Add a close event to MessagePort

[nononokam]

Request for Mozilla Position on an Emerging Web Specification

• Specification title: Add a close event to MessagePort
• Specification or proposal URL (if available):
• Explainer URL (if available):https://github.com/fergald/explainer-messageport-close
• Proposal author(s) (@-mention GitHub accounts): @fergald @nononokam
• Caniuse.com URL (optional):
• Bugzilla URL (optional):
• Mozillians who can provide input (optional): @asutherland

Other information

Currently, there is no timely and reliable way to detect when a MessagePort becomes disentangled, which makes it difficult to release resources associated with ports. This issue has a long discussion of the problem and potential solutions. Some users have used the unload event to detect disconnect, but it is deprecated. Therefore, The approach available now is to hold a MessagePort in WeakRef and periodically check if this port has been garbage collected by using deref() or to make use of a page-hide event, which is quite inefficient.
So, I propose adding a close event. For example, if there are two entangled ports, portA and portB, and portA is closed, the close event is fired on portB.

[asutherland]

I'm very positive on this; I would plan to mark us as positive on this absent any other feedback. There is a lot of good discussion in the issue and explainer, so if anyone has any questions, I would check both of those out first.

[zcorpan]

@smaug---- WDYT?

[smaug----]

Given that one can achieve this through WeakRefs already, I think this is reasonable.
But, this is only about MessagePort, do we want onclose also on Worker?

(unrelated fwiw, unload is not deprecated. That is a link to a chromium internal thing, not a spec level deprecation)

[asutherland]

But, this is only about MessagePort, do we want onclose also on Worker?

whatwg/html#5869 tracks that, so I'd stick to treating that as a separate thing despite the normative and non-normative language around "DedicatedWorkerGlobalScope objects act as if they had an implicit MessagePort associated with them.".