feat(ABR): Add BufferBasedAbrManager for low-latency live streams

[erikherz]

Summary

A buffer-based ABR manager for low-latency live streams (MoQ/MSF) where bandwidth estimation is unreliable due to small segment sizes and push-based delivery via WebTransport.

Instead of estimating bandwidth from segment downloads, this manager monitors buffer health relative to the stream's target latency.

How it works

• Down-switch: immediate when buffer < 30% of target latency, or when dropped frames exceed threshold
• Up-switch: requires 5 seconds of sustained buffer > 150% of target latency
• Quality ladder: built from variants sorted by resolution (pixel count)
• Grace period: 5 seconds after enable before evaluating

Usage

player.configure({
  abrFactory: () => new shaka.abr.BufferBasedAbrManager(),
});

Files

File	Change
lib/abr/buffer_based_abr_manager.js	New file — implements shaka.extern.AbrManager
build/types/core	Include in core build

Test plan

• Tested with live MoQ/MSF multi-quality streams (480p/720p/1080p)
• Verified down-switch on buffer depletion
• Verified up-switch after sustained healthy buffer
• Verified smooth playback during quality transitions
• Unit tests (to be added based on feedback)

Related to #9872

Author: erik@vivoh.com

[matvp91]

Thank you,@erikherz! Glad to see some activity around LL ABR.

Shall we mark this as a draft for the sake of reference?

I'm not entirely sure we'd want to ship a secondary ABR impl for the sake of LL, but EWMA is definitely lacking. I also suspect buffer health is a useful metric unrelated to MSF or even LL but that's an unproven theory.

Hope to hear some opinions, @joeyparrish @avelad @tykus160?

[joeyparrish]

I haven't reviewed the implementation yet, but I would be in favor of including this as an option. (Assuming it's not a large change to the size of Shaka Player.)

@erikherz, you have a linter failure preventing other CI workflows from running, including the size analysis.

[joeyparrish]

/home/runner/work/shaka-player/shaka-player/head-source/lib/abr/buffer_based_abr_manager.js
  339:9  error  Unnecessary return statement  no-useless-return

[avelad]

I think it would be good if this were only included in the experimental build.

[avelad]

@erikherz FYI, we are working on add "monitor dropped frames to influence ABR decisions " #9918

[shaka-bot]

Closing due to inactivity. If the author would like to continue this PR, they can reopen it (preferred) or start a new one (if needed).