Bugfix/live start bugs

[cjpillsbury]

This PR will...

Resolve edge case issues in initial load for live content, which are more likely with LL-HLS:

• Accounts for maxFragLookUpTolerance when determining start position (not just maxBufferHole)
• Accounts for the possibility of incongruous segments on initial load of audio tracks when applying sliding/pdt and instead attempts to align based on Pantos §6.2.4. - Providing Variant Streams (https://datatracker.ietf.org/doc/html/draft-pantos-http-live-streaming#section-6.2.4):

If any Media Playlist in a Master Playlist contains an EXT-X-PROGRAM-DATE-TIME tag, then all Media Playlists in that Master Playlist MUST contain EXT-X-PROGRAM-DATE-TIME tags with consistent mappings of date and time to media timestamps.

Are there any points in the code the reviewer needs to double check?

If possible, confirm there aren't any discontinuity-related issues with new alignMediaPlaylistByPDT , or at least none that wouldn't also occur with the original alignPDT usage in onAudioTrackLoaded & onSubtitleTrackLoaded

Resolves issues:

#4302 (Startup on Live Streams sometimes hangs based on requesting the wrong audio and/or video segment)

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md (N/A)

[itsjamie]

If the problem exists on audio tracks, it will likely exist also on subtitle tracks that exist.

https://github.com/video-dev/hls.js/blob/master/src/controller/subtitle-stream-controller.ts#L254

I'd check if you need to add similar changes on the subtitle stream controller above to ensure these also align using the new mechanism.

[itsjamie]

Can you document the difference between alignByPDT and alignPDT above. Given the closeness in the implementation, having a description for them would be helpful.

That or a better name for each.

Perhaps alignByPDT could be something like; alignMediaPlaylistByPDT.

[cjpillsbury]

Yup can do both.

[cjpillsbury]

If the problem exists on audio tracks, it will likely exist also on subtitle tracks that exist.

https://github.com/video-dev/hls.js/blob/master/src/controller/subtitle-stream-controller.ts#L254

I'd check if you need to add similar changes on the subtitle stream controller above to ensure these also align using the new mechanism.

Good callout. Not sure if I have a stream available to test, but the reasoning tracks.

[cjpillsbury]

@itsjamie @robwalch PR approved but still blocked pending "test_functional_required (chrome-win_10)" task/check. Let me know if there's anything I can do to help get that unblocked.

[itsjamie]

I'm not sure, when I run the job, the sauce labs username/access key isn't present so the check is skipped. @tjenkinson thoughts? I'm not sure where the CI was left here.

[cjpillsbury]

Thanks for the quick response, @itsjamie

[tjenkinson]

Yeh unfortunately sauce can’t run on forks because it doesn’t get the secrets, so the options are admin override and merge and have the checks run on master or merge this into a separate branch in the repo and open a new pr off that one into master, where the secret will then be there.

[itsjamie]

Ran the functional tests in #4321 existing tests all passed except for the LL-HLS due to the test stream being down. Going to merge. Thanks @cjpillsbury.