1.x: fix takeLast() backpressure #3839
Merged
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The counted and timed versions of
takeLast
had a concurrency bug which allowed concurrent emission from the underlying queue that holds onto the values till the upstream completes. When timed correctly, both anonCompleted()
and arequest()
that went from 0 to n could enter the emit() method and break the internal state of the non-concurrent queue. Note that this queue doesn't have to be concurrent by nature because the operator can emit only after the upstream completes (offer()
never runs concurrently withpoll()
unlike inobserveOn
).In addition, the emission didn't immediately complete if the queue was emptied and requested == 0. In this operator, the fact that the queue became empty is the indicator of completion.
Both issues can be fixed with the
postCompleteXXX
mechanics inBackpressureUtils
.Since the operator has to store
null
values, thepostCompleteXXX
had to be extended with an exit transform overload to allow converting theNotificationLite
values back to regularT
/null
.