fix(transcoding): make piped FLAC transcodes seekable

The FLAC muxer writes STREAMINFO before it knows the stream length, then
rewinds at the end to fill total_samples in. Navidrome pipes ffmpeg's stdout
(-f flac -), which is not seekable, so ffmpeg logs "unable to rewrite FLAC
header" and the field stays 0. A decoder needs total_samples to turn a
timestamp into a byte offset, so it reports an unknown duration and refuses to
seek. Online playback hides this because the client re-requests with a new
offset each time, but an offline copy is permanently unseekable, the symptom
reported against Symfonium where seeking a downloaded track jumps back to the
start.

Transcode now wraps its own output and rewrites total_samples as the first
bytes flow past. This lives in core/ffmpeg because the unseekable pipe is that
package's doing: buildDynamicArgs is what appends the trailing '-'. core/stream
only learns a target format and hands back an io.ReadCloser, so compensating
there leaked a transcoder implementation detail one layer up. TranscodeOptions
grows a Duration field alongside the existing Offset, which also puts the
duration-minus-offset arithmetic in the same function that emits -ss.

The wrapper runs on every transcode rather than only FLAC targets: the format
on a transcoding row is a declared target that nothing validates against the
command's actual -f, so a custom command can emit FLAC under any target_format.
The magic-byte check inside the wrapper is the authoritative test and costs a
26-byte peek. The output sample rate is read back out of the header ffmpeg just
wrote rather than taken from the transcode options, so a resampled (-ar) output
still gets the right count. Anything that is not a FLAC stream with an unset
total_samples passes through byte for byte.

Measured on a 177s source: before, total_samples=0 and ffprobe reported
duration N/A; after, total_samples=7807023 and duration 177.03s, with the audio
payload byte-identical. This affects every piped FLAC regardless of the source
format; only FLAC stores an authoritative "unknown", which is why mp3, opus
and aac survive the same pipe.

No SEEKTABLE is synthesised and the MD5 is left zero: both are optional, and
decoders binary-search using total_samples alone.

core/ffmpeg/ffmpeg.go

@@ -27,11 +27,12 @@ type TranscodeOptions struct {
 	Command    string // DB command template (used to detect custom vs default)
 	Format     string // Target format (mp3, opus, aac, flac)
 	FilePath   string
-	BitRate    int // kbps, 0 = codec default
-	SampleRate int // 0 = no constraint
-	Channels   int // 0 = no constraint
-	BitDepth   int // 0 = no constraint; valid values: 16, 24, 32
-	Offset     int // seconds
+	BitRate    int     // kbps, 0 = codec default
+	SampleRate int     // 0 = no constraint
+	Channels   int     // 0 = no constraint
+	BitDepth   int     // 0 = no constraint; valid values: 16, 24, 32
+	Offset     int     // seconds
+	Duration   float32 // seconds; 0 = unknown. Only used to repair a piped FLAC header.
 }
 
 // AudioProbeResult contains authoritative audio stream properties from ffprobe.
@@ -86,7 +87,11 @@ func (e *ffmpeg) Transcode(ctx context.Context, opts TranscodeOptions) (io.ReadC
 	} else {
 		args = buildTemplateArgs(opts)
 	}
-	return e.start(ctx, args)
+	out, err := e.start(ctx, args)
+	if err != nil {
+		return nil, err
+	}
+	return patchFLACDuration(out, opts.Duration-float32(opts.Offset)), nil
 }
 
 func (e *ffmpeg) ConvertAnimatedImage(ctx context.Context, reader io.Reader, maxSize int, quality int) (io.ReadCloser, error) {

