Originally posted on StackOverflow.

The following code crashes with StackOverflowError and the stacktrace shows a long chain of request calls.

import io.reactivex.rxjava3.core.Flowable
import io.reactivex.rxjava3.core.Single
import io.reactivex.rxjava3.schedulers.Schedulers
import java.util.concurrent.TimeUnit.SECONDS

fun main() {
    fun incr(n: Int): Single<Int> = Single.just(n + 1)

    fun numbers(n: Int, max: Int): Flowable<Int> = Flowable.just(n).concatWith(
        if (n < max)
            incr(n)
            .observeOn(Schedulers.single())
            .toFlowable()
            .concatMap { next -> numbers(next, max) }
        else
            Flowable.empty()
    )

    numbers(1, 10_000)
    .blockingForEach(::println)
}

Exception in thread "main" java.lang.StackOverflowError
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.operators.flowable.FlowableConcatMap$ConcatMapImmediate.request(FlowableConcatMap.java:215)
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.operators.flowable.FlowableConcatMap$ConcatMapImmediate.request(FlowableConcatMap.java:215)
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.subscriptions.SubscriptionArbiter.request(SubscriptionArbiter.java:135)
	at io.reactivex.rxjava3.internal.operators.flowable.FlowableConcatMap$ConcatMapImmediate.request(FlowableConcatMap.java:215)

I'm not sure why there is such a chain created and if this is a result of an RxJava bug or not.