fix: cache aes keys for text tracks (#973)

[phloxic]

Description

So far aes keys for vtt are not cached even when cacheEncryptionKeys: true.

Specific Changes proposed

Call segmentKey() if the requested vtt segment includes a key before appending, so key can be cached.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[welcome]

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[codecov]

Codecov Report

Merging #1228 (03047ce) into main (5223427) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1228   +/-   ##
=======================================
  Coverage   86.30%   86.30%           
=======================================
  Files          39       39           
  Lines        9840     9842    +2     
  Branches     2292     2293    +1     
=======================================
+ Hits         8492     8494    +2     
  Misses       1348     1348           

Impacted Files	Coverage Δ
src/vtt-segment-loader.js	81.11% <100.00%> (+0.21%)	⬆️

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[phloxic]

Here's an example.
In network panel filter URLs with .key.
Example before fix.

[phloxic]

@gkatsev the fix is easy and works, but right now chances are low that I will time to dive into the testing rabbit hole anytime soon, sorry.

[phloxic]

@gkatsev - is the lack of a test blocking this?

[gkatsev]

A test would be greatly appreciated!

[phloxic]

@gkatsev - I feared as much ;-)
Unfortunately I have no clue how the tests work, especially the media related ones. For instance test/manifests/subtitles.m3u8 loads ./[0-2].webvtt which I can't find, probably because they are generated somehow, similarly to the manifests referenced by test/manifests/master-subtitles.m3u8 ... and at that point I give up because I am getting too embarrased ;-) but mainly because, as much as I'd like to, I simply do not have the time atm to really dig into it. Sorry.

[gesinger]

Thank you for the PR @phloxic !

Our tests can use a good cleanup, and can be hard to parse, so no worries there. Thankfully, we already have some tests around caching encryption keys over in the segment loader tests, so I made a PR to move those over to loader common: phloxic#1 . The loader common tests are run against all types of loaders (including VTT).

I ran the tests with your change commented out to prove the tests failed. Then ran again with the change included to show that they passed. And it looks good.

Thanks again for the contribution.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