silo-server/internal/playback/prepare_file.go

package playback

import (
	"context"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"time"

	"github.com/Silo-Server/silo-server/internal/models"
)

// PrepareTarget describes the concrete encode target for a prepared download
// artifact (remux or transcode-to-file).
type PrepareTarget struct {
	Container         string
	CodecVideo        string // "copy" for remux, else an encoder codec (e.g. "h264")
	CodecAudio        string // "copy" or "aac"
	Resolution        string // "" = keep source resolution (no scale)
	AudioTrackIndex   int
	TargetBitrateKbps int // 0 = encoder default/CRF; >0 caps video bitrate
}

// ResolvePrepareTarget computes the encode target for a remux/transcode download
// of file, reusing Resolve so a download's encoding matches the streaming
// decision for the same client (no duplicated codec logic).
//
//   - remux: copy video; copy audio unless the client can't decode it (then AAC);
//     keep source resolution.
//   - transcode: H.264/AAC, downscaled to the client's max resolution when the
//     source exceeds it.
func ResolvePrepareTarget(file *models.MediaFile, format string, caps ClientCapabilities, settings AdminSettings) PrepareTarget {
	t := PrepareTarget{Container: "mp4", AudioTrackIndex: -1}
	decision := Resolve(file, caps, settings)
	if format == "remux" {
		t.CodecVideo = "copy"
		if decision.TranscodeAudio {
			t.CodecAudio = "aac"
		} else {
			t.CodecAudio = "copy"
		}
		return t
	}
	// transcode
	t.CodecVideo = "h264"
	t.CodecAudio = "aac"
	if caps.MaxResolution != "" && resolutionOrder(file.Resolution) > resolutionOrder(caps.MaxResolution) {
		t.Resolution = caps.MaxResolution
	}
	return t
}

// PrepareFile encodes a single finalized MP4 (with a relocated moov atom via
// -movflags +faststart, enabling clean seek/resume) from opts.InputPath. It
// writes to outputPath+".part" and atomically renames on success so a partial
// file is never observable at outputPath. The call blocks until ffmpeg exits.
func PrepareFile(ctx context.Context, opts TranscodeOpts, outputPath string) error {
	if outputPath == "" {
		return fmt.Errorf("prepare-file: empty output path")
	}
	if err := os.MkdirAll(filepath.Dir(outputPath), 0o755); err != nil {
		return fmt.Errorf("prepare-file: create output dir: %w", err)
	}
	partPath := outputPath + ".part"
	// A reclaimed job overwrites its own .part; ffmpeg -y handles that, but remove
	// any stale partial first so a failed prior attempt can't be mistaken for output.
	_ = os.Remove(partPath)

	// Resolve a multi-device hw_device list to one concrete GPU for this
	// encode. Run blocks until ffmpeg exits, so the deferred release fires at
	// exactly the process-exit boundary.
	opts.HWAccel = resolveEffectiveTranscodeHWAccel(opts)
	hwDevice, releaseHWDevice := AcquireHWDevice(opts.HWDevice, opts.HWAccel)
	opts.HWDevice = hwDevice
	defer releaseHWDevice()

	args := buildPrepareFileArgs(opts, partPath)
	bin := opts.FFmpegPath
	if bin == "" {
		bin = ffmpegBinary()
	}

	cmd := exec.CommandContext(ctx, bin, args...)
	stderr := newBoundedTailBuffer(stderrTailMaxBytes)
	cmd.Stderr = stderr
	cmd.WaitDelay = 3 * time.Second

	if err := cmd.Run(); err != nil {
		_ = os.Remove(partPath)
		if tail := truncateStderr(stderr.String()); tail != "" {
			return fmt.Errorf("%w: %w (stderr: %s)", ErrTranscodeFailed, err, tail)
		}
		return fmt.Errorf("%w: %w", ErrTranscodeFailed, err)
	}

	if err := os.Rename(partPath, outputPath); err != nil {
		_ = os.Remove(partPath)
		return fmt.Errorf("prepare-file: finalize artifact: %w", err)
	}
	return nil
}

// buildPrepareFileArgs constructs single-file ffmpeg args. It mirrors
// buildFFmpegArgs' input/stream/codec/audio/subtitle handling but emits one
// faststart MP4 instead of HLS segments. Full-file output needs no seek or
// segment-boundary keyframes.
func buildPrepareFileArgs(opts TranscodeOpts, outputPath string) []string {
	opts.HWAccel = resolveEffectiveTranscodeHWAccel(opts)
	isVideoCopy := opts.TargetCodecVideo == "copy"
	isAudioCopy := opts.TargetCodecAudio == "copy"

	args := []string{"-nostdin", "-hide_banner", "-loglevel", "error"}

	if !isVideoCopy {
		args = appendHWAccelArgs(args, opts)
	}
	args = append(args,
		"-fflags", "+genpts+fastseek",
		"-analyzeduration", "3000000",
		"-probesize", "5000000",
	)
	args = append(args, "-i", opts.InputPath)
	args = append(args, "-map_metadata", "-1", "-map_chapters", "-1")
	args = appendStreamSelectionArgs(args, opts)

	if isVideoCopy {
		args = append(args, "-c:v", "copy")
	} else {
		args = appendVideoArgs(args, opts)
	}
	if isVideoCopy && !isAudioCopy {
		args = append(args, "-threads", "1", "-filter_threads", "1", "-filter_complex_threads", "1")
	}
	args = appendAudioArgs(args, opts)

	if !isVideoCopy {
		args = appendVideoFilterArgs(args, opts)
	}

	// One finalized MP4. +faststart relocates the moov atom in a finalization pass
	// (impossible over a pure pipe) so the file is cleanly seekable and resumable.
	args = append(args, "-movflags", "+faststart", "-f", "mp4", "-y", outputPath)
	return args
}