Log error and detach if MediaSource 'sourceopen' is interrupted

[robwalch]

This PR will...

Listen for "emptied" event on media element while attaching MediaSource. If the media src attribute does not match the object URL being attached log an error and detach the media element (this will emit MEDIA_DETACHING).

Why is this Pull Request needed?

#4952 Users are unsure why calling hls.attachMedia(HTMLMediaElement) does not always resolve with the player emitting MEDIA_ATTACHED after MediaSource is attached and open. My best guess is that something is changing the video.src externally while attaching, which interrupts the MediaSource from dispatching the 'sourceopen' event HLS.js depends on to complete the attachMedia operation.

With this change-set, detachMedia will be called if attaching the MediaSource is interrupted ("emptied" event with src change detected). The detaching event following an attach attempt could allow application to retry attaching.

For v2.0, attachMedia could be made to return a Promise, which resolved on attached and is rejected if interrupted or the media element errors.

Resolves issues:

Resolves #4952

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

[itsjamie]

If they've requested the action, and it has failed, giving that information back makes sense.

However, to this question:

In addition to these changes, should the player “auto detach” if the media element is emptied externally after sourceopen?

I don't think that hls.js should start to make changes in addition to what the user has requested, because then the actual state to the average user is non-deterministic and requires far more intimate knowledge of what the underlying API does, breaking the abstraction.

[robwalch]

If they've requested the action, and it has failed, giving that information back makes sense.

In v2.0 attachMedia can return a Promise.

In addition to these changes, should the player “auto detach” if the media element is emptied externally after sourceopen?

I don't think that hls.js should start to make changes in addition to what the user has requested, because then the actual state to the average user is non-deterministic and requires far more intimate knowledge of what the underlying API does, breaking the abstraction.

To be completely safe, I will remove this call to detachMedia. I don't think the detached event provides a reliable mechanism to recover. Changing src while opening a MediaSource can result in the media element entering an error state.

[robwalch]

@itsjamie what do you think?

Suggested change

	logger.error(
	`Media reset while attaching MediaSource, detaching media (${_objectUrl} > ${media.src})`
	);
	this.hls.detachMedia();
	logger.error(
	`Media element src was set while attaching MediaSource (${_objectUrl} > ${media.src})`
	);

The cap-level and fps controllers will use the media element after onMediaAttaching even without the MediaSource completing it's task. Detaching would prevent this.

[itsjamie]

I like the messaging. For the onMediaAttaching continuing to fire thoughts on those controllers using onMediaAttached instead?

[robwalch]

That makes sense for the fps controller, since it's entire operation is based on getVideoPlaybackQuality() dropped frames. For the cap controller, there may be a need to get element dimensions sooner rather than later so that the selected media matches the player size on start. I don't think a change is needed now. Plan is to deliver this in today's v1.3.3 patch. We can continue if there's additional feedback from users in #4952 using the patch.