[Proposal] DRM: Add keySystems[].wantedSessionTypes loadVideo option

[peaBerberian]

Problem

We encountered a complex case with an application where they want to be able to load some contents relying on temporary licenses (the decryption kind) and others relying on persistent licenses, yet still keep our "MediaKeySession cache" intact when doing that (cache which would in the end contains both types).

The main issue with that usage was linked to our API:

• When an application wants to load a content with a persistent license, it will set in its loadVideo call our keySystems[].persistentLicenseConfig option, which would set both:

  • the EME configuration's persistentState requirement to "required" - unless the keySystems[].persistentState option is explicitly set to something else, and

  • the sessionTypes configuration property to ["temporary", "persistent-license"] (NOTE: so able to handle both kinds, weirdly, not sure why we ask for both, it seems to be a mistake here - maybe for another PR).

• When an application wants to load a content with temporary licenses, it will not set the keySystems[].persistentLicense option, and consequently:

  • the persistentState would be set to "optional" -unless the keySystems[].persistentState option is explicitly set to something else, and

  • the sessionTypes configuration property is set to just ["temporary"]

Those results lead to incompatible MediaKeySystemAccess instances which means that going from content with temporary licenses to contents with persistent ones and vice-versa is going go lead to the re-creation of a MediaKeySystemAccess behind the hood at content load.

In turn, this MediaKeySystemAccess change is going to internally reset the MediaKeySession cache (the one we keep to avoid doing license request if we already requested it recently).

Note that this is the only known issue of switching MediaKeySystemAccess here. For cases where that cache is not important, there should be no much of a problem to create another MediaKeySystemAccess (beside perhaps performance issue on really really slow implementations).

Solution proposed here

To fix that very specific issue, I propose here the keySystems[].wantedSessionTypes loadVideo options, which would be explicit about the sessionTypes we want the created MediaKeySystemAccess to handle.

With that and other options like keySystems[].persistentState, an application should now be able to ask for every capabilities it needs the MediaKeySystemAccess to be able to do, without necessarily having to link it to the current content (e.g. it could ask the MediaKeySystemAccess to be compatible with persistent licenses, even if the current content rely on temporary ones).

As this is a very technical need, I had a lot of trouble in making the API approachable in the documentation, I spent a lot more time writing the few lines in the documentation than actually implementing this!

I'm also afraid that most applications won't understand the point of that API, even application developers at Canal+, as it is very linked to both the EME recommendation and the RxPlayer internals (its MediaKeySession cache).