feat(subsonic): support timeOffset in stream.view (#384)

as per
https://github.com/opensubsonic/open-subsonic-api/pull/54
https://github.com/opensubsonic/open-subsonic-api/discussions/21

dont cache partial transcodes

add a transcode seek test

server/ctrlsubsonic/handlers_common.go

@@ -38,7 +38,9 @@ func (c *Controller) ServePing(_ *http.Request) *spec.Response {
 
 func (c *Controller) ServeGetOpenSubsonicExtensions(_ *http.Request) *spec.Response {
 	sub := spec.NewResponse()
-	sub.OpenSubsonicExtensions = &spec.OpenSubsonicExtensions{}
+	sub.OpenSubsonicExtensions = &spec.OpenSubsonicExtensions{
+		{Name: "transcodeOffset", Versions: []int{1}},
+	}
 	return sub
 }
 

server/ctrlsubsonic/handlers_raw.go

@@ -186,6 +186,7 @@ func (c *Controller) ServeStream(w http.ResponseWriter, r *http.Request) *spec.R
 
 	maxBitRate, _ := params.GetInt("maxBitRate")
 	format, _ := params.Get("format")
+	timeOffset, _ := params.GetInt("timeOffset")
 
 	if format == "raw" || maxBitRate >= audioFile.AudioBitrate() {
 		http.ServeFile(w, r, file.AbsPath())
@@ -208,6 +209,9 @@ func (c *Controller) ServeStream(w http.ResponseWriter, r *http.Request) *spec.R
 	if maxBitRate > 0 && int(profile.BitRate()) > maxBitRate {
 		profile = transcode.WithBitrate(profile, transcode.BitRate(maxBitRate))
 	}
+	if timeOffset > 0 {
+		profile = transcode.WithSeek(profile, time.Second*time.Duration(timeOffset))
+	}
 
 	log.Printf("trancoding to %q with max bitrate %dk", profile.MIME(), profile.BitRate())
 

transcode/transcode_test.go

@@ -64,3 +64,39 @@ func TestTranscode(t *testing.T) {
 	// we should have 5 seconds of PCM data
 	require.Equal(t, testFileLen*bytesPerSec, buf.Len())
 }
+
+// TestTranscodeWithSeek starts a web server that transcodes a 5s FLAC file to PCM audio, but with a 2 second offset.
+// A client consumes the result over a 3 second period.
+func TestTranscodeWithSeek(t *testing.T) {
+	t.Parallel()
+
+	testFile := "testdata/5s.flac"
+	testFileLen := 5
+
+	seekSecs := 2
+	profile := transcode.WithSeek(testProfile, time.Duration(seekSecs)*time.Second)
+
+	tr := transcode.NewFFmpegTranscoder()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.NoError(t, tr.Transcode(r.Context(), profile, testFile, w))
+		w.(http.Flusher).Flush()
+	}))
+	defer server.Close()
+
+	resp, err := server.Client().Get(server.URL)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	var buf bytes.Buffer
+	for {
+		n, err := io.Copy(&buf, io.LimitReader(resp.Body, bytesPerSec))
+		require.NoError(t, err)
+		if n == 0 {
+			break
+		}
+		time.Sleep(1 * time.Second)
+	}
+
+	// since we seeked 2 seconds, we should have 5-2 = 3 seconds of PCM data
+	require.Equal(t, (testFileLen-seekSecs)*bytesPerSec, buf.Len())
+}

transcode/transcoder_caching.go

@@ -23,6 +23,11 @@ func NewCachingTranscoder(t Transcoder, cachePath string) *CachingTranscoder {
 }
 
 func (t *CachingTranscoder) Transcode(ctx context.Context, profile Profile, in string, out io.Writer) error {
+	// don't try cache partial transcodes
+	if profile.Seek() > 0 {
+		return t.transcoder.Transcode(ctx, profile, in, out)
+	}
+
 	if err := os.MkdirAll(t.cachePath, perm^0o111); err != nil {
 		return fmt.Errorf("make cache path: %w", err)
 	}