first() produce a lot more than 1 data in some recursive situation.

[kasiditi]

It is pretty hard to describe in text. The code below might be more understandable.

I have a Subject that is connected with first() and subscribed with a observer that will fire a data synchronously to the same Subject with next(). It turns out that the observer receives a lot of data even though it is chained with first().

I found this bug in more personal complicated code, but below is a simplified repro one. Also, I'm not sure if this is expected behavior or not.

RxJS version:
5.0.0-beta.12

Code to reproduce:

const valueSubject = new Subject();
const result = [];

valueSubject.first().subscribe(val => {
    result.push(val);
    valueSubject.next(false);
});

valueSubject.next(true);

// Expect result.length to be 1.

Expected behavior:
Data should be received once.

Actual behavior:
Large amount of data fired to observer. Sometimes, the Error Maximum call stack size exceeded shows up.

Additional information:
When data is pushed asynchronously, the test will pass. Here is the asynchronous test in Jasmine:

it('should behave correctly', (done) => {
    const valueSubject = new Subject<boolean>();
    const result: boolean[] = [];

    valueSubject.first().subscribe(val => {
        result.push(val);
        setTimeout(() => valueSubject.next(false), 0);
    });

    valueSubject.next(true);

    setTimeout(() => {
        expect(result.length).toBe(1);
        done();
    }, 100);
});

In this case, the test passes.

[kwonoj]

I think this is similar case to #1759 (comment) to try synchronous reentrant causes those behavior. As your last snippet does, if you explicitly make it asynchronous to trap out from synchronous boundary it'll work, as similar observeOn or other different scheduling approach does.

/cc @trxcllnt to confirm in case if I understood this incorrectly and this is unexpected behavior.

[trxcllnt]

Yes we should likely guard after the first emission, similar in spirit to #1967

[benlesh]

I'll have a look

[lock]

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