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Commit5231ed8

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Use replay LSN as target for cascading logical WAL senders
A cascading WAL sender doing logical decoding (as known as doing itswork on a standby) has been using as flush LSN the value returned byGetStandbyFlushRecPtr() (last position safely flushed to disk). This isincorrect as such processes are only able to decode changes up to theLSN that has been replayed by the startup process.This commit changes cascading logical WAL senders to use the replay LSN,as returned by GetXLogReplayRecPtr(). This distinction is importantparticularly during shutdown, when WAL senders need to send anyremaining available data to their clients, switching WAL senders to acaught-up state. Using the latest flush LSN rather than the replay LSNcould cause the WAL senders to be stuck in an infinite loop preventingthem to shut down, as the startup process does not run when WAL sendersattempt to catch up, so they could keep waiting for work that wouldnever happen.Backpatch down to v16, where logical decoding on standbys has beenintroduced.Author: Alexey Makhmutov <a.makhmutov@postgrespro.ru>Reviewed-by: Ajin Cherian <itsajin@gmail.com>Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>Reviewed-by: Michael Paquier <michael@paquier.xyz>Discussion:https://postgr.es/m/52138028-7246-421c-9161-4fa108b88070@postgrespro.ruBackpatch-through: 16
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‎src/backend/replication/walsender.c

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -3449,8 +3449,16 @@ XLogSendLogical(void)
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if (flushPtr==InvalidXLogRecPtr||
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logical_decoding_ctx->reader->EndRecPtr >=flushPtr)
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{
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/*
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* For cascading logical WAL senders, we use the replay LSN instead of
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* the flush LSN, since logical decoding on a standby only processes
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* WAL that has been replayed. This distinction becomes particularly
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* important during shutdown, as new WAL is no longer replayed and the
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* last replayed LSN marks the furthest point up to which decoding can
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* proceed.
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*/
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if (am_cascading_walsender)
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flushPtr=GetStandbyFlushRecPtr(NULL);
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flushPtr=GetXLogReplayRecPtr(NULL);
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else
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flushPtr=GetFlushRecPtr(NULL);
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}

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