Node: `dns.promises.Resolver` does not have a `.cancel()` method

[szmarczak]

Is your feature request related to a problem? Please describe.

You cannot cancel pending DNS queries using dns.promises.Resolver. Note that dns.Resolver has a .cancel() method, but the async version does not.

Describe alternatives you've considered

There are no alternatives yet.

[rickyes]

Yes, just mount the cancel function to the async Resolver.prototype. I'm working on this.

[rickyes]

I'm not sure can add cancel in the promise API directly. Reference:21264#discussion_r196582158

/cc @cjihrig

[cjihrig]

It can be done, but people have opinions there. I'm personally OK with it, but you should probably check with others.

[jasnell]

We cannot add a cancel to the Promises API directly but there are ways cancel can be accomplished...

The following is an example from a workshop I deliver to customers:

const EventEmitter = require('events');

// When using Race or All, if the underlying async API
// provides a mechanism for cancelation the pending
// operation, then a signaling mechanism like event
// emitter can be used to interject the ability to
// cancel. This, however, can be subject to race
// conditions that are not unlike thread synchronization

function createPromise(monitor, timeout, value) {
  return new Promise((res, rej) => {
    const t = setTimeout(() => {
      res(value);
      monitor.emit('resolved');
    }, timeout);
    monitor.on('resolved', () => {
      clearTimeout(t);
      rej('canceled')
    });
  });
}

function Foo(monitor) {
  return createPromise(monitor, 1000, 'B');
}

function Bar(monitor) {
  return createPromise(monitor, 10, 'A');
}

const monitor = new EventEmitter();

Promise.race([Foo(monitor), Bar(monitor)]).then(console.log);

Specifically, while Promises themselves cannot be canceled, if the underlying mechanism being used to schedule the asynchronous activity provides a means for canceling, then EventEmitter (or some other signaling mechanism) can be passed in to the Promise to trigger an early resolve or reject.

[cjihrig]

If I'm remembering the linked discussion correctly, the issue was just about using the name "cancel" not actually implementing promise cancellation.

[szmarczak]

Specifically, while Promises themselves cannot be canceled

From the docs:

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code ECANCELLED.

So I see no reason why can't we just reject(queryCancelledError).

[jasnell]

When user code is using dnsPromises.Resolver directly that should be possible, with the caveat that it would have to cancel all outstanding Promises initiated by that Resolver in order to keep the API and implementation as simple as possible. For Promises returned by the default Resolver, there really wouldn't be a way to do it because the default Resolver is not exposed to user code via the API.

That is, this would work:

const { Resolver } = require('dns').promises;
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

resolver.resolve4('example.org').then((addresses) => {});

resolver.cancel();

But canceling this would not:

const { resolve4 } = require('dns').promises;
resolve4('example.org').then((addresses) => {});

[szmarczak]

You can't cancel this either. What's your point?

const { resolve4 } = require('dns');
resolve4('example.org', (error, addresses) => {});

[szmarczak]

If we need to be worried about cancelling these, it should be another issue...

[jasnell]

You can't cancel this either. What's your point?

Be explicitly clear about expectations. I'm agreeing with you that implementing this should be possible. Just needs a PR.