ensures onLastAssembly does not break fusion chain

[OlegDokuka]

closes #3137

as it was observed, onLastOperator may add extra operators to the chain. In the case of scalar fusion at flatMap operators ( https://github.com/reactor/reactor-core/blob/main/reactor-core/src/main/java/reactor/core/publisher/FluxFlatMap.java#L118) the inner chain may be fused with the outer downstream. Since onLastOperator must be applied to the inner chain, modification added by the call may break fusion which leads to the ClastCastException

Signed-off-by: OlegDokuka odokuka@vmware.com

[simonbasle]

@OlegDokuka there's 2 usages of onLastAssembly in ParallelFlux, have you reviewed them see if that fix should also apply there ?

[OlegDokuka]

@OlegDokuka there's 2 usages of onLastAssembly in ParallelFlux, have you reviewed them see if that fix should also apply there ?

Yes, looks like fusión is always off for parallel Flux and already using Hide subscriber

[reactorbot]

@OlegDokuka this PR seems to have been merged on a maintenance branch, please ensure the change is merge-forwarded to intermediate maintenance branches and up to main 🙇