HasPrefix(line, errorPrefix) {
+				ytErrCh <- errors.New(line[len(errorPrefix):])
+				return
+			}
+		}
+		ytErrCh <- nil
+		_, _ = io.Copy(io.Discard, stderrR)
+	}()
+
+	return dr, <-ytErrCh
 }
 
 func (dr *DownloadResult) Read(p []byte) (n int, err error) {

goutubedl_test.go

@@ -552,3 +552,29 @@ func TestDownloadPlaylistEntry(t *testing.T) {
 		t.Error("not the same content between the playlist index entry and the direct link entry")
 	}
 }
+
+func TestFormatDownloadError(t *testing.T) {
+	defer leaktest.Check(t)()
+
+	ydl, ydlErr := goutubedl.New(
+		context.Background(),
+		"https://www.reddit.com/r/newsbabes/s/92rflI0EB0",
+		goutubedl.Options{},
+	)
+
+	if ydlErr != nil {
+		// reddit seems to not like github action hosts
+		if strings.Contains(ydlErr.Error(), "HTTPError 403: Blocked") {
+			t.Skip()
+		}
+		t.Error(ydlErr)
+	}
+
+	// no pre-muxed audio/video format available
+	_, ytDlErr := ydl.Download(context.Background(), "best")
+	expectedErr := "Requested format is not available"
+	if ydlErr != nil && !strings.Contains(ytDlErr.Error(), expectedErr) {
+		t.Errorf("expected error prefix %q got %q", expectedErr, ytDlErr.Error())
+
+	}
+}