Add condition variable for recovery resume.

Replace a sleep loop with a CV, to get a fast reaction time when
recovery is resumed or the postmaster exits via standard infrastructure.
Unfortunately we still need to wake up every second to perform extra
polling during the recovery pause loop.

Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com

src/backend/access/transam/xlog.c

@@ -724,6 +724,7 @@ typedef struct XLogCtlData
 	TimestampTz currentChunkStartTime;
 	/* Recovery pause state */
 	RecoveryPauseState	recoveryPauseState;
+	ConditionVariable recoveryNotPausedCV;
 
 	/*
 	 * lastFpwDisableRecPtr points to the start of the last replayed
@@ -5228,6 +5229,7 @@ XLOGShmemInit(void)
 	SpinLockInit(&XLogCtl->info_lck);
 	SpinLockInit(&XLogCtl->ulsn_lck);
 	InitSharedLatch(&XLogCtl->recoveryWakeupLatch);
+	ConditionVariableInit(&XLogCtl->recoveryNotPausedCV);
 }
 
 /*
@@ -6040,10 +6042,6 @@ recoveryStopsAfter(XLogReaderState *record)
  * endOfRecovery is true if the recovery target is reached and
  * the paused state starts at the end of recovery because of
  * recovery_target_action=pause, and false otherwise.
- *
- * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
- * Probably not worth the trouble though.  This state shouldn't be one that
- * anyone cares about server power consumption in.
  */
 static void
 recoveryPausesHere(bool endOfRecovery)
@@ -6071,7 +6069,6 @@ recoveryPausesHere(bool endOfRecovery)
 		HandleStartupProcInterrupts();
 		if (CheckForStandbyTrigger())
 			return;
-		pgstat_report_wait_start(WAIT_EVENT_RECOVERY_PAUSE);
 
 		/*
 		 * If recovery pause is requested then set it paused.  While we are in
@@ -6079,9 +6076,15 @@ recoveryPausesHere(bool endOfRecovery)
 		 */
 		ConfirmRecoveryPaused();
 
-		pg_usleep(1000000L);	/* 1000 ms */
-		pgstat_report_wait_end();
+		/*
+		 * We wait on a condition variable that will wake us as soon as the
+		 * pause ends, but we use a timeout so we can check the above exit
+		 * condition periodically too.
+		 */
+		ConditionVariableTimedSleep(&XLogCtl->recoveryNotPausedCV, 1000,
+									WAIT_EVENT_RECOVERY_PAUSE);
 	}
+	ConditionVariableCancelSleep();
 }
 
 /*
@@ -6118,6 +6121,9 @@ SetRecoveryPause(bool recoveryPause)
 		XLogCtl->recoveryPauseState = RECOVERY_PAUSE_REQUESTED;
 
 	SpinLockRelease(&XLogCtl->info_lck);
+
+	if (!recoveryPause)
+		ConditionVariableBroadcast(&XLogCtl->recoveryNotPausedCV);
 }
 
 /*
@@ -6357,10 +6363,15 @@ RecoveryRequiresIntParameter(const char *param_name, int currValue, int minValue
 				 */
 				ConfirmRecoveryPaused();
 
-				pgstat_report_wait_start(WAIT_EVENT_RECOVERY_PAUSE);
-				pg_usleep(1000000L);	/* 1000 ms */
-				pgstat_report_wait_end();
+				/*
+				 * We wait on a condition variable that will wake us as soon as
+				 * the pause ends, but we use a timeout so we can check the
+				 * above conditions periodically too.
+				 */
+				ConditionVariableTimedSleep(&XLogCtl->recoveryNotPausedCV, 1000,
+											WAIT_EVENT_RECOVERY_PAUSE);
 			}
+			ConditionVariableCancelSleep();
 		}
 
 		ereport(FATAL,