javascript - Is there a way to short circuit async/await flow?

Question

All four functions called below in update return promises.

async function update() {
   var urls = await getCdnUrls();
   var metadata = await fetchMetaData(urls);
   var content = await fetchContent(metadata);
   await render(content);
   return;
}

What if we want to abort the sequence from outside, at any given time?

For example, while fetchMetaData is being executed, we realize we no longer need to render the component and we want to cancel the remaining operations (fetchContent and render). Is there a way to abort/cancel these operations from outside the update function?

We could check against a condition after each await, but that seems like an inelegant solution, and even then we will have to wait for the current operation to finish.

Answer 1

I just gave a talk about this - this is a lovely topic but sadly you're not really going to like the solutions I'm going to propose as they're gateway-solutions.

What the spec does for you

Getting cancellation "just right" is actually very hard. People have been working on just that for a while and it was decided not to block async functions on it.

There are two proposals attempting to solve this in ECMAScript core:

• Cancellation tokens - which adds cancellation tokens that aim to solve this issue.
• Cancelable promise - which adds catch cancel (e) { syntax and throw.cancel syntax which aims to address this issue.

Both proposals changed substantially over the last week so I wouldn't count on either to arrive in the next year or so. The proposals are somewhat complimentary and are not at odds.

What you can do to solve this from your side

Cancellation tokens are easy to implement. Sadly the sort of cancellation you'd really want (aka "third state cancellation where cancellation is not an exception) is impossible with async functions at the moment since you don't control how they're run. You can do two things:

• Use coroutines instead - bluebird ships with sound cancellation using generators and promises which you can use.
• Implement tokens with abortive semantics - this is actually pretty easy so let's do it here

CancellationTokens

Well, a token signals cancellation:

class Token {
   constructor(fn) {
      this.isCancellationRequested = false; 
      this.onCancelled = []; // actions to execute when cancelled
      this.onCancelled.push(() => this.isCancellationRequested = true);
      // expose a promise to the outside
      this.promise = new Promise(resolve => this.onCancelled.push(resolve));
      // let the user add handlers
      fn(f => this.onCancelled.push(f));
   }
   cancel() { this.onCancelled.forEach(x => x); }
}

This would let you do something like:

async function update(token) {
   if(token.isCancellationRequested) return;
   var urls = await getCdnUrls();
   if(token.isCancellationRequested) return;
   var metadata = await fetchMetaData(urls);
   if(token.isCancellationRequested) return;
   var content = await fetchContent(metadata);
   if(token.isCancellationRequested) return;
   await render(content);
   return;
}

var token = new Token(); // don't ned any special handling here
update(token);
// ...
if(updateNotNeeded) token.cancel(); // will abort asynchronous actions

Which is a really ugly way that would work, optimally you'd want async functions to be aware of this but they're not (yet).

Optimally, all your interim functions would be aware and would throw on cancellation (again, only because we can't have third-state) which would look like:

async function update(token) {
   var urls = await getCdnUrls(token);
   var metadata = await fetchMetaData(urls, token);
   var content = await fetchContent(metadata, token);
   await render(content, token);
   return;
}

Since each of our functions are cancellation aware, they can perform actual logical cancellation - getCdnUrls can abort the request and throw, fetchMetaData can abort the underlying request and throw and so on.

Here is how one might write getCdnUrl (note the singular) using the XMLHttpRequest API in browsers:

function getCdnUrl(url, token) {
    var xhr = new XMLHttpRequest();
    xhr.open("GET", url);
    var p = new Promise((resolve, reject) => {
      xhr.onload = () => resolve(xhr);
      xhr.onerror = e => reject(new Error(e));
      token.promise.then(x => { 
        try { xhr.abort(); } catch(e) {}; // ignore abort errors
        reject(new Error("cancelled"));
      });
   });
   xhr.send();
   return p;
}

This is as close as we can get with async functions without coroutines. It's not very pretty but it's certainly usable.

Note that you'd want to avoid cancellations being treated as exceptions. This means that if your functions throw on cancellation you need to filter those errors on the global error handlers process.on("unhandledRejection", e => ... and such.