keep hls chunk source from getting redundant playlists

[TakuSemba]

Overview

when playing live hls stream, ExoPlayer periodically getting all playlists which has been loaded at least once. Because of this, ExoPlayer keeps sending network requests more than necessary.

Environment

since version 2.9.0

This is the url that I tested, and it is reproducible on ExoPlayer demo.

You will see ExoPlayer keeps fetching all variants of mediaPlaylist which ExoPlayer has loaded during the playback.

Cause

Inside the HlsChunkSource, HlsChunkSource#createMediaChunkIterators() provides MediaChunkIterator[] that contains information for upcoming media chunks. However, in createMediaChunkIterators(), playlistTracker.getPlaylistSnapshot(variantUrl) is called and it sends an network request to fetch an MediaPlaylist. It's called for every variant that has playlistSnapshot.

I think unselected playlists should no be loaded through network calls from the viewpoint of reducing network requests.

[AquilesCanta]

This does look like a bug. But it needs to be fixed on the HlsPlaylistTracker side. I have filed #5059 to track its fix. Any pushed commits will be referenced there.