Fix usage of WaitEventSetWait() with timeout (#10947)

WaitEventSetWait() returns the number of "events" that happened, and
only that many events in the WaitEvent array are updated. When the
timeout is reached, it returns 0 and does not modify the WaitEvent array
at all. We were reading 'event.events' without checking the return
value, which would be uninitialized when the timeout was hit.

No test included, as this is harmless at the moment. But this caused the
test I'm including in PR #10882 to fail. That PR changes the logic to
loop back to retry the PQgetCopyData() call if WL_SOCKET_READABLE was
set. Currently, making an extra call to PQconsumeInput() is harmless,
but with that change in logic, it turns into a busy-wait.

pgxn/neon/libpagestore.c

@@ -716,20 +716,21 @@ call_PQgetCopyData(shardno_t shard_no, char **buffer)
 
 	if (ret == 0)
 	{
-		WaitEvent	event;
+		WaitEvent	occurred_event;
+		int			noccurred;
 		long		timeout;
 
 		timeout = Max(0, LOG_INTERVAL_MS - INSTR_TIME_GET_MILLISEC(since_last_log));
 
 		/* Sleep until there's something to do */
-		(void) WaitEventSetWait(shard->wes_read, timeout, &event, 1,
-								WAIT_EVENT_NEON_PS_READ);
+		noccurred = WaitEventSetWait(shard->wes_read, timeout, &occurred_event, 1,
+									 WAIT_EVENT_NEON_PS_READ);
 		ResetLatch(MyLatch);
 
 		CHECK_FOR_INTERRUPTS();
 
 		/* Data available in socket? */
-		if (event.events & WL_SOCKET_READABLE)
+		if (noccurred > 0 && (occurred_event.events & WL_SOCKET_READABLE) != 0)
 		{
 			if (!PQconsumeInput(pageserver_conn))
 			{