Reducing inner observable blows stack

For my use case I have the reduce function return an inner observable. A simple test reveals that creating a large enough sequence will result in stack overflow.

RxJS version: 5.5.6

Code to reproduce:

const Observable  = require("rxjs/Rx").Observable;

function add($a,$b) {
	$a = $a instanceof Observable ? $a : Observable.of($a);
	$b = $b instanceof Observable ? $b : Observable.of($b);
	return $a.mergeMap(a => $b.reduce((a,b) => a + b,a));
}

Observable.range(1,1000).reduce(add).mergeAll().subscribe(console.log);

Expected behavior: Log result

Actual behavior: UnsubscriptionError: 1 errors occurred during unsubscription:

1. RangeError: Maximum call stack size exceeded

Additional information: Tested in Node.js 8.5

This may not be a bug if there’s a workaround, but I couldn’t find / come up with one.

[cartant]

I think there’s a bit more to this, as the reduce composes an extremely long dot-chained observable and even if the queue scheduler is used, the eventual unsubscription will effect a stack overflow error.

The good news is that you can solve the problem using the queue scheduler, but you need to write a user-land unsubscribeOn operator to ensure that unsubscription does not happen recursively, like this:

Rx.Observable
    .range(0, 5000).reduce(
        (acc$, x) => acc$
          .map(acc => acc + x)
          .subscribeOn(Rx.Scheduler.queue)
          .let(unsubscribeOn(Rx.Scheduler.queue)),
        Rx.Observable.of(0)
    )
    .mergeAll()
    .subscribe({
        next: x => console.log(x),
        error: err => console.error(err),
        complete: () => console.log("complete")
    });

function unsubscribeOn(scheduler) {
    return source => new Rx.Observable(observer => {
        const subscription = source.subscribe(observer);
        return () => scheduler.schedule(() => subscription.unsubscribe());
    });
}

[lock[bot]]

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