Use WaitLatch() for condition variables.

Previously, condition_variable.c created a long lived WaitEventSet to
avoid extra system calls.  WaitLatch() now uses something similar
internally, so there is no point in wasting an extra kernel descriptor.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/storage/lmgr/condition_variable.c

@@ -30,9 +30,6 @@
 /* Initially, we are not prepared to sleep on any condition variable. */
 static ConditionVariable *cv_sleep_target = NULL;
 
-/* Reusable WaitEventSet. */
-static WaitEventSet *cv_wait_event_set = NULL;
-
 /*
  * Initialize a condition variable.
  */
@@ -62,23 +59,6 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
 {
 	int			pgprocno = MyProc->pgprocno;
 
-	/*
-	 * If first time through in this process, create a WaitEventSet, which
-	 * we'll reuse for all condition variable sleeps.
-	 */
-	if (cv_wait_event_set == NULL)
-	{
-		WaitEventSet *new_event_set;
-
-		new_event_set = CreateWaitEventSet(TopMemoryContext, 2);
-		AddWaitEventToSet(new_event_set, WL_LATCH_SET, PGINVALID_SOCKET,
-						  MyLatch, NULL);
-		AddWaitEventToSet(new_event_set, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
-						  NULL, NULL);
-		/* Don't set cv_wait_event_set until we have a correct WES. */
-		cv_wait_event_set = new_event_set;
-	}
-
 	/*
 	 * If some other sleep is already prepared, cancel it; this is necessary
 	 * because we have just one static variable tracking the prepared sleep,
@@ -135,6 +115,7 @@ ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
 	long		cur_timeout = -1;
 	instr_time	start_time;
 	instr_time	cur_time;
+	int			wait_events;
 
 	/*
 	 * If the caller didn't prepare to sleep explicitly, then do so now and
@@ -166,19 +147,20 @@ ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
 		INSTR_TIME_SET_CURRENT(start_time);
 		Assert(timeout >= 0 && timeout <= INT_MAX);
 		cur_timeout = timeout;
+		wait_events = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
 	}
+	else
+		wait_events = WL_LATCH_SET | WL_EXIT_ON_PM_DEATH;
 
 	while (true)
 	{
-		WaitEvent	event;
 		bool		done = false;
 
 		/*
 		 * Wait for latch to be set.  (If we're awakened for some other
 		 * reason, the code below will cope anyway.)
 		 */
-		(void) WaitEventSetWait(cv_wait_event_set, cur_timeout, &event, 1,
-								wait_event_info);
+		(void) WaitLatch(MyLatch, wait_events, cur_timeout, wait_event_info);
 
 		/* Reset latch before examining the state of the wait list. */
 		ResetLatch(MyLatch);