fix(DASH): segments being fetched out of the range of the timeline

[dave-nicholas]

fixes: #3952

There was an issue that would occur that caused segments to be fetched that are out of the range of the manifest.

[avelad]

@littlespex For me the change is fine, can you check if it does not introduce any regression in your streams? Thanks!

[shaka-bot]

Incremental code coverage: 100.00%

[littlespex]

I've tested a few streams and do not see any issues with this PR. I did notice that we lost captions on some of our streams, but this issue occurs in main as well. Unfortunately I don't have a lot of time to investigate right now.

[nrcrast]

@avelad @littlespex For some more info on this fix (I'm on a team with @dave-nicholas :) )

Using numEvicted is not ideal for the segmentReplacement calculation, as that value is not reset or consistent when you close and reopen segment indexes. If you let the stream play on one variant with ABR disabled, it will work fine. It's when variant switches come into the picture that numEvicted becomes problematic. You can reproduce fairly easily by turning ABR off, playing a linear stream for a couple minutes, and then switching to a different variant and immediately switching back.

Furthermore, architecturally numEvicted has one single purpose, and that's to keep consistent positioning while you're playing a stream. It's not there to be used for absolute URI calculations or to be consistent between closing and re-opening of a Segment Index. This dual purpose of using numEvicted to calculate the Segment Replacement is both architecturally confusing and causes an actual behavioral bug.

I think, architecturally, it makes sense that all info required to build the Segment URI should be contained within the TemplateInfo structure. Mixing in a variable internal to the Segment Index just makes it all the more confusing.