starr-philproperties/services/ffmpeg.service.js

// services/ffmpeg.service.js
//
// Fast container remux for video assets whose original format loads slowly
// in the browser. Not a transcode — the video/audio streams are copied
// byte-for-byte (`-c copy`), only the container is swapped:
//
//   .mov  — often exported without "fast start" (common for OBS/QuickTime
//           screen recordings), which puts the moov atom — the index the
//           browser needs before it can render anything — at the END of the
//           file. Playback can't begin until that's reached.
//   .mkv  — same class of problem with its Cues index, plus native browser
//           support for Matroska demuxing/seeking is inconsistent to begin
//           with.
//
// Remuxing into a faststart .mp4 (moov moved to the front) makes both play
// exactly like this app's already-fast .mp4 uploads. .mp4/.mp3 are untouched
// — they don't have this problem.
//
// Reads directly from a presigned S3 URL — ffmpeg's own HTTP client handles
// that (same as ffprobe.service.js's probeUrl()), this process never buffers
// the original file. The mp4 muxer needs a seekable *output* to rewrite the
// moov atom after the fact, so the result is written to a local temp file —
// see services/assetTranscode.service.js for streaming that back to storage
// without buffering it into memory either.

const ffmpeg = require("fluent-ffmpeg");
const ffprobeStatic = require("ffprobe-static");
const os = require("os");
const path = require("path");
const fs = require("fs");
const crypto = require("crypto");

try {
  const { execSync } = require("child_process");
  execSync("which ffprobe", { stdio: "ignore" });
} catch {
  ffmpeg.setFfprobePath(ffprobeStatic.path);
}

const REMUXABLE_EXTENSIONS = new Set(["mov", "mkv"]);

function needsRemux(extension = "") {
  return REMUXABLE_EXTENSIONS.has((extension || "").toLowerCase());
}

function tempOutputPath() {
  return path.join(os.tmpdir(), `remux_${Date.now()}_${crypto.randomBytes(4).toString("hex")}.mp4`);
}

// input:  presigned GET URL for the original .mov/.mkv object
// output: local filesystem path to the remuxed .mp4 (caller owns cleanup)
//
// -map 0:v:0 -map 0:a:0? — take the first video stream and, if present, the
// first audio stream only. Drops subtitle/data streams some mkv/mov files
// carry, which the mp4 muxer either can't hold or chokes on.
// -max_muxing_queue_size — defensive bump; large copy-remuxes of files with
// bursty interleaving can otherwise hit "Too many packets buffered for
// output stream" and abort.
function remuxToFaststartMp4(inputUrl, { timeoutMs = 30 * 60 * 1000 } = {}) {
  const outputPath = tempOutputPath();

  return new Promise((resolve, reject) => {
    let settled = false;

    const command = ffmpeg(inputUrl)
      .outputOptions([
        "-map 0:v:0",
        "-map 0:a:0?",
        "-c:v copy",
        "-c:a copy",
        "-movflags +faststart",
        "-max_muxing_queue_size 9999",
      ])
      .format("mp4");

    const timer = setTimeout(() => {
      if (settled) return;
      settled = true;
      command.kill("SIGKILL");
      fs.promises.unlink(outputPath).catch(() => {});
      reject(new Error(`Remux timed out after ${timeoutMs}ms`));
    }, timeoutMs);

    command
      .on("error", (err) => {
        if (settled) return;
        settled = true;
        clearTimeout(timer);
        fs.promises.unlink(outputPath).catch(() => {});
        reject(err);
      })
      .on("end", () => {
        if (settled) return;
        settled = true;
        clearTimeout(timer);
        resolve(outputPath);
      })
      .save(outputPath);
  });
}

module.exports = { needsRemux, remuxToFaststartMp4 };