Assert that WaitLatchOrSocket callers cannot wait only for writability.

Since we have chosen to report socket EOF and error conditions via the
WL_SOCKET_READABLE flag bit, it's unsafe to wait only for
WL_SOCKET_WRITEABLE; the caller would never be notified of the socket
condition, and in some of these implementations WaitLatchOrSocket would
busy-wait until something else happens.  Add this restriction to the API
specification, and add Asserts to check that callers don't try to do that.

At some point we might want to consider adjusting the API to relax this
restriction, but until we have an actual use case for waiting on a
write-only socket, it seems premature to design a solution.

src/backend/port/unix_latch.c

@@ -172,6 +172,10 @@ WaitLatch(volatile Latch *latch, int wakeEvents, long timeout)
 /*
  * Like WaitLatch, but with an extra socket argument for WL_SOCKET_*
  * conditions.
+ *
+ * When waiting on a socket, WL_SOCKET_READABLE *must* be included in
+ * 'wakeEvents'; WL_SOCKET_WRITEABLE is optional.  The reason for this is
+ * that EOF and error conditions are reported only via WL_SOCKET_READABLE.
  */
 int
 WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
@@ -195,6 +199,8 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);
 
 	Assert(wakeEvents != 0);	/* must have at least one wake event */
+	/* Cannot specify WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE */
+	Assert((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) != WL_SOCKET_WRITEABLE);
 
 	if ((wakeEvents & WL_LATCH_SET) && latch->owner_pid != MyProcPid)
 		elog(ERROR, "cannot wait on a latch owned by another process");
@@ -295,7 +301,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		if ((wakeEvents & WL_SOCKET_READABLE) &&
 			(pfds[0].revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
 		{
-			/* data available in socket */
+			/* data available in socket, or EOF/error condition */
 			result |= WL_SOCKET_READABLE;
 		}
 		if ((wakeEvents & WL_SOCKET_WRITEABLE) &&
@@ -373,7 +379,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		}
 		if ((wakeEvents & WL_SOCKET_READABLE) && FD_ISSET(sock, &input_mask))
 		{
-			/* data available in socket */
+			/* data available in socket, or EOF */
 			result |= WL_SOCKET_READABLE;
 		}
 		if ((wakeEvents & WL_SOCKET_WRITEABLE) && FD_ISSET(sock, &output_mask))

src/backend/port/win32_latch.c

@@ -106,6 +106,8 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);
 
 	Assert(wakeEvents != 0);	/* must have at least one wake event */
+	/* Cannot specify WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE */
+	Assert((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) != WL_SOCKET_WRITEABLE);
 
 	if ((wakeEvents & WL_LATCH_SET) && latch->owner_pid != MyProcPid)
 		elog(ERROR, "cannot wait on a latch owned by another process");