“The script will never generate a response” on Cloudflare Workers

Whenever a Worker initiates some sort of external I/O -- such as invoking `fetch()` -- the I/O task is associated with the incoming request that was running at the time. If the incoming request is canceled, then all outgoing I/O tasks associated with it are also canceled. This all happens within the runtime implementation, without actually invoking any JavaScript, and so is not constrained by JavaScript Promise semantics. Technically, what ends up happening from the JS side is that the Promise that `fetch()` returned simply never resolves, because the runtime throws away the Resolver without invoking it. V8 is smart enough that when the Resolver for a Promise is garbage-collected, V8 can also GC all of the downstream continuations that were waiting on it, since they will never be called. So all the state ends up being GC'd nicely without ever running any more code.

Related: The way the runtime knows which request is running is that it always knows what I/O event caused it to start executing JavaScript, and every time it starts executing JavaScript, it always keeps running until either the microtask queue is empty, or the time limit is hit (which aborts the request). All these microtasks are necessarily downstream effects of the original I/O event that started them, and therefore the runtime concludes that they are all executing code on behalf of the request with which that I/O was associated. Any new I/O initiated by these microtasks is associated with the same request.

These assumptions break, though, when requests share Promises via the global scope. If request A awaits a promise, and request B later resolves that promise, the code in request A continues running... but the runtime still thinks it is running on behalf of request B. Any new I/O initiated at this point is accounted to request B. If the code tries to access any pre-existing I/O objects than belong to request A, it'll throw an exception. That implies in particular that it cannot cause Request A to complete and return a response.

"The script will never generate a response" happens when the runtime sees that it has finished executing all the JavaScript microtasks for a particular request, and no I/O operations have been scheduled. In this case, the runtime correctly concludes that there is no possible way for new microtasks to be executed on behalf of this request ever again, because there is nothing that would trigger them. This can be trivially reproduced by doing `await new Promise(() => {})` -- awaiting a promise that never resolves. The confusing this is it can also be reproduced by awaiting a promise which is going to be resolved by some other request. Since, as described above, this scenario will cause the subsequent code to execute on behalf of that other request, and such execution cannot possibly cause the original request to produce a response, the error is correct -- the first request will never produce a response. But, it's quite confusing if you don't realize that cross-request promise sharing is a no-no.

The easiest and best way to avoid it is to never modify anything in the global scope. If a request never modifies global scope, then there's no way another request could get ahold of its objects, particularly its promises.

That said, the situation obviously isn't intuitive and we'd like to improve here. What I'm really hoping is that we can find a way to efficiently run every request in a "fresh state", so requests cannot see each other's effects at all. But, literally creating an isolate per request would be too expensive; we need something a bit more clever than that.

(I'm the tech lead for Workers and this strange behavior is my fault.)