8a906204ae
replorigin_drop() misunderstood the API for condition variables: it had ConditionVariablePrepareToSleep and ConditionVariableCancelSleep inside its test-and-sleep loop, rather than outside the loop as intended. The net effect is a narrow race-condition window wherein, if the process using a replication slot releases it immediately after replorigin_drop() releases the ReplicationOriginLock, replorigin_drop() would get into the condition variable's wait list too late and then wait indefinitely for a signal that won't come. Because there's a different CV for each replication slot, we can't just move the ConditionVariablePrepareToSleep call to above the test-and-sleep loop. What we can do, in the wake of commit 13db3b936, is drop the ConditionVariablePrepareToSleep call entirely. This fix depends on that commit because (at least in principle) the slot matching the target replication origin might move around, so that once in a blue moon successive loop iterations might involve different CVs. We can now cope with such a scenario, at the cost of an extra trip through the retry loop. (There are ways we could fix this bug without depending on that commit, but they're all a lot more complicated than this way.) While at it, upgrade the rather skimpy comments in this function. Back-patch to v10 where this code came in. Discussion: https://postgr.es/m/19947.1515455433@sss.pgh.pa.us
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