silk/server-go/internal/service/transcode.go

package service

import (
	"bufio"
	"context"
	"fmt"
	"io"
	"log/slog"
	"os/exec"

	"silk-server-go/internal/model"

	"gorm.io/gorm"
)

// TranscodeService ffmpeg 实时转码服务
type TranscodeService struct {
	db  *gorm.DB
	s3  *S3Service
	sem chan struct{} // 信号量，限制并发转码数为 4
}

// NewTranscodeService 创建转码服务，信号量默认容量 4
func NewTranscodeService(db *gorm.DB, s3 *S3Service) *TranscodeService {
	return &TranscodeService{
		db:  db,
		s3:  s3,
		sem: make(chan struct{}, 4),
	}
}

// StreamLive 通过 ffmpeg 实时转码直播流（H264 re-encode for browser compatibility）
func (s *TranscodeService) StreamLive(sourceURL string, writer io.Writer) error {
	s.sem <- struct{}{}
	defer func() { <-s.sem }()

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	cmd := exec.CommandContext(ctx, "ffmpeg",
		"-i", sourceURL,
		"-c:v", "libx264",
		"-preset", "ultrafast",
		"-tune", "zerolatency",
		"-b:v", "1M",
		"-maxrate", "1.5M",
		"-bufsize", "1M",
		"-g", "30",
		"-an",
		"-f", "flv",
		"pipe:1",
	)

	stdoutPipe, err := cmd.StdoutPipe()
	if err != nil {
		return fmt.Errorf("创建 stdout 管道失败: %w", err)
	}
	stderrPipe, err := cmd.StderrPipe()
	if err != nil {
		return fmt.Errorf("创建 stderr 管道失败: %w", err)
	}

	if err := cmd.Start(); err != nil {
		return fmt.Errorf("启动 ffmpeg 失败: %w", err)
	}

	defer func() {
		if cmd.ProcessState == nil || !cmd.ProcessState.Exited() {
			_ = cmd.Process.Kill()
		}
	}()

	go func() {
		scanner := bufio.NewScanner(stderrPipe)
		for scanner.Scan() {
			slog.Info("ffmpeg-live", "msg", scanner.Text())
		}
	}()

	if _, err := io.Copy(writer, stdoutPipe); err != nil {
		cancel()
		return fmt.Errorf("流式传输失败: %w", err)
	}

	if err := cmd.Wait(); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("ffmpeg 异常退出: %w", err)
	}

	return nil
}

// StreamClip 通过 ffmpeg 实时转码视频片段并流式写入 writer
// 流程：查库获取 S3 信息 → 生成 presigned URL → ffmpeg -c copy 转封装为 fragmented MP4 → 管道输出
func (s *TranscodeService) StreamClip(clipId string, writer io.Writer) error {
	// 1. 查库获取 clip 的 s3Bucket + s3Key
	var clip model.VideoClip
	if err := s.db.Where("id = ?", clipId).First(&clip).Error; err != nil {
		return fmt.Errorf("视频片段不存在: %w", err)
	}
	if clip.S3Bucket == nil || clip.S3Key == nil || *clip.S3Bucket == "" || *clip.S3Key == "" {
		return fmt.Errorf("该片段没有 S3 对象")
	}

	// 2. 生成 S3 presigned URL
	presignedURL, err := s.s3.GetPresignedURL(*clip.S3Bucket, *clip.S3Key)
	if err != nil {
		return fmt.Errorf("生成 presigned URL 失败: %w", err)
	}

	// 3. 获取信号量（并发限制 4）
	s.sem <- struct{}{}
	defer func() { <-s.sem }()

	// 4. 启动 ffmpeg: ffmpeg -i <url> -c copy -movflags frag_keyframe+empty_moov -f mp4 pipe:1
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	cmd := exec.CommandContext(ctx, "ffmpeg",
		"-i", presignedURL,
		"-c", "copy",
		"-movflags", "frag_keyframe+empty_moov",
		"-f", "mp4",
		"pipe:1",
	)

	stdoutPipe, err := cmd.StdoutPipe()
	if err != nil {
		return fmt.Errorf("创建 stdout 管道失败: %w", err)
	}
	stderrPipe, err := cmd.StderrPipe()
	if err != nil {
		return fmt.Errorf("创建 stderr 管道失败: %w", err)
	}

	if err := cmd.Start(); err != nil {
		return fmt.Errorf("启动 ffmpeg 失败: %w", err)
	}

	// 确保 ffmpeg 进程被清理（如果还在运行则 kill）
	defer func() {
		if cmd.ProcessState == nil || !cmd.ProcessState.Exited() {
			_ = cmd.Process.Kill()
		}
	}()

	// 5. 记录 ffmpeg stderr 日志
	go func() {
		scanner := bufio.NewScanner(stderrPipe)
		for scanner.Scan() {
			slog.Info("ffmpeg", "clipId", clipId, "msg", scanner.Text())
		}
	}()

	// 6. 流式传输：io.Copy(writer, ffmpeg.Stdout)
	if _, err := io.Copy(writer, stdoutPipe); err != nil {
		cancel() // 取消 context，终止 ffmpeg
		return fmt.Errorf("流式传输失败: %w", err)
	}

	// 7. 等待 ffmpeg 结束，检查退出码
	if err := cmd.Wait(); err != nil {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		return fmt.Errorf("ffmpeg 异常退出: %w", err)
	}

	return nil
}