Could Mono.block(Duration timeout) throw java.util.concurrent.TimeoutException?

[Mahoney]

Mono.block(Duration timeout) currently throws IllegalStateException if the timeout expires. This seems odd as a) it isn't an illegal state, it's a perfectly possible state that we're explicitly anticipating and b) the more correct java.util.concurrent.TimeoutException is thrown in similar circumstances when Mono.timeout(Duration timeout).subscribe() times out.

The code that throws the IllegalStateException is:

reactor-core/reactor-core/src/main/java/reactor/core/publisher/BlockingSingleSubscriber.java

Line 123 in 232fe2e

throw new IllegalStateException("Timeout on blocking read for " + timeout + " " + unit);

[simonbasle]

TimeoutException is a checked exception. It is not a problem on the timeout operator because it is not thrown but propagated through onError, but on block it would force us to add it to the signature of the method (and all current callers to declare a catch block).

[simonbasle]

Note that

Mono.never()
    .timeout(Duration.ofSeconds(1))
    .block();

Throws a Runtime(Reactive)Exception, with the TimeoutException as the cause.
Maybe we could align on that for block(Duration)?

This would at least bring consistency in how to deal with timeout exceptions in blocking code:

try {
    longMono.block();
}
catch (SpecificRuntimeException sre) {
    //do something specific
}
catch (RuntimeException e) {
    if (Exceptions.unwrap(e) instanceof TimeoutException) {
        //deal with timeout
    }
    else {
        //generic exception
    }
}

[Mahoney]

Working in Kotlin, so I'd forgotten about the checked / unchecked problem. Though I guess it's a breaking change anyway as people might have code expecting an IllegalStateException for a timeout now...

I don't really have a clear solution, just wanted to raise it as something that confused me as a reactor newbie.

[simonbasle]

I'm inclined to leave it as is. The breaking change (or the breaking behavior that I suggested above) are IMO not worth it, especially considering that blocking is for corner cases only and is generally strongly discouraged.