Ensure that ER_CONNECTION_KILLED error message is not lost

my_real_read() detects if the connection was killed and sets error to
ER_CONNECTION_KILLED. However net_real_write() overrides the error
with ER_NET_READ_INTERRUPTED or ER_NET_READ_ERROR when it tries to
send the 'Connection was killed' to the user on a closed connection.

Fixed by not overwriting the original error code if the connection was
shutdown.

Rewiewed-by: Kristian Nielsen <knielsen@knielsen-hq.org>

Author: montywi

sql/net_serv.cc

@@ -773,18 +773,22 @@ net_real_write(NET *net,const uchar *packet, size_t len)
       }
 #endif /* !defined(MYSQL_SERVER) */
       net->error= 2;				/* Close socket */
-      net->last_errno= (interrupted ? ER_NET_WRITE_INTERRUPTED :
-                        ER_NET_ERROR_ON_WRITE);
-#ifdef MYSQL_SERVER
-      if (global_system_variables.log_warnings > 3)
+
+      if (net->vio->state != VIO_STATE_SHUTDOWN || net->last_errno == 0)
       {
-        sql_print_warning("Could not write packet: fd: %lld  state: %d  "
-                          "errno: %d  vio_errno: %d  length: %ld",
-                          (longlong) vio_fd(net->vio), (int) net->vio->state,
-                          vio_errno(net->vio), net->last_errno,
-                          (ulong) (end-pos));
-      }
+        net->last_errno= (interrupted ? ER_NET_WRITE_INTERRUPTED :
+                          ER_NET_ERROR_ON_WRITE);
+#ifdef MYSQL_SERVER
+        if (global_system_variables.log_warnings > 3)
+        {
+          sql_print_warning("Could not write packet: fd: %lld  state: %d  "
+                            "errno: %d  vio_errno: %d  length: %ld",
+                            (longlong) vio_fd(net->vio), (int) net->vio->state,
+                            vio_errno(net->vio), net->last_errno,
+                            (ulong) (end-pos));
+        }
 #endif
+      }
       MYSQL_SERVER_my_error(net->last_errno, MYF(0));
       break;
     }