Introduce a WaitEventSet for the stats collector.

This avoids avoids some epoll/kqueue system calls for every wait.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -4458,6 +4458,8 @@ PgstatCollectorMain(int argc, char *argv[])
 	int			len;
 	PgStat_Msg	msg;
 	int			wr;
+	WaitEvent	event;
+	WaitEventSet *wes;
 
 	/*
 	 * Ignore all signals usually bound to some action in the postmaster,
@@ -4485,6 +4487,12 @@ PgstatCollectorMain(int argc, char *argv[])
 	pgStatRunningInCollector = true;
 	pgStatDBHash = pgstat_read_statsfiles(InvalidOid, true, true);
 
+	/* Prepare to wait for our latch or data in our socket. */
+	wes = CreateWaitEventSet(CurrentMemoryContext, 3);
+	AddWaitEventToSet(wes, WL_LATCH_SET, PGINVALID_SOCKET, MyLatch, NULL);
+	AddWaitEventToSet(wes, WL_POSTMASTER_DEATH, PGINVALID_SOCKET, NULL, NULL);
+	AddWaitEventToSet(wes, WL_SOCKET_READABLE, pgStatSock, NULL, NULL);
+
 	/*
 	 * Loop to process messages until we get SIGQUIT or detect ungraceful
 	 * death of our parent postmaster.
@@ -4672,10 +4680,7 @@ PgstatCollectorMain(int argc, char *argv[])
 
 		/* Sleep until there's something to do */
 #ifndef WIN32
-		wr = WaitLatchOrSocket(MyLatch,
-							   WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_SOCKET_READABLE,
-							   pgStatSock, -1L,
-							   WAIT_EVENT_PGSTAT_MAIN);
+		wr = WaitEventSetWait(wes, -1L, &event, 1, WAIT_EVENT_PGSTAT_MAIN);
 #else
 
 		/*
@@ -4688,10 +4693,7 @@ PgstatCollectorMain(int argc, char *argv[])
 		 * to not provoke "using stale statistics" complaints from
 		 * backend_read_statsfile.
 		 */
-		wr = WaitLatchOrSocket(MyLatch,
-							   WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_SOCKET_READABLE | WL_TIMEOUT,
-							   pgStatSock,
-							   2 * 1000L /* msec */ ,
+		wr = WaitEventSetWait(wes, 2 * 1000L /* msec */ , &event, 1,
 							  WAIT_EVENT_PGSTAT_MAIN);
 #endif
 
@@ -4699,7 +4701,7 @@ PgstatCollectorMain(int argc, char *argv[])
 		 * Emergency bailout if postmaster has died.  This is to avoid the
 		 * necessity for manual cleanup of all postmaster children.
 		 */
-		if (wr & WL_POSTMASTER_DEATH)
+		if (wr == 1 && event.events == WL_POSTMASTER_DEATH)
 			break;
 	}							/* end of outer loop */
 
@@ -4708,6 +4710,8 @@ PgstatCollectorMain(int argc, char *argv[])
 	 */
 	pgstat_write_statsfiles(true, true);
 
+	FreeWaitEventSet(wes);
+
 	exit(0);
 }