Re: [ServiceWorker] Allow respondWith() to be called asynchronously or allow some way to cancel the response. (#836)

@mkruisselbrink you're right. Perhaps the solution of cancelling the `respondWith()` by using a special response could do the trick if we block threads at `respondWith()` call:

```js
// handler 1:
// 1. Gets the lock
event.respondWith(new Promise(resolve => {
  // 3. After 1s, cancel the respond and release the lock.
  setTimeout(() => { resolve(event.cancelRespond()); }, 1000); 
}));

// handler 2
// 2. Blocks and enqueue itself.
// 4. After 1s, unblocks and get the lock.
event.respondWith(new Promise(resolve => { 
  // 5. After 2s, it responds with \0/ making other enqueued handlers to throw.
  setTimeout(() => { resolve(new Response("\0/")); }, 2000); 
}));
```
When respond's promise is resolved with a non `cancelRespond()` result, all other enqueued `respondWith()` throw:
```js
Uncaught InvalidStateError: Failed to execute 'respondWith' on 'FetchEvent': The fetch event has already been responded to.
```

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/836#issuecomment-186393274