fm-rds-tx/internal/ingest/decoder/fallback/ffmpeg.go

package fallback

import (
	"context"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"os/exec"
	"strings"
	"sync"
	"time"

	"github.com/jan/fm-rds-tx/internal/ingest"
	"github.com/jan/fm-rds-tx/internal/ingest/decoder"
)

type FFmpegDecoder struct{}

func NewFFmpeg() *FFmpegDecoder { return &FFmpegDecoder{} }

func (d *FFmpegDecoder) Name() string { return "ffmpeg-fallback" }

func (d *FFmpegDecoder) DecodeStream(ctx context.Context, r io.Reader, meta decoder.StreamMeta, emit func(ingest.PCMChunk) error) error {
	if r == nil {
		return fmt.Errorf("%w: ffmpeg decoder stream reader is nil", decoder.ErrUnsupported)
	}
	if emit == nil {
		return fmt.Errorf("%w: ffmpeg decoder emit callback is nil", decoder.ErrUnsupported)
	}

	sampleRate := meta.SampleRateHz
	if sampleRate <= 0 {
		sampleRate = 44100
	}
	channels := meta.Channels
	if channels <= 0 {
		channels = 2
	}

	cmd := exec.CommandContext(ctx,
		"ffmpeg",
		"-hide_banner", "-loglevel", "error",
		"-i", "pipe:0",
		"-f", "s16le",
		"-acodec", "pcm_s16le",
		"-ac", fmt.Sprintf("%d", channels),
		"-ar", fmt.Sprintf("%d", sampleRate),
		"pipe:1",
	)

	stdin, err := cmd.StdinPipe()
	if err != nil {
		return fmt.Errorf("ffmpeg stdin pipe: %w", err)
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return fmt.Errorf("ffmpeg stdout pipe: %w", err)
	}
	stderr, err := cmd.StderrPipe()
	if err != nil {
		return fmt.Errorf("ffmpeg stderr pipe: %w", err)
	}

	if err := cmd.Start(); err != nil {
		if errorsIsNotFound(err) {
			return fmt.Errorf("%w: ffmpeg executable not found in PATH", decoder.ErrUnsupported)
		}
		return fmt.Errorf("ffmpeg start: %w", err)
	}

	errCh := make(chan error, 2)
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		defer wg.Done()
		_, copyErr := io.Copy(stdin, r)
		_ = stdin.Close()
		if copyErr != nil && ctx.Err() == nil {
			errCh <- fmt.Errorf("ffmpeg stdin copy: %w", copyErr)
		}
	}()

	// DEADLOCK FIX: stderr and stdout must be drained concurrently.
	// Reading stderr synchronously before readPCM means ffmpeg blocks when
	// stdout's pipe buffer fills (typically 64KB), which prevents it from
	// closing stderr, which prevents ReadAll from returning — deadlock.
	var stderrData []byte
	wg.Add(1)
	go func() {
		defer wg.Done()
		stderrData, _ = io.ReadAll(stderr)
	}()
	readErr := d.readPCM(ctx, stdout, sampleRate, channels, meta.SourceID, emit)
	waitErr := cmd.Wait()
	wg.Wait()
	close(errCh)

	for e := range errCh {
		if e != nil {
			return e
		}
	}
	if readErr != nil {
		return readErr
	}
	if waitErr != nil && ctx.Err() == nil {
		msg := strings.TrimSpace(string(stderrData))
		if msg != "" {
			return fmt.Errorf("ffmpeg decode: %w (%s)", waitErr, msg)
		}
		return fmt.Errorf("ffmpeg decode: %w", waitErr)
	}
	return nil
}

func (d *FFmpegDecoder) readPCM(ctx context.Context, r io.Reader, sampleRate, channels int, sourceID string, emit func(ingest.PCMChunk) error) error {
	const chunkFrames = 1024
	frameBytes := channels * 2
	buf := make([]byte, chunkFrames*frameBytes)
	seq := uint64(0)
	for {
		select {
		case <-ctx.Done():
			return nil
		default:
		}
		n, err := io.ReadAtLeast(r, buf, frameBytes)
		if err != nil {
			if err == io.EOF || err == io.ErrUnexpectedEOF {
				if n > 0 {
					if emitErr := emitPCM(buf[:n], seq, sampleRate, channels, sourceID, emit); emitErr != nil {
						return emitErr
					}
				}
				return nil
			}
			return fmt.Errorf("ffmpeg read pcm: %w", err)
		}
		if emitErr := emitPCM(buf[:n], seq, sampleRate, channels, sourceID, emit); emitErr != nil {
			return emitErr
		}
		seq++
	}
}

func emitPCM(data []byte, seq uint64, sampleRate, channels int, sourceID string, emit func(ingest.PCMChunk) error) error {
	samples := make([]int32, 0, len(data)/2)
	for i := 0; i+1 < len(data); i += 2 {
		v := int16(binary.LittleEndian.Uint16(data[i : i+2]))
		samples = append(samples, int32(v)<<16)
	}
	return emit(ingest.PCMChunk{
		Samples:      samples,
		Channels:     channels,
		SampleRateHz: sampleRate,
		Sequence:     seq,
		Timestamp:    time.Now(),
		SourceID:     sourceID,
	})
}

func errorsIsNotFound(err error) bool {
	var execErr *exec.Error
	return err != nil && (errors.As(err, &execErr) || strings.Contains(strings.ToLower(err.Error()), "executable file not found"))
}