Be clear about whether a recovery pause has taken effect.

Previously, the code and documentation seem to have essentially
assumed than a call to pg_wal_replay_pause() would take place
immediately, but that's not the case, because we only check for a
pause in certain places. This means that a tool that uses this
function and then wants to do something else afterward that is
dependent on the pause having taken effect doesn't know how long it
needs to wait to be sure that no more WAL is going to be replayed.

To avoid that, add a new function pg_get_wal_replay_pause_state()
which returns either 'not paused', 'paused requested', or 'paused'.
After calling pg_wal_replay_pause() the status will immediate change
from 'not paused' to 'pause requested'; when the startup process
has noticed this, the status will change to 'pause'.  For backward
compatibility, pg_is_wal_replay_paused() still exists and returns
the same thing as before: true if a pause has been requested,
whether or not it has taken effect yet; and false if not.
The documentation is updated to clarify.

To improve the changes that a pause request is quickly confirmed
effective, adjust things so that WaitForWALToBecomeAvailable will
swiftly reach a call to recoveryPausesHere() when a pause request
is made.

Dilip Kumar, reviewed by Simon Riggs, Kyotaro Horiguchi, Yugo Nagata,
Masahiko Sawada, and Bharath Rupireddy.

Discussion: http://postgr.es/m/CAFiTN-vcLLWEm8Zr%3DYK83rgYrT9pbC8VJCfa1kY9vL3AUPfu6g%40mail.gmail.com

1 files changed, 43 insertions, 2 deletions

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index d8c5bf6dc29..daa4a113b7d 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -517,7 +517,7 @@ pg_walfile_name(PG_FUNCTION_ARGS)
 }
 
 /*
- * pg_wal_replay_pause - pause recovery now
+ * pg_wal_replay_pause - Request to pause recovery
  *
  * Permission checking for this function is managed through the normal
  * GRANT system.
@@ -540,6 +540,9 @@ pg_wal_replay_pause(PG_FUNCTION_ARGS)
 
 	SetRecoveryPause(true);
 
+	/* wake up the recovery process so that it can process the pause request */
+	WakeupRecovery();
+
 	PG_RETURN_VOID();
 }
 
@@ -582,7 +585,45 @@ pg_is_wal_replay_paused(PG_FUNCTION_ARGS)
 				 errmsg("recovery is not in progress"),
 				 errhint("Recovery control functions can only be executed during recovery.")));
 
-	PG_RETURN_BOOL(RecoveryIsPaused());
+	PG_RETURN_BOOL(GetRecoveryPauseState() != RECOVERY_NOT_PAUSED);
+}
+
+/*
+ * pg_get_wal_replay_pause_state - Returns the recovery pause state.
+ *
+ * Returned values:
+ *
+ * 'not paused' - if pause is not requested
+ * 'pause requested' - if pause is requested but recovery is not yet paused
+ * 'paused' - if recovery is paused
+ */
+Datum
+pg_get_wal_replay_pause_state(PG_FUNCTION_ARGS)
+{
+	char	*statestr = NULL;
+
+	if (!RecoveryInProgress())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("recovery is not in progress"),
+				 errhint("Recovery control functions can only be executed during recovery.")));
+
+	/* get the recovery pause state */
+	switch(GetRecoveryPauseState())
+	{
+		case RECOVERY_NOT_PAUSED:
+			statestr = "not paused";
+			break;
+		case RECOVERY_PAUSE_REQUESTED:
+			statestr = "pause requested";
+			break;
+		case RECOVERY_PAUSED:
+			statestr = "paused";
+			break;
+	}
+
+	Assert(statestr != NULL);
+	PG_RETURN_TEXT_P(cstring_to_text(statestr));
 }
 
 /*