Error parsing DASHEventMessageBox (emsg) without message_data.

[damienlevin]

Have you read the FAQ and checked for duplicate open issues?:
Yes
What version of Shaka Player are you using?:
Shaka Player v2.4.3-uncompiled
Can you reproduce the issue with our latest release version?:
Yes
Can you reproduce the issue with the latest code from master?:
Yes
Are you using the demo app or your own custom app?:
Demo app
If custom app, can you reproduce the issue using our demo app?:
N/A
What browser and OS are you using?:
Chrome/Mac
What are the manifest and license server URIs?:
https://storage.googleapis.com/dlevin/dash_id3/manifest_id3_v0_no_data.mpd
What did you do?
Play a stream with emsg with empty message_data.
What did you expect to happen?
Working playback since according to the spec:

message_data: body of the message, which fills the remainder of the message box. This may be empty depending on the above information. The syntax and semantics of this field must be defined by the owner of the scheme identified in the scheme_id_uri field.

Note that it works with:
http://reference.dashif.org/dash.js/v2.9.0/samples/dash-if-reference-player/index.html

What actually happened?
Playback error and following stack:

  | shaka.media.StreamingEngine.parseEMSG_ | @ | streaming_engine.js:1780
  | shaka.util.Mp4Parser.parseNext | @ | mp4_parser.js:233
  | shaka.util.Mp4Parser.parse | @ | mp4_parser.js:161
  | shaka.media.StreamingEngine.append_ | @ | streaming_engine.js:1740
  | (anonymous) | @ | streaming_engine.js:1509
  | Promise.then (async) |   |  
  | shaka.media.StreamingEngine.fetchAndAppend_ | @ | streaming_engine.js:1507
  | shaka.media.StreamingEngine.update_ | @ | streaming_engine.js:1247
  | shaka.media.StreamingEngine.onUpdate_ | @ | streaming_engine.js:1103
  | setTimeout (async) |   |  
  | shaka.media.StreamingEngine.scheduleUpdate_ | @ | streaming_engine.js:2209
  | mediaSourceEngine.init.then.then | @ | streaming_engine.js:885
  | Promise.then (async) |   |  
  | shaka.media.StreamingEngine.initStreams_ | @ | streaming_engine.js:875
  | shaka.media.StreamingEngine.init | @ | streaming_engine.js:418
  | shaka.Player.load | @ | player.js:779
  | async function (async) |   |  
  | shaka.Player.load | @ | player.js:676
  | (anonymous) | @ | asset_section.js:323
  | Promise.then (async) |   |  
  | shakaDemo.load

[theodab]

I have an experimental CL that handles EMSG boxes with empty message_data. Easy enough.

Shaka Player is also having trouble skipping the MDAT box, though. Looking at the range requests we are making, it seems like it's only downloading part of the MDAT box for some reason? Like, it doesn't download the last ~250ish bytes, and our mp4 parser tries to seek past the end of the data and errors. The DASH-IF reference player also downloads that range, so apparently that is intentional.

[TheModMaker]

We will definitely support the empty data field; but we have to consider if we want to support this kinds of bad content or not.

[TheModMaker]

Actually I was incorrect, I didn't see the range requests. Your sidx is incorrect. @theodab was correct and we aren't requesting the whole mdat. The reason is because your sidx is slightly off, by 265 bytes.

The first entry in the sidx doesn't account for the 5 emsg boxes, so the referenced_size attribute is 265 bytes too small, so the first segment is too small. This plays fine in a streaming situation since the browser will just append the segments one after another, but if you seek, you'll get a segment in the middle of where it should be.

[damienlevin]

Very good catch on the bad segment - I'll try to update that sample content I posted.
Also, by doing a quick look at the code there doesn't seems to be support for emsg version 1, is that correct ? If so, I'm happy to create a sample stream and open an issue.

Thanks.

[TheModMaker]

The emsg box is defined by the DASH spec in ISO/IEC 23009-1; from the 2014 version, I only see version 0 specified. Could you provide a spec for emsg version 1? And yes, you can file another issue for that, we don't support it.

[damienlevin]

Until I file a ticket with a sample stream, you can look at CMAF spec: https://www.iso.org/standard/71975.html and https://www.iso.org/standard/68960.html