Implement Request.signal to detect client disconnects

[npaun]

How it works:

• An AbortController is added as a member of WorkerEntrypoint.
• Its AbortSignal is passed through via ServiceWorkerGlobalScope and Request through to JS, so it can be accessed via Request.signal.
• If the client disconnects, and we've both not produced a response or thrown an exception, the AbortSignal is triggered.

[kentonv]

In #3033 I made an observation which I don't think has been covered here.

One gotcha regarding request.signal that we'll need to work out: What happens in the common case that the request is passed along to an outgoing fetch()? If the response from that fetch is immediately returned by the worker, then it makes sense that if the client disconnects, the outgoing fetch should be canceled. But what if the response of that fetch is going to be used in some other logic, maybe involving a waitUntil() that writes to cache, etc.? Then, if we cancel the subrequest based on the client disconnecting, we might break that logic.

I think we might have to say: If the signal on an outgoing fetch() is the exact same signal from the original incoming request that started the Worker, it is ignored. If that feels wrong, then we would have to introduce a compat flag to fix it without breaking anyone. But I think it's fine really... if the client disconnects and there are no waitUntil()s, then all subrequests are canceled implicitly. So this only makes a difference if there are waitUntil()s, in which case you probably don't actually want to cancel the subrequest?

[kentonv]

It looks like you force-pushed changes in response to review comments and a rebase at the same time. Unfortunately this means we reviewers are unable to see what changed since we last reviewed.

Could you please rebase this back to e5b3760 so that we can directly diff against 3afd1ad? If you need to rebase forward to fix merge conflicts, please do it as a second push, and write a comment in between, so that GitHub doesn't collapse them. (GitHub is, unfortunately, extremely annoying in this regard...)

[npaun]

@kentonv Sorry for making a mess out of the diff. I'll be more careful about this next time.
Here's a link to the diff: https://github.com/cloudflare/workerd/compare/3afd1ad..4ad11a390f1d4d1f1ee2245c71804549083e940d

[kentonv]

Still need to address the passthrough issue? (The big blockquote in my earlier comment.)

[npaun]

Yes, still to address:

• Passthrough
• ASan issue

[npaun]

The latest push takes care of the passthrough subrequest case.

[npaun]

...

[jasnell]

Overall LGTM but let's also have @kentonv review. Given the complexity of the change it's easy to miss something in this