feat: add configurable transcoding cancellation (#4411)

* feat: add configurable transcoding cancellation

Implemented EnableTranscodingCancellation configuration option to control whether
FFmpeg transcoding processes can be interrupted when client requests are cancelled.
This addresses resource management issues on low-power hardware where transcoding
processes would accumulate and cause CPU spikes.

Key changes:
- Added EnableTranscodingCancellation bool to configuration (default: false)
- Added CLI flag --enabletranscodingcancellation and TOML/env support
- Modified FFmpeg package to always use exec.CommandContext for consistency
- Implemented conditional context handling in NewTranscodingCache function
- When enabled: uses request context directly (allows cancellation)
- When disabled: uses background context with request metadata preserved
- Added comprehensive tests for both FFmpeg and transcoding layers
- Maintained backward compatibility with existing behavior as default

The implementation follows proper layered architecture with policy decisions
at the media streaming layer and execution utilities remaining focused on
their core responsibilities.

Signed-off-by: Deluan <deluan@navidrome.org>

* test: refactor FFmpeg context cancellation tests for improved clarity and reliability

Signed-off-by: Deluan <deluan@navidrome.org>

* test: reset FFmpeg initialization

Signed-off-by: Deluan <deluan@navidrome.org>

* test: improve FFmpeg context cancellation tests for cross-platform compatibility

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/ffmpeg/ffmpeg.go

@@ -112,7 +112,7 @@ func (e *ffmpeg) start(ctx context.Context, args []string) (io.ReadCloser, error
 	log.Trace(ctx, "Executing ffmpeg command", "cmd", args)
 	j := &ffCmd{args: args}
 	j.PipeReader, j.out = io.Pipe()
-	err := j.start()
+	err := j.start(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -127,8 +127,8 @@ type ffCmd struct {
 	cmd  *exec.Cmd
 }
 
-func (j *ffCmd) start() error {
-	cmd := exec.Command(j.args[0], j.args[1:]...) // #nosec
+func (j *ffCmd) start(ctx context.Context) error {
+	cmd := exec.CommandContext(ctx, j.args[0], j.args[1:]...) // #nosec
 	cmd.Stdout = j.out
 	if log.IsGreaterOrEqualTo(log.LevelTrace) {
 		cmd.Stderr = os.Stderr

core/ffmpeg/ffmpeg_test.go

@@ -1,7 +1,11 @@
 package ffmpeg
 
 import (
+	"context"
+	"runtime"
+	sync "sync"
 	"testing"
+	"time"
 
 	"github.com/navidrome/navidrome/log"
 	"github.com/navidrome/navidrome/tests"
@@ -65,4 +69,98 @@ var _ = Describe("ffmpeg", func() {
 			Expect(args).To(Equal([]string{"/usr/bin/with spaces/ffmpeg.exe", "-i", "one.mp3", "-f", "ffmetadata"}))
 		})
 	})
+
+	Describe("FFmpeg", func() {
+		Context("when FFmpeg is available", func() {
+			var ff FFmpeg
+
+			BeforeEach(func() {
+				ffOnce = sync.Once{}
+				ff = New()
+				// Skip if FFmpeg is not available
+				if !ff.IsAvailable() {
+					Skip("FFmpeg not available on this system")
+				}
+			})
+
+			It("should interrupt transcoding when context is cancelled", func() {
+				ctx, cancel := context.WithTimeout(GinkgoT().Context(), 5*time.Second)
+				defer cancel()
+
+				// Use a command that generates audio indefinitely
+				// -f lavfi uses FFmpeg's built-in audio source
+				// -t 0 means no time limit (runs forever)
+				command := "ffmpeg -f lavfi -i sine=frequency=1000:duration=0 -f mp3 -"
+
+				// The input file is not used here, but we need to provide a valid path to the Transcode function
+				stream, err := ff.Transcode(ctx, command, "tests/fixtures/test.mp3", 128, 0)
+				Expect(err).ToNot(HaveOccurred())
+				defer stream.Close()
+
+				// Read some data first to ensure FFmpeg is running
+				buf := make([]byte, 1024)
+				_, err = stream.Read(buf)
+				Expect(err).ToNot(HaveOccurred())
+
+				// Cancel the context
+				cancel()
+
+				// Next read should fail due to cancelled context
+				_, err = stream.Read(buf)
+				Expect(err).To(HaveOccurred())
+			})
+
+			It("should handle immediate context cancellation", func() {
+				ctx, cancel := context.WithCancel(GinkgoT().Context())
+				cancel() // Cancel immediately
+
+				// This should fail immediately
+				_, err := ff.Transcode(ctx, "ffmpeg -i %s -f mp3 -", "tests/fixtures/test.mp3", 128, 0)
+				Expect(err).To(MatchError(context.Canceled))
+			})
+		})
+
+		Context("with mock process behavior", func() {
+			var longRunningCmd string
+			BeforeEach(func() {
+				// Use a long-running command for testing cancellation
+				switch runtime.GOOS {
+				case "windows":
+					// Use PowerShell's Start-Sleep
+					ffmpegPath = "powershell"
+					longRunningCmd = "powershell -Command Start-Sleep -Seconds 10"
+				default:
+					// Use sleep on Unix-like systems
+					ffmpegPath = "sleep"
+					longRunningCmd = "sleep 10"
+				}
+			})
+
+			It("should terminate the underlying process when context is cancelled", func() {
+				ff := New()
+				ctx, cancel := context.WithTimeout(GinkgoT().Context(), 5*time.Second)
+				defer cancel()
+
+				// Start a process that will run for a while
+				stream, err := ff.Transcode(ctx, longRunningCmd, "tests/fixtures/test.mp3", 0, 0)
+				Expect(err).ToNot(HaveOccurred())
+				defer stream.Close()
+
+				// Give the process time to start
+				time.Sleep(50 * time.Millisecond)
+
+				// Cancel the context
+				cancel()
+
+				// Try to read from the stream, which should fail
+				buf := make([]byte, 100)
+				_, err = stream.Read(buf)
+				Expect(err).To(HaveOccurred(), "Expected stream to be closed due to process termination")
+
+				// Verify the stream is closed by attempting another read
+				_, err = stream.Read(buf)
+				Expect(err).To(HaveOccurred())
+			})
+		})
+	})
 })

core/media_streamer.go

@@ -204,7 +204,20 @@ func NewTranscodingCache() TranscodingCache {
 				log.Error(ctx, "Error loading transcoding command", "format", job.format, err)
 				return nil, os.ErrInvalid
 			}
-			out, err := job.ms.transcoder.Transcode(ctx, t.Command, job.filePath, job.bitRate, job.offset)
+
+			// Choose the appropriate context based on EnableTranscodingCancellation configuration.
+			// This is where we decide whether transcoding processes should be cancellable or not.
+			var transcodingCtx context.Context
+			if conf.Server.EnableTranscodingCancellation {
+				// Use the request context directly, allowing cancellation when client disconnects
+				transcodingCtx = ctx
+			} else {
+				// Use background context with request values preserved.
+				// This prevents cancellation but maintains request metadata (user, client, etc.)
+				transcodingCtx = request.AddValues(context.Background(), ctx)
+			}
+
+			out, err := job.ms.transcoder.Transcode(transcodingCtx, t.Command, job.filePath, job.bitRate, job.offset)
 			if err != nil {
 				log.Error(ctx, "Error starting transcoder", "id", job.mf.ID, err)
 				return nil, os.ErrInvalid