Allow WES's events to listen to no events at all

This enables/simplifies Neon's dynamic socket switching.

Author: Matthias van de Meent

src/backend/storage/ipc/waiteventset.c

@@ -757,9 +757,10 @@ WaitEventAdjustEpoll(WaitEventSet *set, WaitEvent *event, int action)
 	else
 	{
 		Assert(event->fd != PGINVALID_SOCKET);
-		Assert(event->events & (WL_SOCKET_READABLE |
-								WL_SOCKET_WRITEABLE |
-								WL_SOCKET_CLOSED));
+		Assert((event->events & (WL_SOCKET_READABLE |
+								 WL_SOCKET_WRITEABLE |
+								 WL_SOCKET_CLOSED)) ||
+				event->events == 0);
 
 		if (event->events & WL_SOCKET_READABLE)
 			epoll_ev.events |= EPOLLIN;
@@ -805,9 +806,12 @@ WaitEventAdjustPoll(WaitEventSet *set, WaitEvent *event)
 	}
 	else
 	{
-		Assert(event->events & (WL_SOCKET_READABLE |
-								WL_SOCKET_WRITEABLE |
-								WL_SOCKET_CLOSED));
+		Assert(event->fd >= 0);
+		Assert((event->events & (WL_SOCKET_READABLE |
+								 WL_SOCKET_WRITEABLE |
+								 WL_SOCKET_CLOSED)) ||
+				event->events == 0);
+
 		pollfd->events = 0;
 		if (event->events & WL_SOCKET_READABLE)
 			pollfd->events |= POLLIN;
@@ -891,7 +895,8 @@ WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events)
 		   event->events == WL_POSTMASTER_DEATH ||
 		   (event->events & (WL_SOCKET_READABLE |
 							 WL_SOCKET_WRITEABLE |
-							 WL_SOCKET_CLOSED)));
+							 WL_SOCKET_CLOSED)) ||
+		   (event->fd >= 0 && event->events == 0));
 
 	if (event->events == WL_POSTMASTER_DEATH)
 	{