2.x: Fix boundary fusion of concatMap and publish operator #6145
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This PR fixes the lack of boundary fusion marker in the
concatMap
andpublish
operators that can break the thread-confinement expectation when usingobserveOn
upstream and a thread-sensitive operator:By default, these operators may call
poll
from any of the participating threads which would otherwise dequeueobserveOn
and executemap
on that thread.The fix adds the necessary flag to both operator's
onSubscribe
logic. For a temporary workaround, one should usehide()
before such operators to make sure fusion doesn't happen.Fixes: #6144