Different behavior of takeUntil in RxJS 5 and 6

[d3lm]

RxJS version: 6.2.0

Code to reproduce:

import { from, interval, of } from 'rxjs';
import { share, filter, takeUntil, tap } from 'rxjs/operators';

const rules = [true, true, true];

const results$ = from(rules).pipe(
  share()
);

const fails$ = results$.pipe(
  filter(value => !value),
  tap(value => console.log('FAIL'))
);

results$.pipe(
  takeUntil(fails$)
).subscribe(value => console.log('SUBSCRIBE', value));

Expected behavior: There are a two core streams involved in the code above, results$ and fails$. fails$ is based off of results$ and filters out all truthy source values, meaning it only emits falsy values. results$ should now emit values until fails$ produces a value. So when results$ synchronously emits 3 values [true, true, true], fails$ should never emit and it doesn't because of the filter operator. Hence the observer passed to subscribe should be called 3 times.

Actual behavior:

The next callback should be called 3 times. However, no values are propagated via the observer passed to results$.subscribe(...). When takeUntil is removed, everything works as expected. Even if I leave takeUntil in and only change the notifier to something like interval(1000), it works as expected.

Here's a StackBlitz that demonstrates the issue.

It somehow makes sense in this situation as everything is happning synchronously. This means that takeUntil creates a subscription to fails$, which then subscribes to results$. The thing is that results$ emits synchronously when takeUntil subscibres and it completes so the outer subscription on results$ will never receive any values. This is just a guess and my shot at explaining this behavior. The weird thing is that it works when using RxJS 5.5.11. Is this some edge case that was "fixed" in RxJS 6 or which one is incorrect?

Here's a runnable demo on StackBlitz.

We have noticed the issue while upgrading MachineLabs to RxJS 6.x. Here's the corresponding PR. Due to failing tests we were able to spot a different behavior, so we started to investigate.

[benlesh]

I think the fix to keep sources from being subscribed to if the notifier synchronously emits is what you're setting here: #3504

Yes, this is expected behavior.

Thank you for the detailed issue, @d3lm .. it's appreciated.

[d3lm]

Thanks for replying to this issue and pointing me to the corresponding PR.