Prefer standby promotion over recovery pause.

Previously if a promotion was triggered while recovery was paused,
the paused state continued. Also recovery could be paused by executing
pg_wal_replay_pause() even while a promotion was ongoing. That is,
recovery pause had higher priority over a standby promotion.
But this behavior was not desirable because most users basically wanted
the recovery to complete as soon as possible and the server to become
the master when they requested a promotion.

This commit changes recovery so that it prefers a promotion over
recovery pause. That is, if a promotion is triggered while recovery
is paused, the paused state ends and a promotion continues. Also
this commit makes recovery pause functions like pg_wal_replay_pause()
throw an error if they are executed while a promotion is ongoing.

Internally, this commit adds new internal function PromoteIsTriggered()
that returns true if a promotion is triggered. Since the name of
this function and the existing function IsPromoteTriggered() are
confusingly similar, the commit changes the name of IsPromoteTriggered()
to IsPromoteSignaled, as more appropriate name.

Author: Fujii Masao
Reviewed-by: Atsushi Torikoshi, Sergei Kornilov
Discussion: https://postgr.es/m/00c194b2-dbbb-2e8a-5b39-13f14048ef0a@oss.nttdata.com

1 files changed, 14 insertions, 0 deletions

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index 20316539b6f..b84ba572596 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -531,6 +531,13 @@ pg_wal_replay_pause(PG_FUNCTION_ARGS)
 				 errmsg("recovery is not in progress"),
 				 errhint("Recovery control functions can only be executed during recovery.")));
 
+	if (PromoteIsTriggered())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("standby promotion is ongoing"),
+				 errhint("%s cannot be executed after promotion is triggered.",
+						 "pg_wal_replay_pause()")));
+
 	SetRecoveryPause(true);
 
 	PG_RETURN_VOID();
@@ -551,6 +558,13 @@ pg_wal_replay_resume(PG_FUNCTION_ARGS)
 				 errmsg("recovery is not in progress"),
 				 errhint("Recovery control functions can only be executed during recovery.")));
 
+	if (PromoteIsTriggered())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("standby promotion is ongoing"),
+				 errhint("%s cannot be executed after promotion is triggered.",
+						 "pg_wal_replay_resume()")));
+
 	SetRecoveryPause(false);
 
 	PG_RETURN_VOID();