AbstractUaaTokenProvider violates Publisher Spec. Rule 1.3

Start in AbstractUaaTokenProvider.token(...) at AbstractUaaTokenProvider.java:261 and review the surrounding token-provider flow and Reactor sink usage. Reproduce concurrent tryEmitNext calls that lead to FAIL_NON_SERIALIZED, then verify that token subscribers receive signals serially and that the reported error no longer occurs.

Hi,

I have observed below error related to Publisher Spec. Rule 1.3, while trying to get token in AbstractUaaTokenProvider.token(…) method -

reactor.core.Exceptions$ErrorCallbackNotImplemented: reactor.core.publisher.Sinks$EmissionException: Spec. Rule 1.3 - onSubscribe, onNext, onError and onComplete signaled to a Subscriber MUST be signaled serially.

Stack trace -
reactor.core.Exceptions$ErrorCallbackNotImplemented: reactor.core.publisher.Sinks$EmissionException: Spec. Rule 1.3 - onSubscribe, onNext, onError and onComplete signaled to a Subscriber MUST be signaled serially.
Caused by: reactor.core.publisher.Sinks$EmissionException: Spec. Rule 1.3 - onSubscribe, onNext, onError and onComplete signaled to a Subscriber MUST be signaled serially.
at reactor.core.publisher.InternalManySink.emitNext(InternalManySink.java:56)
Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below:
Assembly trace from producer [reactor.core.publisher.MonoCacheTime] :
reactor.core.publisher.Mono.checkpoint(Mono.java:2206)
org.cloudfoundry.reactor.tokenprovider.AbstractUaaTokenProvider.token(AbstractUaaTokenProvider.java:261)
Error has been observed at the following site(s):
*__checkpoint() ⇢ at org.cloudfoundry.reactor.tokenprovider.AbstractUaaTokenProvider.token(AbstractUaaTokenProvider.java:261)

While checking the flow with this stacktrace, it seems this error is getting thrown at InternalManySink.emitNext(...) level. It is failing due to state 'FAIL_NON_SERIALIZED', as tryEmitNext(..) is getting triggered by different threads at the same time.

It will be helpful if someone can share some insights on the same, and share any document for more idea on tokenprovider feature of cf java client or share any already existing issue on the same line for more details

Thanks.