BizGaze_Remote/server/media.js

'use strict';
//
// media.js — web playback renditions for uploaded videos.
//
// WHY THIS EXISTS (measured, not guessed):
//   Real uploads on this box were probed at 19.4 Mbps and 19.0 Mbps (1080p screen recordings) and
//   3.6 Mbps (phone portrait). To play a 19 Mbps file the client must SUSTAIN a 19 Mbps download for
//   the whole clip; no mobile link does, so the <video> buffer drains every few seconds and you get
//   the classic "buffers, plays, buffers, plays". Server disk and CPU were idle throughout — the
//   bottleneck is the media, not the delivery.
//   Separately, phone MP4s often store `moov` AFTER `mdat` (not faststart), so the player has to
//   fetch the tail before it can begin at all.
//
// WHAT WE DO:
//   Leave the uploaded bytes untouched — that is what the download button serves, at full quality.
//   Alongside it build <id>.web.mp4: longest side capped at 1280, ~2.5 Mbps ceiling, +faststart.
//   /stream/<id> prefers that rendition and falls back to the original until it is ready, so a video
//   is never unplayable while it transcodes.
//
const fs = require('fs');
const path = require('path');
const { execFile } = require('child_process');
const { UPLOADS_DIR } = require('./config');

const MAX_CONCURRENT = 2;        // transcoding is never urgent; leave the cores to the app
const OK_BITRATE = 2500000;      // ≤2.5 Mbps streams fine over mobile
const OK_DIMENSION = 1280;       // ...provided it isn't oversized as well
const PROBE_TIMEOUT = 20000;
const XCODE_TIMEOUT = 30 * 60 * 1000;

let running = 0;
const queue = [];
const pending = new Set();       // ids queued or transcoding right now
const failed = new Set();        // ffmpeg couldn't handle it — don't retry forever

const webPath = (id) => path.join(UPLOADS_DIR, id + '.web.mp4');

function hasWebRendition(id) {
  try { return fs.statSync(webPath(id)).size > 0; } catch (e) { return false; }
}

// Walk the top-level MP4 box headers. If `mdat` comes before `moov` the index lives at the end of
// the file and the player must seek there before it can start — that is what +faststart fixes.
function isFastStart(fp) {
  let fd;
  try {
    fd = fs.openSync(fp, 'r');
    const buf = Buffer.alloc(4096);
    const read = fs.readSync(fd, buf, 0, 4096, 0);
    let off = 0;
    while (off + 8 <= read) {
      let size = buf.readUInt32BE(off);
      const type = buf.toString('latin1', off + 4, off + 8);
      if (type === 'moov') return true;
      if (type === 'mdat') return false;
      if (size === 1) {                                   // 64-bit largesize follows the header
        if (off + 16 > read) return true;
        size = Number(buf.readBigUInt64BE(off + 8));
      } else if (size === 0) return true;                 // box runs to EOF
      if (size < 8) return true;                          // malformed — leave it alone
      off += size;
    }
    return true;                                          // couldn't tell; assume fine
  } catch (e) {
    return true;
  } finally {
    if (fd !== undefined) { try { fs.closeSync(fd); } catch (e) {} }
  }
}

function probe(fp, cb) {
  execFile('ffprobe', ['-v', 'error', '-select_streams', 'v:0',
    '-show_entries', 'stream=width,height,codec_name',
    '-show_entries', 'format=bit_rate,duration',
    '-of', 'json', fp], { timeout: PROBE_TIMEOUT, maxBuffer: 1 << 20 }, (err, stdout) => {
    if (err) return cb(err);
    let info;
    try { info = JSON.parse(stdout); } catch (e) { return cb(e); }
    const s = (info.streams || [])[0], f = info.format || {};
    if (!s || !s.width) return cb(new Error('no video stream'));
    cb(null, {
      width: +s.width, height: +s.height, codec: s.codec_name || '',
      bitrate: +f.bit_rate || 0, duration: +f.duration || 0,
    });
  });
}

