Recursive task instantiation errors

[tzellman]

For starters, this twiddle demonstrates the issue. I will also state that this may not be a very popular or even valid use case. 😄

The error is mainly benign, but does get logged in the console:

Error: tried to advance finished generator
    at Class._takeSafeStep (addon.js:1683)
    at Class._syncResume (addon.js:1785)
    at Class._handleResolvedContinueValue (addon.js:1769)
    at Class._proceed (addon.js:1737)
    at Class.proceed (addon.js:1725)
    at Promise.handleYieldedRsvpPromise [as __ec_yieldable__] (addon.js:1305)
    at cb (addon.js:1310)
    at tryCatch (VM399 ember.debug.js:59069)
    at invokeCallback (VM399 ember.debug.js:59084)
    at publish (VM399 ember.debug.js:59052)
defaultDispatch @ VM399 ember.debug.js:18015
dispatchError @ VM399 ember.debug.js:17994
onerrorDefault @ VM399 ember.debug.js:31641
trigger @ VM399 ember.debug.js:58725
(anonymous) @ VM399 ember.debug.js:59626
invokeWithOnError @ VM399 ember.debug.js:346
flush @ VM399 ember.debug.js:405
flush @ VM399 ember.debug.js:529
end @ VM399 ember.debug.js:599
run @ VM399 ember.debug.js:713
_runExpiredTimers @ VM399 ember.debug.js:1107
Backburner._boundRunExpiredTimers @ VM399 ember.debug.js:568

The use case in mind is similar to how a polling live-reload may work :

• create a timer-like task that gets kicked off by the app
• the task likely needs to wait for a period of time, perform some work (e.g. check for new data), then instantiate a new instance so it can do it again

As a workaround either a while(true) could be used inside the task function, or a separate function could be called that defers, then re-instantiates the task.

One thing that I noted from twiddle is that the 0.7.19 version didn't generate the error that 0.8.1 does.

Since the example code to replicate is so small, here it is:

import Ember from 'ember';
import {task, timeout} from 'ember-concurrency';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  count: 0,
  
  recursiveTask: task(function*(){
    yield timeout(1000);
    this.incrementProperty("count", 1);
    this.get("recursiveTask").perform();
  }).restartable(),
  
  init(){
    this.get("recursiveTask").perform();
  }
});

[JackCA]

I believe this is the same as the issue I had in this thread: #138 but I closed -- so you're not alone @tzellman

[tzellman]

Ha! I searched the issues but only the open ones.. Looks like you suggested the same workaround.

I'll leave this open for a bit to possibly get some discussion, even though I'm fine with this not being a supported use case.

I may investigate some alternative approaches as well.

[courajs]

Yeah, this happens when a task cancels itself synchronously:

foo: task(function * () {
  this.get('foo').cancelAll();
})