java - skipUntil and flatMap with concurrency 1 hangs in Reactor

Question

I have experienced a strange behavior in Reactor (v. 3.4.2) when using skipUntil and flatMap(fn, concurrency) operators with concurrency=1. The pipeline hangs.

To demonstrate what I mean here is a short example that works as I would expect:

Flux.range(1, 5)
    .skipUntil(v -> v > 1)
    .flatMap(v -> Mono.just(v))
    .doOnNext(System.out::println)
    .blockLast();

It prints 2, 3, 4 and 5 to a separate line. However, if I add a concurrency argument equal to 1 to the flatMap operator then the execution hangs:

Flux.range(1, 5)
    .skipUntil(v -> v > 1)
    .flatMap(v -> Mono.just(v), 1)
    .doOnNext(System.out::println)
    .blockLast();

I don't know if I missed something or there is some erroneous behavior of skipUntil that after it skips the first item it does not request next from the upstream. Is the second example valid?

question from:https://stackoverflow.com/questions/65927400/skipuntil-and-flatmap-with-concurrency-1-hangs-in-reactor

Answer 1

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