// Cap the LONGEST side at 1280 (so 1920x1080 → 1280x720, and portrait 1080x2340 → 591x1280) while
// preserving aspect. force_divisible_by=2 keeps dimensions legal for H.264.
const SCALE = 'scale=w=' + OK_DIMENSION + ':h=' + OK_DIMENSION +
  ':force_original_aspect_ratio=decrease:force_divisible_by=2';

function buildArgs(src, out, meta) {
  const maxDim = Math.max(meta.width, meta.height);
  const lightEnough = meta.bitrate > 0 && meta.bitrate <= OK_BITRATE && maxDim <= OK_DIMENSION;
  // Already small enough and only the atom order is wrong → remux, no re-encode. Seconds, not minutes.
  if (lightEnough && /^(h264|avc1)$/i.test(meta.codec)) {
    return ['-y', '-i', src, '-c', 'copy', '-movflags', '+faststart', '-f', 'mp4', out];
  }
  return ['-y', '-i', src,
    '-map', '0:v:0', '-map', '0:a:0?',                    // audio optional — silent clips exist
    '-vf', SCALE,
    '-c:v', 'libx264', '-preset', 'veryfast', '-profile:v', 'high', '-level', '4.0',
    '-crf', '24', '-maxrate', '2500k', '-bufsize', '5000k',
    '-g', '48', '-pix_fmt', 'yuv420p',                    // 2s keyframes: smooth seeking
    '-c:a', 'aac', '-b:a', '128k', '-ac', '2',
    '-movflags', '+faststart', '-f', 'mp4', out];
}

function pump() {
  while (running < MAX_CONCURRENT && queue.length) {
    const id = queue.shift();
    running++;
    transcode(id, () => { running--; pending.delete(id); pump(); });
  }
}

function transcode(id, done) {
  const src = path.join(UPLOADS_DIR, id);
  const out = webPath(id);
  const tmp = out + '.part';
  if (!src.startsWith(UPLOADS_DIR)) return done();
  fs.stat(src, (e, srcStat) => {
    if (e) return done();
    probe(src, (perr, meta) => {
      if (perr) { failed.add(id); return done(); }
      const maxDim = Math.max(meta.width, meta.height);
      // Nothing to gain: already light, already sized, already faststart.
      if (meta.bitrate > 0 && meta.bitrate <= OK_BITRATE && maxDim <= OK_DIMENSION && isFastStart(src)) {
        failed.add(id); // "no rendition needed" — same effect: stop reconsidering it
        return done();
      }
      execFile('ffmpeg', buildArgs(src, tmp, meta), { timeout: XCODE_TIMEOUT, maxBuffer: 1 << 20 }, (xerr) => {
        if (xerr) {
          try { fs.unlinkSync(tmp); } catch (_) {}
          failed.add(id);
          return done();
        }
        let outStat;
        try { outStat = fs.statSync(tmp); } catch (_) { failed.add(id); return done(); }
        // A rendition bigger than the original helps nobody — throw it away and stream the original.
        if (!outStat.size || outStat.size >= srcStat.size) {
          try { fs.unlinkSync(tmp); } catch (_) {}
          failed.add(id);
          return done();
        }
        try { fs.renameSync(tmp, out); } catch (_) { try { fs.unlinkSync(tmp); } catch (__) {} }
        done();
      });
    });
  });
}

// Queue a freshly uploaded (or first-played) video. Cheap and idempotent: safe to call on every
// /stream hit, which is also how pre-existing uploads get backfilled.
function ensureWebRendition(id, mime) {
  if (!/^video\//.test(mime || '')) return;
  if (pending.has(id) || failed.has(id) || hasWebRendition(id)) return;
  pending.add(id);
  queue.push(id);
  pump();
}

// Drop derived files when the attachment goes away.
function dropDerived(id) {
  for (const p of [webPath(id), webPath(id) + '.part', path.join(UPLOADS_DIR, id + '.thumb.jpg')]) {
    try { fs.unlinkSync(p); } catch (e) {}
  }
  pending.delete(id); failed.delete(id);
}

// One-off catch-up for videos uploaded before this module existed (and after a restore). Renditions
// persist on the data volume, so on a normal restart this finds nothing to do and costs one query.
// Deliberately delayed and rate-limited by the same 2-at-a-time queue — boot must not stall on it.
function backfill() {
  setTimeout(() => {
    let rows = [];
    try { rows = require('./repos').attachments.allVideos(); } catch (e) { return; }
    let queued = 0;
    for (const r of rows) {
      if (hasWebRendition(r.id)) continue;
      try { if (!fs.statSync(path.join(UPLOADS_DIR, r.id)).size) continue; } catch (e) { continue; }
      ensureWebRendition(r.id, r.mime); queued++;
    }
    if (queued) console.log('[media] backfilling streaming renditions for ' + queued + ' video(s)');
  }, 15000).unref();
}

module.exports = { ensureWebRendition, hasWebRendition, webPath, dropDerived, backfill };