2.x: Fix Observable.flatMap to sustain concurrency level #6283
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If the
Observable.flatMap
operator run in limited concurrency mode and multiple sources completed at the same time while the operator was in its drain loop, the operator only started with one new inner source instead of trying to run replace all the completed inner sources with new ones.The solution is to count the completed sources and replenish them in a loop.
(The
Flowable
variant works properly because it uses backpressure and inner source counting already to replenish those completed inner sources. TheObservable
doesn't have backpressure so it has to emulate it via the work-in-progress counting and the secondary queue for available inner sources.)Fixes: #6282