2.x Observable#concatMapEager doesn't always preserve order

[ansman]

Sample code (using RxJava 2.0.4):

Observable.just(1, 2, 3, 4, 5)
        .concatMapEager(i -> {
            System.out.println("Processing " + i);
            return i == 3 ? Observable.just(i) : Observable
                    .just(i)
                    .delay(1, TimeUnit.MILLISECONDS, Schedulers.io());
        })
        .observeOn(Schedulers.io())
        .subscribe(i -> System.out.println("Value: " + i));

Expected output:

Processing 1
Processing 2
Processing 3
Processing 4
Processing 5
Value: 1
Value: 2
Value: 3
Value: 4
Value: 5

Actual output:

Processing 1
Processing 2
Processing 3
Processing 4
Processing 5
Value: 3
Value: 1
Value: 2
Value: 4
Value: 5

As you can see the order is incorrect. This is due to the fact that the item 3 is mapped to a Callable source which won't wait for the previous observables.

[akarnokd]

Thanks for the feedback. This is indeed a bug with Observable.concatMapEager(). Flowable works properly.

I'll post a fix shortly. A workaround is to use just(i).hide() to break the bad/false internal optimization.

[akarnokd]

Closing via #4982