Re: [fetch] Aborting a fetch (#27)

I just ran into this. I'm trying to write a ServiceWorker that takes existing long-polling requests - possibly from multiple tabs - and multiplex all the tabs into a single long-polling request chain.
This fits extremely neatly into the "progressive enhancement" story of ServiceWorkers - if the worker is not installed, things will work normally, just with extra network traffic.

As ServiceWorkers do *not* have access to XMLHttpRequest, when a client subscribes to a new channel by abort()ing the previous request and starting a new one, the worker is forced to let the old (stale) fetch continue but ignore the result.

In the WIP code's architecture, it would be much more convenient to store a controller / context object like so:

```
function restartPolling() {
  // ...
  if (previousRequest) {
    previousRequest.abort();
    previousRequest = null;
  }
  // ...
  fetch(...)
  // ...
}
```

I hope that giving an actual use-case will help to make a decision.

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https://github.com/whatwg/fetch/issues/27#issuecomment-137924142