Fix WaitEventSet resource leak in WaitLatchOrSocket().

This function would have the same issue we solved in commit 501cfd0:
If an error is thrown after calling CreateWaitEventSet(), the file
descriptor (on epoll- or kqueue-based systems) or handles (on Windows)
that the WaitEventSet contains are leaked.

Like that commit, use PG_TRY-PG_FINALLY (PG_TRY-PG_CATCH in v12) to make
sure the WaitEventSet is freed properly.

Back-patch to all supported versions, but as we do not have this issue
in HEAD (cf. commit 50c67c2), no need to apply this patch to it.

Discussion: https://postgr.es/m/CAPmGK16MqdDoD8oatp8SQWaEa4vS3nfQqDN_Sj9YRuu5J3Lj9g%40mail.gmail.com

Author: Etsuro Fujita

src/backend/storage/ipc/latch.c

@@ -374,46 +374,55 @@ WaitLatchOrSocket(Latch *latch, int wakeEvents, pgsocket sock,
 	WaitEvent	event;
 	WaitEventSet *set = CreateWaitEventSet(CurrentMemoryContext, 3);
 
-	if (wakeEvents & WL_TIMEOUT)
-		Assert(timeout >= 0);
-	else
-		timeout = -1;
+	PG_TRY();
+	{
+		if (wakeEvents & WL_TIMEOUT)
+			Assert(timeout >= 0);
+		else
+			timeout = -1;
 
-	if (wakeEvents & WL_LATCH_SET)
-		AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,
-						  latch, NULL);
+		if (wakeEvents & WL_LATCH_SET)
+			AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,
+							  latch, NULL);
 
-	/* Postmaster-managed callers must handle postmaster death somehow. */
-	Assert(!IsUnderPostmaster ||
-		   (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
-		   (wakeEvents & WL_POSTMASTER_DEATH));
+		/* Postmaster-managed callers must handle postmaster death somehow. */
+		Assert(!IsUnderPostmaster ||
+			   (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
+			   (wakeEvents & WL_POSTMASTER_DEATH));
 
-	if ((wakeEvents & WL_POSTMASTER_DEATH) && IsUnderPostmaster)
-		AddWaitEventToSet(set, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,
-						  NULL, NULL);
+		if ((wakeEvents & WL_POSTMASTER_DEATH) && IsUnderPostmaster)
+			AddWaitEventToSet(set, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,
+							  NULL, NULL);
 
-	if ((wakeEvents & WL_EXIT_ON_PM_DEATH) && IsUnderPostmaster)
-		AddWaitEventToSet(set, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
-						  NULL, NULL);
+		if ((wakeEvents & WL_EXIT_ON_PM_DEATH) && IsUnderPostmaster)
+			AddWaitEventToSet(set, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
+							  NULL, NULL);
 
-	if (wakeEvents & WL_SOCKET_MASK)
-	{
-		int			ev;
+		if (wakeEvents & WL_SOCKET_MASK)
+		{
+			int			ev;
 
-		ev = wakeEvents & WL_SOCKET_MASK;
-		AddWaitEventToSet(set, ev, sock, NULL, NULL);
-	}
+			ev = wakeEvents & WL_SOCKET_MASK;
+			AddWaitEventToSet(set, ev, sock, NULL, NULL);
+		}
 
-	rc = WaitEventSetWait(set, timeout, &event, 1, wait_event_info);
+		rc = WaitEventSetWait(set, timeout, &event, 1, wait_event_info);
 
-	if (rc == 0)
-		ret |= WL_TIMEOUT;
-	else
+		if (rc == 0)
+			ret |= WL_TIMEOUT;
+		else
+		{
+			ret |= event.events & (WL_LATCH_SET |
+								   WL_POSTMASTER_DEATH |
+								   WL_SOCKET_MASK);
+		}
+	}
+	PG_CATCH();
 	{
-		ret |= event.events & (WL_LATCH_SET |
-							   WL_POSTMASTER_DEATH |
-							   WL_SOCKET_MASK);
+		FreeWaitEventSet(set);
+		PG_RE_THROW();
 	}
+	PG_END_TRY();
 
 	FreeWaitEventSet(set);