Automatically resize signalling socket buffer if full

If the socketpair used by signaler_t fills up, this can lead to deadlock.
This patch provides partial resolution by attempting to resize SO_SNDBUF on
the writer side, and if that fails we shall at least assert rather than
hang.

I've also refactored the signaler_t code to make the platform-dependent
parts clearer and have tested both the MSG_DONTWAIT and standard POSIX path
in recv.

The Win32 implementation currently does not implement resizing as I'm not
convinced that it's safe, but it will also assert like other platforms if
signaler_t::send() cannot succeed.

The OpenVMS implementation has been carried forward but is untested.
Signed-off-by: Martin Lucina <mato@kotelna.sk>

src/config.hpp

@@ -35,10 +35,6 @@ namespace zmq
         //  memory allocation by approximately 99.6%
         message_pipe_granularity = 256,
 
-        //  Socketpair send buffer size used by signaler. The default value of
-        //  zero means leave it at the system default.
-        signaler_sndbuf_size = 0,
-
         //  Determines how often does socket poll for new commands when it
         //  still has unprocessed messages to handle. Thus, if it is set to 100,
         //  socket will process 100 inbound messages before doing the poll.

src/signaler.hpp

@@ -44,24 +44,14 @@ namespace zmq
         
     private:
 
-#if defined ZMQ_HAVE_OPENVMS
-
-        //  Whilst OpenVMS supports socketpair - it maps to AF_INET only.
-        //  Further, it does not set the socket options TCP_NODELAY and
-        //  TCP_NODELACK which can lead to performance problems. We'll
-        //  overload the socketpair function for this class.
-        //
-        //  The bug will be fixed in V5.6 ECO4 and beyond.  In the
-        //  meantime, we'll create the socket pair manually.
-        static int socketpair (int domain_, int type_, int protocol_,
-            int sv_ [2]);
-#endif
-
         //  Write & read end of the socketpair.
         fd_t w;
         fd_t r;
 
-        //  Disable copying of fd_signeler object.
+        //  Platform-dependent function to create a socketpair.
+        static int make_socketpair (fd_t *r_, fd_t *w_);
+
+        //  Disable copying of signaler_t object.
         signaler_t (const signaler_t&);
         void operator = (const signaler_t&);
     };