core/ffmpeg/ffmpeg_test.go

@@ -3,6 +3,7 @@ package ffmpeg
 import (
 	"context"
 	"errors"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -684,6 +685,40 @@ var _ = Describe("ffmpeg", func() {
 				})
 				Expect(err).To(MatchError(context.Canceled))
 			})
+
+			It("fills in total_samples on a piped FLAC transcode", func() {
+				stream, err := ff.Transcode(GinkgoT().Context(), TranscodeOptions{
+					Command:  "ffmpeg -i %s -map 0:a:0 -v 0 -c:a flac -f flac -",
+					Format:   "flac",
+					FilePath: "tests/fixtures/test.flac",
+					Duration: 1, // the fixture is exactly 1s at 44100Hz
+				})
+				Expect(err).ToNot(HaveOccurred())
+				defer stream.Close()
+
+				out, err := io.ReadAll(stream)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(string(out[:4])).To(Equal("fLaC"))
+				Expect(readTotalSamples(out)).To(Equal(uint64(44100)))
+			})
+
+			It("patches the duration net of the requested offset", func() {
+				// The command has no %t, so ffmpeg still emits the whole fixture.
+				// What is under test is the header arithmetic, not the audio.
+				stream, err := ff.Transcode(GinkgoT().Context(), TranscodeOptions{
+					Command:  "ffmpeg -i %s -map 0:a:0 -v 0 -c:a flac -f flac -",
+					Format:   "flac",
+					FilePath: "tests/fixtures/test.flac",
+					Duration: 3,
+					Offset:   1,
+				})
+				Expect(err).ToNot(HaveOccurred())
+				defer stream.Close()
+
+				out, err := io.ReadAll(stream)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(readTotalSamples(out)).To(Equal(uint64(2 * 44100)))
+			})
 		})
 
 		Context("stderr capture", func() {

core/ffmpeg/flac_streaminfo.go

@@ -0,0 +1,66 @@
+package ffmpeg
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"io"
+	"math"
+)
+
+const (
+	flacPrefixLen       = 26 // through the last total_samples byte
+	flacMaxTotalSamples = 1<<36 - 1
+)
+
+// patchFLACDuration fills in the STREAMINFO total_samples that ffmpeg leaves at 0
+// when writing to a pipe, since a decoder cannot seek a cached FLAC without it.
+func patchFLACDuration(r io.ReadCloser, duration float32) io.ReadCloser {
+	if duration <= 0 {
+		return r
+	}
+	return &flacPatcher{ReadCloser: r, duration: duration}
+}
+
+type flacPatcher struct {
+	io.ReadCloser
+	duration float32
+	// Peeking here rather than in the constructor keeps Transcode from blocking
+	// until ffmpeg has emitted its first bytes.
+	stream io.Reader
+}
+
+func (f *flacPatcher) Read(p []byte) (int, error) {
+	if f.stream == nil {
+		prefix := make([]byte, flacPrefixLen)
+		n, err := io.ReadFull(f.ReadCloser, prefix)
+		if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrUnexpectedEOF) {
+			return 0, err
+		}
+		prefix = prefix[:n]
+		if err == nil {
+			setFLACTotalSamples(prefix, f.duration)
+		}
+		f.stream = io.MultiReader(bytes.NewReader(prefix), f.ReadCloser)
+	}
+	return f.stream.Read(p)
+}
+
+// setFLACTotalSamples takes the rate from the header rather than the transcode
+// options, so a resampled (-ar) output still gets the right count.
+func setFLACTotalSamples(prefix []byte, duration float32) {
+	if string(prefix[:4]) != "fLaC" || prefix[4]&0x7F != 0 {
+		return
+	}
+	// 20-bit rate | 3-bit channels | 5-bit depth | 36-bit total_samples
+	info := binary.BigEndian.Uint64(prefix[18:])
+	rate := info >> 44
+	if rate == 0 || info&flacMaxTotalSamples != 0 {
+		return
+	}
+	total := math.Round(float64(duration) * float64(rate))
+	if total > flacMaxTotalSamples {
+		return
+	}
+	binary.BigEndian.PutUint64(prefix[18:], info|uint64(total))
+}

