2.x: Fix Flowable.concatMap backpressure w/ scalars #7091
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In
concatMap, there is a shortcut for when the mappedFlowableturns out to be a scalar value and thus the full subscription process can be skipped. This used a so-called weak subscription that expected non-concurrent requesting to emits its single value.Unfortunately, there is a race condition for when the downstream requests exactly when this weak subscription is activated, resulting in either double emission or no emission at all. The fix is to do the proper
compareAndSetto ensure the emission happens exactly once.Backport of #7089