feat(DASH): Add MPD Patch support

[dave-nicholas]

Closes #2228

[github-actions]

Incremental code coverage: 87.44%

[joeyparrish]

Thank you for contributing!

I don't understand the patch adapter, though. I worry that it will not be representative of actual patch streams. I also see it being applied in the demo to a VOD manifest, which should have no need of patching. Patching is only useful for updates to live streams, correct?

[dave-nicholas]

Thank you for contributing!

I don't understand the patch adapter, though. I worry that it will not be representative of actual patch streams. I also see it being applied in the demo to a VOD manifest, which should have no need of patching. Patching is only useful for updates to live streams, correct?

Hi @joeyparrish

I agree with you that patch manifest adapter is not ideal, but I wanted to give the ability to test the feature in the demo app as open patch compliant streams are not easy to come by (at least I haven't seen any 😆 ). The reason I chose the ad enabled VOD is because of its behaviour of presenting new segments and periods regularly like a live stream.

I can remove remove the manifest adapter if you wish.

[dave-nicholas]

Hi @joeyparrish I have removed the patch adapter.
Please let me know if there any concerns or changes in approach that you would like me to make.
We will be taking this change into production shortly.

[Iragne]

Happy to support in this kink of new feature.
Let me know if you need help

[joeyparrish]

Overall, I would be comfortable with an "experimental" flag to enable this, but I want to ensure that the existing flow isn't broken. I'm most concerned about extra calls to combinePeriods (which can be expensive) and missing calls to set up caption streams.

[tykus160]

Hi! This PR now supports DASH IF test streams, though there are few gotchas @tobbee

• SegmentTemplate with $Number$ has publishTime set to 1970, so we do regular updates there instead of patches
• on multiperiod, whenever new period arrives, following patch requests will return HTTP 500, something similar has been reported on dash.js board as well MPD Patching with multiperiod manifest fails Dash-Industry-Forum/dash.js#4487

I'm gonna focus on adjusting unit tests now.

[avelad]

@shaka-bot test

[shaka-bot]

@avelad: Lab tests started with arguments:

• pr=5247

[tobbee]

@tykus160 The patch issues in livesim2 should be fixed now including the multiperiod case for SegmentTimeline.

Regarding the publishTime for SegmentTemplate with $Number$ it changes every time the MPD changes, which is every minute for one-minute period, but never if there are no periods. The reason behind that is that it the publishTime is an "id" of the manifest so it shouldn't change every segment if there is no change.

In general, I think that MPD Patch and SegmentTemplate with Number is a bad match, so one should rather focus on SegmentTimeline, which is also the case with long MPDs. I think that your choice of regular updates is the most reasonable in that case.

As far as I understood from Daniel Silhavy, dash.js fallbacks to normal MPD requests if there is no update of MPD patch.