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When WalSndCaughtUp, sleep only in WalSndWaitForWal().

Before sleeping, WalSndWaitForWal() sends a keepalive if MyWalSnd->write< sentPtr. That is important in logical replication. When the latestphysical LSN yields no logical replication messages (a common case),that keepalive elicits a reply, and processing the reply updatespg_stat_replication.replay_lsn. WalSndLoop() lacks that; whenWalSndLoop() slept, replay_lsn advancement could stall untilwal_receiver_status_interval elapsed. This sometimes stalledsrc/test/subscription/t/001_rep_changes.pl for up to 10s.Discussion:https://postgr.es/m/20200406063649.GA3738151@rfd.leadboat.com

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src/backend/replication
- walsender.c

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`‎src/backend/replication/walsender.c`

Lines changed: 8 additions & 13 deletions

Original file line number	Diff line number	Diff line change
`@@ -1428,8 +1428,10 @@ WalSndWaitForWal(XLogRecPtr loc)`
`1428`	`1428`	`/*`
`1429`	`1429`	`* We only send regular messages to the client for full decoded`
`1430`	`1430`	`* transactions, but a synchronous replication and walsender shutdown`
`1431`		`- * possibly are waiting for a later location. So we send pings`
`1432`		`- * containing the flush location every now and then.`
	`1431`	`+ * possibly are waiting for a later location. So, before sleeping, we`
	`1432`	`+ * send a ping containing the flush location. If the receiver is`
	`1433`	`+ * otherwise idle, this keepalive will trigger a reply. Processing the`
	`1434`	`+ * reply will update these MyWalSnd locations.`
`1433`	`1435`	`*/`
`1434`	`1436`	`if (MyWalSnd->flush<sentPtr&&`
`1435`	`1437`	`MyWalSnd->write<sentPtr&&`
`@@ -2314,30 +2316,23 @@ WalSndLoop(WalSndSendDataCallback send_data)`
`2314`	`2316`	`WalSndKeepaliveIfNecessary();`
`2315`	`2317`
`2316`	`2318`	`/*`
`2317`		`- * We don't block if not caught up, unless there is unsent data`
`2318`		`- * pending in which case we'd better block until the socket is`
`2319`		`- * write-ready. This test is only needed for the case where the`
`2320`		`- * send_data callback handled a subset of the available data but then`
`2321`		`- * pq_flush_if_writable flushed it all --- we should immediately try`
`2322`		`- * to send more.`
	`2319`	`+ * Block if we have unsent data. Let WalSndWaitForWal() handle any`
	`2320`	`+ * other blocking; idle receivers need its additional actions.`
`2323`	`2321`	`*/`
`2324`		`-if ((WalSndCaughtUp&& !streamingDoneSending)\|\|pq_is_send_pending())`
	`2322`	`+if (pq_is_send_pending())`
`2325`	`2323`	`{`
`2326`	`2324`	`longsleeptime;`
`2327`	`2325`	`intwakeEvents;`
`2328`	`2326`
`2329`	`2327`	`wakeEvents=WL_LATCH_SET \|WL_EXIT_ON_PM_DEATH \|WL_TIMEOUT \|`
`2330`		`-WL_SOCKET_READABLE;`
	`2328`	`+WL_SOCKET_READABLE \|WL_SOCKET_WRITEABLE;`
`2331`	`2329`
`2332`	`2330`	`/*`
`2333`	`2331`	`* Use fresh timestamp, not last_processing, to reduce the chance`
`2334`	`2332`	`* of reaching wal_sender_timeout before sending a keepalive.`
`2335`	`2333`	`*/`
`2336`	`2334`	`sleeptime=WalSndComputeSleeptime(GetCurrentTimestamp());`
`2337`	`2335`
`2338`		`-if (pq_is_send_pending())`
`2339`		`-wakeEvents \|=WL_SOCKET_WRITEABLE;`
`2340`		`-`
`2341`	`2336`	`/* Sleep until something happens or we time out */`
`2342`	`2337`	`(void)WaitLatchOrSocket(MyLatch,wakeEvents,`
`2343`	`2338`	`MyProcPort->sock,sleeptime,`

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`‎src/backend/replication/walsender.c`

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