core/ffmpeg/flac_streaminfo_test.go

@@ -0,0 +1,142 @@
+package ffmpeg
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"os"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+// Decoded independently so the specs do not mirror the production bit-twiddling.
+func readSampleRate(b []byte) int {
+	return int(b[18])<<12 | int(b[19])<<4 | int(b[20])>>4
+}
+
+func readTotalSamples(b []byte) uint64 {
+	return uint64(b[21]&0x0F)<<32 | uint64(b[22])<<24 | uint64(b[23])<<16 | uint64(b[24])<<8 | uint64(b[25])
+}
+
+var _ = Describe("patchFLACDuration", func() {
+	var fileFLAC []byte
+
+	// Zeroing total_samples reproduces what a piped transcode emits.
+	pipedFLAC := func() []byte {
+		b := bytes.Clone(fileFLAC)
+		b[21] &= 0xF0
+		clear(b[22:26])
+		return b
+	}
+
+	readAll := func(in []byte, duration float32) []byte {
+		out, err := io.ReadAll(patchFLACDuration(io.NopCloser(bytes.NewReader(in)), duration))
+		Expect(err).ToNot(HaveOccurred())
+		return out
+	}
+
+	BeforeEach(func() {
+		var err error
+		fileFLAC, err = os.ReadFile("tests/fixtures/test.flac")
+		Expect(err).ToNot(HaveOccurred())
+		Expect(readSampleRate(fileFLAC)).To(Equal(44100)) // specs below hard-code this rate
+	})
+
+	It("fills in total_samples from the duration", func() {
+		out := readAll(pipedFLAC(), 1.0)
+		Expect(readTotalSamples(out)).To(Equal(uint64(44100)))
+	})
+
+	It("takes the sample rate from the header, not from the source file", func() {
+		in := pipedFLAC()
+		// Rewrite the header's rate to 48000, as -ar would.
+		in[18], in[19] = 0x0B, 0xB8
+		in[20] &= 0x0F
+
+		out := readAll(in, 2.0)
+
+		Expect(readSampleRate(out)).To(Equal(48000))
+		Expect(readTotalSamples(out)).To(Equal(uint64(96000)))
+	})
+
+	It("rounds to the nearest sample rather than truncating", func() {
+		// float32(0.7)*44100 is 30869.9995, so truncation would lose a sample.
+		out := readAll(pipedFLAC(), 0.7)
+		Expect(readTotalSamples(out)).To(Equal(uint64(30870)))
+	})
+
+	It("passes through when the duration overflows the 36-bit field", func() {
+		in := pipedFLAC()
+		Expect(readAll(in, 2e6)).To(Equal(in))
+	})
+
+	It("leaves everything after the header untouched", func() {
+		in := pipedFLAC()
+		out := readAll(in, 1.0)
+		Expect(out).To(HaveLen(len(in)))
+		Expect(out[26:]).To(Equal(in[26:]))
+		Expect(out[:18]).To(Equal(in[:18]))
+	})
+
+	It("leaves an already-populated total_samples alone", func() {
+		out := readAll(fileFLAC, 99.0)
+		Expect(out).To(Equal(fileFLAC))
+	})
+
+	It("passes through a stream that is not FLAC", func() {
+		in := []byte("ID3\x04\x00\x00\x00\x00\x00\x00 not a flac stream at all, just bytes")
+		Expect(readAll(in, 1.0)).To(Equal(in))
+	})
+
+	It("passes through when the first metadata block is not STREAMINFO", func() {
+		in := pipedFLAC()
+		in[4] = 0x04 // VORBIS_COMMENT
+		Expect(readAll(in, 1.0)).To(Equal(in))
+	})
+
+	It("passes through a stream shorter than the STREAMINFO fields it patches", func() {
+		in := pipedFLAC()[:20]
+		Expect(readAll(in, 1.0)).To(Equal(in))
+	})
+
+	It("passes through an empty stream", func() {
+		Expect(readAll(nil, 1.0)).To(BeEmpty())
+	})
+
+	It("passes through when the duration is zero or negative", func() {
+		in := pipedFLAC()
+		Expect(readAll(in, 0)).To(Equal(in))
+		Expect(readAll(in, -5)).To(Equal(in))
+	})
+
+	It("passes through when the header declares no sample rate", func() {
+		in := pipedFLAC()
+		in[18], in[19] = 0, 0
+		in[20] &= 0x0F
+		Expect(readAll(in, 1.0)).To(Equal(in))
+	})
+
+	It("propagates a read error from the underlying stream", func() {
+		_, err := io.ReadAll(patchFLACDuration(io.NopCloser(io.MultiReader(
+			bytes.NewReader(pipedFLAC()[:10]), &errReader{})), 1.0))
+		Expect(err).To(MatchError("boom"))
+	})
+
+	It("closes the underlying stream", func() {
+		c := &closeSpy{Reader: bytes.NewReader(pipedFLAC())}
+		Expect(patchFLACDuration(c, 1.0).Close()).To(Succeed())
+		Expect(c.closed).To(BeTrue())
+	})
+})
+
+type errReader struct{}
+
+func (e *errReader) Read([]byte) (int, error) { return 0, errors.New("boom") }
+
+type closeSpy struct {
+	io.Reader
+	closed bool
+}
+
+func (c *closeSpy) Close() error { c.closed = true; return nil }

core/stream/media_streamer.go

@@ -268,6 +268,7 @@ func NewTranscodingCache() TranscodingCache {
 				BitDepth:   job.bitDepth,
 				Channels:   job.channels,
 				Offset:     job.offset,
+				Duration:   job.mf.Duration,
 			})
 			if err != nil {
 				release()