Add clip extension from the last frame with FFmpeg stitching.

Let a finished player continue into a new MiniMax segment, then losslessly concat the parts and discard the intermediate files so the library keeps one extended MP4.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/utils/ffmpeg.ts

@@ -0,0 +1,156 @@
+import { existsSync, writeFileSync, readFileSync, unlinkSync } from 'node:fs'
+import { dirname, join } from 'node:path'
+import { spawn } from 'node:child_process'
+
+function collectError(stderr: string, fallback: string) {
+  const text = stderr.replace(/\r/g, '').trim()
+  const lines = text.split('\n').map(line => line.trim()).filter(Boolean)
+  return lines.slice(-6).join(' ') || fallback
+}
+
+export function runFfmpeg(args: string[], timeoutMs = 120000) {
+  return new Promise<void>((resolve, reject) => {
+    const proc = spawn('ffmpeg', args, { stdio: ['ignore', 'ignore', 'pipe'] })
+    let stderr = ''
+    proc.stderr?.on('data', (chunk: Buffer | string) => { stderr += String(chunk) })
+    const timer = setTimeout(() => {
+      try { proc.kill('SIGKILL') } catch { /* ignore */ }
+      reject(new Error(`FFmpeg timed out after ${Math.round(timeoutMs / 1000)}s`))
+    }, timeoutMs)
+    proc.on('error', (error) => {
+      clearTimeout(timer)
+      const message = (error as NodeJS.ErrnoException).code === 'ENOENT'
+        ? 'FFmpeg is not installed on this server'
+        : error.message
+      reject(new Error(message))
+    })
+    proc.on('close', (code) => {
+      clearTimeout(timer)
+      if (code === 0) resolve()
+      else reject(new Error(collectError(stderr, `FFmpeg exited with code ${code}`)))
+    })
+  })
+}
+
+function runFfprobe(args: string[], timeoutMs = 15000) {
+  return new Promise<string>((resolve, reject) => {
+    const proc = spawn('ffprobe', args, { stdio: ['ignore', 'pipe', 'pipe'] })
+    let out = ''
+    let err = ''
+    proc.stdout?.on('data', (chunk: Buffer | string) => { out += String(chunk) })
+    proc.stderr?.on('data', (chunk: Buffer | string) => { err += String(chunk) })
+    const timer = setTimeout(() => {
+      try { proc.kill('SIGKILL') } catch { /* ignore */ }
+      reject(new Error('ffprobe timed out'))
+    }, timeoutMs)
+    proc.on('error', (error) => {
+      clearTimeout(timer)
+      reject(error)
+    })
+    proc.on('close', (code) => {
+      clearTimeout(timer)
+      if (code === 0) resolve(out.trim())
+      else reject(new Error(collectError(err, `ffprobe exited with code ${code}`)))
+    })
+  })
+}
+
+function concatPath(filePath: string) {
+  return filePath.replace(/\\/g, '/').replace(/'/g, "'\\''")
+}
+
+export async function extractLastFrame(inputPath: string, outputPath: string) {
+  if (!existsSync(inputPath)) {
+    throw new Error('Source video is missing')
+  }
+  await runFfmpeg([
+    '-y',
+    '-sseof', '-1',
+    '-i', inputPath,
+    '-update', '1',
+    '-frames:v', '1',
+    '-q:v', '1',
+    outputPath
+  ], 30000)
+  if (!existsSync(outputPath)) {
+    throw new Error('FFmpeg did not write the last frame')
+  }
+}
+
+export async function probeHasAudio(filePath: string) {
+  try {
+    const out = await runFfprobe([
+      '-v', 'error',
+      '-select_streams', 'a:0',
+      '-show_entries', 'stream=codec_type',
+      '-of', 'csv=p=0',
+      filePath
+    ])
+    return out.includes('audio')
+  } catch {
+    return false
+  }
+}
+
+export async function concatMp4(part1Path: string, part2Path: string, outputPath: string) {
+  const listPath = join(dirname(outputPath), 'concat_list.txt')
+  writeFileSync(listPath, `file '${concatPath(part1Path)}'\nfile '${concatPath(part2Path)}'\n`)
+  try {
+    await runFfmpeg(['-y', '-f', 'concat', '-safe', '0', '-i', listPath, '-c', 'copy', outputPath], 180000)
+  } catch (copyError) {
+    const audio1 = await probeHasAudio(part1Path)
+    const audio2 = await probeHasAudio(part2Path)
+    try {
+      if (audio1 && audio2) {
+        await runFfmpeg([
+          '-y',
+          '-i', part1Path,
+          '-i', part2Path,
+          '-filter_complex', '[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]',
+          '-map', '[v]',
+          '-map', '[a]',
+          '-c:v', 'libx264',
+          '-preset', 'fast',
+          '-crf', '18',
+          '-pix_fmt', 'yuv420p',
+          '-c:a', 'aac',
+          '-movflags', '+faststart',
+          outputPath
+        ], 300000)
+      } else {
+        await runFfmpeg([
+          '-y',
+          '-i', part1Path,
+          '-i', part2Path,
+          '-filter_complex', '[0:v][1:v]concat=n=2:v=1:a=0[v]',
+          '-map', '[v]',
+          '-c:v', 'libx264',
+          '-preset', 'fast',
+          '-crf', '18',
+          '-pix_fmt', 'yuv420p',
+          '-movflags', '+faststart',
+          outputPath
+        ], 300000)
+      }
+    } catch (encodeError) {
+      const copyMessage = copyError instanceof Error ? copyError.message : String(copyError)
+      const encodeMessage = encodeError instanceof Error ? encodeError.message : String(encodeError)
+      throw new Error(`FFmpeg concat failed (${copyMessage}). Re-encode also failed (${encodeMessage}).`)
+    }
+  }
+  if (!existsSync(outputPath)) {
+    throw new Error('FFmpeg did not write the stitched video')
+  }
+}
+
+export async function stitchExtension(params: { part1Path: string; part2: Buffer; tmpDir: string }) {
+  const part2Path = join(params.tmpDir, 'part2.mp4')
+  const finalPath = join(params.tmpDir, 'final_stitched_video.mp4')
+  writeFileSync(part2Path, params.part2)
+  await concatMp4(params.part1Path, part2Path, finalPath)
+  const stitched = readFileSync(finalPath)
+  for (const extra of [part2Path, join(params.tmpDir, 'concat_list.txt'), join(params.tmpDir, 'last_frame.png')]) {
+    try { unlinkSync(extra) } catch { /* ignore */ }
+  }
+  return stitched
+}