BUG#2845 client fails to reconnect if using TCP/IP

 - Use 'poll' if available
 - Check that sd <= FD_SETSIZE if using 'select'
 - Handle case when 'net_data_is_ready' returns -1, ie. sd > FD_SETSIZE and 'select' is used 


sql/net_serv.cc:
  Use 'poll' in favor of 'select' if avaliable
  This is to avoid the limitation with 'select' only being able to handle fd's with numbers <= 1024 as default.
  If 'poll' is not available use 'select' but check that we are not having a number higher than FD_SETSIZE
  Handle the case when 'net_data_is_ready' can't check if there is data to read, since the sd number is too high

sql/net_serv.cc

@@ -208,23 +208,40 @@ my_bool net_realloc(NET *net, ulong length)
   RETURN VALUES
     0	No data to read
     1	Data or EOF to read
+    -1  Don't know if data is ready or not
 */
 
-static my_bool net_data_is_ready(my_socket sd)
+static int net_data_is_ready(my_socket sd)
 {
+#ifdef HAVE_POLL
+  struct pollfd ufds;
+  int res;
+
+  ufds.fd= sd;
+  ufds.events= POLLIN | POLLPRI;
+  if (!(res= poll(&ufds, 1, 0)))
+    return 0;
+  if (res < 0 || !(ufds.revents & (POLLIN | POLLPRI)))
+    return 0;
+  return 1;
+#else
   fd_set sfds;
   struct timeval tv;
   int res;
 
+  if (sd >= FD_SETSIZE)
+    return -1;
+
   FD_ZERO(&sfds);
   FD_SET(sd, &sfds);
 
   tv.tv_sec= tv.tv_usec= 0;
 
   if ((res= select(sd+1, &sfds, NULL, NULL, &tv)) < 0)
-    return FALSE;
+    return 0;
   else
     return test(res ? FD_ISSET(sd, &sfds) : 0);
+#endif
 }
 
 
@@ -251,10 +268,10 @@ static my_bool net_data_is_ready(my_socket sd)
 
 void net_clear(NET *net)
 {
-  int count;
+  int count, ready;
   DBUG_ENTER("net_clear");
 #if !defined(EMBEDDED_LIBRARY)
-  while(net_data_is_ready(net->vio->sd))
+  while((ready= net_data_is_ready(net->vio->sd)) != 0)
   {
     /* The socket is ready */
     if ((count= vio_read(net->vio, (char*) (net->buff),
@@ -269,6 +286,10 @@ void net_clear(NET *net)
     }
     else
     {
+      /* No data to read and 'net_data_is_ready' returned "don't know" */
+      if (ready == -1)
+        break;
+
       DBUG_PRINT("info",("socket ready but only EOF to read - disconnected"));
       net->error= 2;
       break;