FluxEmpty does not throw NoSuchElementException

[Danny02]

Expected behavior

The JavaDoc of Flux#last states:

[...] or emit NoSuchElementException error if the source was empty

Steps to reproduce

Flux.empty().last().block()

Flux.fromIterable(List.of()).block()

Actual behavior

The first line does not throw an Exception and returns null. The second example throws a NoSuchElementException.

Reactor Core version

3.1.6

JVM version (e.g. java -version)

9.0.4

[simonbasle]

Thanks, indeed this is due to an overeager optimization that detect the Flux.empty() is a form of Callable and converts it to a Mono in a generic way, forgetting about the NoSuchElementException-on-empty aspect.

If you turn it into a Flux.empty().hide() it works as expected.

I'll prepare a fix for the 3.2.0 line

[simonbasle]

Note: I wonder if we should try to fix it in the 3.1.x line as well, people may have come to rely on this behavior so a fix during a maintenance release is maybe not ideal 🤔

[smaldini]

@simonbasle i think it should go to 3.1 too.