How to correctly handle duplicate ftyp/moov in Pinterest streams (error Cannot read properties of undefined (reading 'mdia'))

[databorodata]

Hello! I ran into a problem when assembling a full-fledged MP4 from a fragmented Pinterest stream. We intercept all SourceBuffer.appendBuffer calls on the page and collect them into one ArrayBuffer, which we pass to mp4box.js for analysis and subsequent remultiplexing. MP4BOX stack trace:

TypeError: Cannot read properties of undefined (reading 'mdia')
    at ISOFile.updateSampleLists (libs/mp4box.all.js:7294)
    at ISOFile.appendBuffer (libs/mp4box.all.js:6332)

Contents of the captured buffer (first atoms):

offset=0    type=ftyp size=36
offset=36   type=free size=64
offset=100  type=moov size=914
offset=1014 type=ftyp size=36     ← second init-segment
offset=1050 type=free size=64
offset=1114 type=moov size=800    ← third init-segment
offset=1914 type=moof size=568
offset=2482 type=mdat size=200786
offset=203268 type=moof size=284
offset=203552 type=mdat size=16146
…

It can be seen that Pinterest sends ftyp+moov before each rather large fragment (in fact, each HLS segment begins with an init segment). After the third moov, mp4box tries to update the tables, but it encounters a trak without mdia and crashes.

Minimum code

// Chrome OffscreenDocument (Service Worker) – mp4box.js v2.1.0
import * as MP4Box from 'mp4box';

async function parseCapturedBuffer(arrayBuffer) {
  const mp4box = MP4Box.createFile(true);
  mp4box.onError = (e) => console.error('mp4box error', e);
  mp4box.onReady = (info) => {
    console.log('tracks', info.tracks.map(t => ({
      id: t.id,
      codec: t.codec,
      nb_samples: t.nb_samples,
    })));
  };

  const buf = arrayBuffer.slice(0);
  buf.fileStart = 0;
  mp4box.appendBuffer(buf);   // crashes here _ISOFile.updateSampleLists
  mp4box.flush();
}

What we have already checked
If you manually delete all repeated ftyp+moov and leave only the first init segment + all moof/mdat, the file is parsed correctly.
If you delay the start of the interception (wait 2 seconds before feeding the mp4box buffer), the situation does not change — Pinterest still sends init segments again, and mp4box crashes.
In the examples from examples/ mp4box, it is used on progressive MP4 or streams without a repeated init segment, so such a case is not shown there.

Questions
Is there in mp4box.is js a regular way to ignore (or automatically process) repeated ftyp/moov when we transfer the combined buffer?
Is it recommended to filter init segments manually before appendBuffer (leaving only the first ftyp+moov), or is there a more correct API for working with such a stream?
Is it possible to transfer a sequence of "pieces" to mp4box (each ArrayBuffer with its own fileStart) so that the library itself correctly builds the structure?

I use mp4box.js: 2.1.0 (mp4box.all.js)

I would be grateful for recommendations or hints on how to properly feed a stream with duplicate init segments without deleting them manually.

[DenizUgur]

Not sure if we've tested this but afaik mdia is mandatory, I have to check the spec again. In the meantime can you send me a file or zip with all the buffers you intercept sequentially?