Race condition with RxJava's PublishSubject - rx-java2

Question

When using a PublishSubject in conjunction with blockingGet(), there seems to be a race condition where a subscriber does not receive the events.
I attached a basic JUnit test in Kotlin, which has 2 methods.
rxTestBroken() shows the broken behavior with a PublishSubject.
rxTestOk() shows that everything works fine with a BehaviorSubject, because the latter replays the last event in case the subscriber is not subscribed in time.
Where does this race condition come from and is using a BehaviorSubject the correct fix?
import io.reactivex.Single
import io.reactivex.subjects.BehaviorSubject
import io.reactivex.subjects.PublishSubject
import io.reactivex.subjects.Subject
import org.assertj.core.api.Assertions.assertThat
import org.junit.Test
import java.util.concurrent.TimeUnit
class StateMachine(val stateSubject: Subject<Int>) {
companion object {
val STATE_IDLE = 1
val STATE_READY = 2
}
val output = 10L
var currentState = STATE_IDLE
fun scheduleNextState(nextState: Int) {
Thread(Runnable {
currentState = nextState
stateSubject.onNext(currentState)
}).start()
}
fun start() = scheduleNextState(STATE_READY)
fun stop() = scheduleNextState(STATE_IDLE)
}
class RxTest {
fun stateOutput(stateSubject: Subject<Int>): Single<Int> {
val stateMachine = StateMachine(stateSubject)
val waitForIdle = stateSubject
.startWith(stateMachine.currentState)
.doOnNext {
if (it != StateMachine.STATE_IDLE) { stateMachine.stop() }
}
.filter { it == StateMachine.STATE_IDLE }
.firstOrError()
val scanFile = stateSubject
.doOnSubscribe { stateMachine.start() }
.filter {
when (it) {
StateMachine.STATE_READY -> true
StateMachine.STATE_IDLE -> false
else -> throw RuntimeException("Wrong state $it")
}
}
.firstOrError()
.map { stateMachine.output.toInt() }
.doFinally { stateMachine.stop() }
return waitForIdle.flatMap { scanFile }.timeout(1, TimeUnit.SECONDS).onErrorReturnItem(-1)
}
#Test
fun rxTestBroken() {
for (i in 1..10000) {
assertThat(stateOutput(PublishSubject.create<Int>()).blockingGet())
.withFailMessage("worked $i times")
.isEqualTo(10)
}
}
#Test
fun rxTestOk() {
for (i in 1..10000) {
assertThat(stateOutput(BehaviorSubject.createDefault(StateMachine.STATE_IDLE)).blockingGet())
.withFailMessage("worked $i times")
.isEqualTo(10)
}
}
}