Re: [w3c/ServiceWorker] ServiceWorker lifetime and respondWith() with a user-constructed ReadableStream (#882)

> When a stream is GC'd that cannot complete, we need to figure out if this is an error or a cancellation. My gut feeling is error, as if a server simply cut off its response & reset the connection, but I'd like to hear from @domenic.

I'm not sure GCed is the right word here, since the reference is being held on to (by the implicit promise chain created by the for-await-of). I'm also not sure if the right approach here is to, on termination of the service worker, reach into the objects owned and created by the web developer and move them into a new state (i.e. error them).

Maybe the level this should be handled at is when we "pipe" across the boundary to the main thread. (https://w3c.github.io/ServiceWorker/#dom-fetchevent-respondwith step 8.3.2.5.) The ReadableStream created on the main thread is owned by the user agent, who could decide to error it. In the service worker thread, either the ReadableStream should be canceled (using the reader that's being read from), or the thread should just shut down entirely, destroying all objects, not running any more JavaScript code, and forcibly releasing any resources like network socket handles. I guess you would spec that as a cancel anyway.

Then the question becomes whether you want to error the main-thread ReadableStream, or just close it prematurely. I agree this should act as if the server simply cut off its response and reset the connection, but I'm not 100% sure this would result in an error today. But I trust you if you are sure that is the behavior.

> I'm curious about this case:

It seems like as written the service worker should be able to close, since you didn't wait for the pipeTo to finish. But whether or not the stream will complete successfully is then up to the browser, I think. If the browser decides to destroy the service worker thread, the behind-the-scenes stream (and the identity transform it is passing through) will be killed, so the process of piping it to the main thread will be interrupted.

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