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The regression tests contain numerous cases where we do some activity on a master server and then wait till the slave has ack'd flushing its copy of that transaction. Because WAL flush on the slave is asynchronous to the logicalrep worker process, the worker cannot send such a feedback message during the LogicalRepApplyLoop iteration where it processes the last data from the master. In the previous coding, the feedback message would come out only when the loop's WaitLatchOrSocket call returned WL_TIMEOUT. That requires one full second of delay (NAPTIME_PER_CYCLE); and to add insult to injury, it could take more than that if the WaitLatchOrSocket was interrupted a few times by latch-setting events. In reality we can expect the slave's walwriter process to have flushed the WAL data after, more or less, WalWriterDelay (typically 200ms). Hence, if there are unacked transactions pending, make the wait delay only that long rather than the full NAPTIME_PER_CYCLE. Also, move one of the send_feedback() calls into the loop main line, so that we'll check for the need to send feedback even if we were woken by a latch event and not either socket data or timeout. It's not clear how much this matters for production purposes, but it's definitely helpful for testing. Discussion: https://postgr.es/m/30864.1498861103@sss.pgh.pa.us
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