[PATCH] Fix poll/select signal socket as write ready on connect failure

[Corinna Vinschen]

Hi Marc,

On Jul 17 20:56, Marc Hoersken via Cygwin wrote:
> Hi Corinna,
> 
> Am 16.07.2020 um 11:25 schrieb Corinna Vinschen:
> [...]
> > So, rather than setting FD_WRITE at some later point in the code, what
> > about handling this where the other FD_CONNECT stuff is handled, by
> > not erasing the FD_CONNECT bit, just like with FD_CLOSE?
> 
> I think this makes more sense, yes. I am just not sure if the socket should
> also be write ready in case of a socket error. Looking at the description of
> EINPROGRESS on the man page of connect [1], it seems like writability is
> given regardless of the connection being successful or not, but as soon as
> the connection attempt is no longer pending. For successful connections
> FD_WRITE will be given already, so we will only need to set it for failed
> connections regardless of a socket error in wsock_events->connect_errorcode.
> Therefore I suggest to move the line setting FD_WRITE [2] one level up
> outside of the else branch.

Sounds right to me.

> [...]
> I already tested your diff successfully, so this could be an alternative
> approach to the issue. I just think the wsock_events->connect_errorcode
> should also only be reset if FD_CONNECT is removed, right? So the if branch
> would need to be extended to include the second line [3] as well.

I don't agree here.  The sole purpose for connect_errorcode is to set
SOL_SOCKET/SO_ERROR in case a caller requests FD_CONNECT and FD_CONNECT
is available.  After being set once, SOL_SOCKET/SO_ERROR should not be
rewritten, given the description of SO_ERROR in `man 7 socket':

    SO_ERROR
	  Get and clear the pending socket error.  This socket  option  is
	      ^^^^^^^^^
	  read-only.  Expects an integer.

Therefore I'm inclined to push this:

diff --git a/winsup/cygwin/fhandler_socket_inet.cc b/winsup/cygwin/fhandler_socket_inet.cc
index e5b0d2d1443e..2b50671e533d 100644
--- a/winsup/cygwin/fhandler_socket_inet.cc
+++ b/winsup/cygwin/fhandler_socket_inet.cc
@@ -352,9 +352,13 @@ fhandler_socket_wsock::evaluate_events (const long event_mask, long &events,
 	      WSASetLastError (wsa_err);
 	      ret = SOCKET_ERROR;
 	    }
-	  else
-	    wsock_events->events |= FD_WRITE;
-	  wsock_events->events &= ~FD_CONNECT;
+	  /* Since FD_CONNECT is only given once, we have to keep FD_CONNECT
+	     for connection failed sockets to have consistent behaviour in
+	     programs calling poll/select multiple times.  Example test to
+	     non-listening port: curl -v 127.0.0.1:47 */
+	  if (connect_state () != connect_failed)
+	    wsock_events->events &= ~FD_CONNECT;
+	  wsock_events->events |= FD_WRITE;
 	  wsock_events->connect_errorcode = 0;
 	}
       /* This test makes accept/connect behave as on Linux when accept/connect
@@ -376,12 +380,6 @@ fhandler_socket_wsock::evaluate_events (const long event_mask, long &events,
       if (erase)
 	wsock_events->events &= ~(events & ~(FD_WRITE | FD_CLOSE));
     }
-  /* Since FD_CONNECT is only given once, we manually need to set
-     FD_WRITE for connection failed sockets to have consistent
-     behaviour in programs calling poll/select multiple times.
-     Example test to non-listening port: curl -v 127.0.0.1:47 */
-  if ((connect_state () == connect_failed) && (event_mask & FD_WRITE))
-    wsock_events->events |= FD_WRITE;
   UNLOCK_EVENTS;
 
   return ret;


Make sense?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer