Bugfix: fix missing load stats for LL-HLS

[Oleksandr0xB]

This PR will...

Fix problem with missing load stats for LL-HLS.

Why is this Pull Request needed?

With missing stats ABR for LL-HLS usually stuck on lowers channels. This PR will fix it.

Problem with existing code is following.

1. When new manifest parsed oldPart stats copied to newPart
   

   hls.js/src/controller/level-helper.ts

   Lines 260 to 268 in 1ea75a2

   	// Merge parts
   	mapPartIntersection(
   	oldDetails.partList,
   	newDetails.partList,
   	(oldPart: Part, newPart: Part) => {
   	newPart.elementaryStreams = oldPart.elementaryStreams;
   	newPart.stats = oldPart.stats;
   	}
   	);

2. If part loading will start after that loader stats stats copied to oldPart
   

   hls.js/src/loader/fragment-loader.ts

   Lines 172 to 173 in 1ea75a2

   	// Assign part stats to the loader's stats reference
   	part.stats = loader.stats;

   
   As result ABR will use empty newPart stats.

Changing direction of assign fix this problem

Are there any points in the code the reviewer needs to double check?

Maybe the same logic can be applied to fragment loading stats

Resolves issues:

#3578

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically closed because it has not had recent activity. If this issue is still valid, please ping a maintainer and ask them to label it accordingly.

[alienpavlov]

Why this PR didn't go through? It looks good to me

[robwalch]

This creates a new problem. It is not clear how to reproduce the issue described or how this fix is valid. stats are accumulated from loaders, which is why HLS.js copies stats from the (part) loader to the part.

Please file an issue with steps to reproduce. I could not reproduce the issue following the details provided in this PR.

[robwalch]

Hi @Oleksandr0xB,

Thanks for the fix. I confirmed the issue and have another way to fix it that involved always selecting parts to load from level.details.partList rather than reusing the same partList present when selecting the first part in a fragment.