Improve Windows implementation of WaitLatch/WaitLatchOrSocket.

Ensure that signal handlers are serviced before this function returns.
This should make the behavior more like Unix.  Also, add some more
error checking, and make some other cosmetic improvements.

No back-patch since it's not clear whether this is fixing any live bug
that would affect 9.1.  I'm more concerned about 9.2 anyway given our
considerable recent expansions in the usage of WaitLatch.

src/backend/port/win32_latch.c

@@ -115,14 +115,19 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 	else
 		timeout = INFINITE;
 
-	/* Construct an array of event handles for WaitforMultipleObjects() */
+	/*
+	 * Construct an array of event handles for WaitforMultipleObjects().
+	 *
+	 * Note: pgwin32_signal_event should be first to ensure that it will be
+	 * reported when multiple events are set.  We want to guarantee that
+	 * pending signals are serviced.
+	 */
 	latchevent = latch->event;
 
-	events[0] = latchevent;
-	events[1] = pgwin32_signal_event;
+	events[0] = pgwin32_signal_event;
+	events[1] = latchevent;
 	numevents = 2;
-	if ((wakeEvents & WL_SOCKET_READABLE) ||
-		(wakeEvents & WL_SOCKET_WRITEABLE))
+	if (wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
 	{
 		int			flags = 0;
 
@@ -132,7 +137,13 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 			flags |= FD_WRITE;
 
 		sockevent = WSACreateEvent();
-		WSAEventSelect(sock, sockevent, flags);
+		if (sockevent == WSA_INVALID_EVENT)
+			elog(ERROR, "failed to create event for socket: error code %u",
+				 WSAGetLastError());
+		if (WSAEventSelect(sock, sockevent, flags) != 0)
+			elog(ERROR, "failed to set up event for socket: error code %u",
+				 WSAGetLastError());
+
 		events[numevents++] = sockevent;
 	}
 	if (wakeEvents & WL_POSTMASTER_DEATH)
@@ -141,6 +152,9 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		events[numevents++] = PostmasterHandle;
 	}
 
+	/* Ensure that signals are serviced even if latch is already set */
+	pgwin32_dispatch_queued_signals();
+
 	do
 	{
 		/*
@@ -151,6 +165,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		 */
 		if (!ResetEvent(latchevent))
 			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());
+
 		if ((wakeEvents & WL_LATCH_SET) && latch->is_set)
 		{
 			result |= WL_LATCH_SET;
@@ -164,21 +179,20 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		rc = WaitForMultipleObjects(numevents, events, FALSE, timeout);
 
 		if (rc == WAIT_FAILED)
-			elog(ERROR, "WaitForMultipleObjects() failed: error code %lu", GetLastError());
-
-		/* Participate in Windows signal emulation */
-		else if (rc == WAIT_OBJECT_0 + 1)
-			pgwin32_dispatch_queued_signals();
-
+			elog(ERROR, "WaitForMultipleObjects() failed: error code %lu",
+				 GetLastError());
 		else if (rc == WAIT_TIMEOUT)
 		{
 			result |= WL_TIMEOUT;
 		}
-		else if ((wakeEvents & WL_POSTMASTER_DEATH) &&
-				 rc == WAIT_OBJECT_0 + pmdeath_eventno)
+		else if (rc == WAIT_OBJECT_0)
 		{
-			/* Postmaster died */
-			result |= WL_POSTMASTER_DEATH;
+			/* Service newly-arrived signals */
+			pgwin32_dispatch_queued_signals();
+		}
+		else if (rc == WAIT_OBJECT_0 + 1)
+		{
+			/* Latch is set, we'll handle that on next iteration of loop */
 		}
 		else if ((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) &&
 				 rc == WAIT_OBJECT_0 + 2)	/* socket is at event slot 2 */
@@ -187,9 +201,8 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 
 			ZeroMemory(&resEvents, sizeof(resEvents));
 			if (WSAEnumNetworkEvents(sock, sockevent, &resEvents) == SOCKET_ERROR)
-				ereport(FATAL,
-						(errmsg_internal("failed to enumerate network events: error code %lu", GetLastError())));
-
+				elog(ERROR, "failed to enumerate network events: error code %lu",
+					 GetLastError());
 			if ((wakeEvents & WL_SOCKET_READABLE) &&
 				(resEvents.lNetworkEvents & FD_READ))
 			{
@@ -201,8 +214,13 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 				result |= WL_SOCKET_WRITEABLE;
 			}
 		}
-		/* Otherwise it must be the latch event */
-		else if (rc != WAIT_OBJECT_0)
+		else if ((wakeEvents & WL_POSTMASTER_DEATH) &&
+				 rc == WAIT_OBJECT_0 + pmdeath_eventno)
+		{
+			/* Postmaster died */
+			result |= WL_POSTMASTER_DEATH;
+		}
+		else
 			elog(ERROR, "unexpected return code from WaitForMultipleObjects(): %lu", rc);
 	}
 	while (result == 0);