MXS-1921: Explain why session was closed

When a client connection is closed by MaxScale before the client initiates
a controlled closing of the connection, an error message is sent. This
error message now also explains why the connection was closed to make
problem resolution easier.

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.c

@@ -583,6 +583,7 @@ static void do_handle_error(DCB *dcb, mxs_error_action_t action, const char *err
      */
     if (!succp)
     {
+        session->close_reason = SESSION_CLOSE_HANDLEERROR_FAILED;
         poll_fake_hangup_event(session->client_dcb);
     }
 }

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1136,12 +1136,14 @@ gw_read_finish_processing(DCB *dcb, GWBUF *read_buffer, uint64_t capabilities)
     if (return_code != 0)
     {
         /** Routing failed, close the client connection */
+        dcb->session->close_reason = SESSION_CLOSE_ROUTING_FAILED;
         dcb_close(dcb);
         MXS_ERROR("Routing the query failed. Session will be closed.");
     }
     else if (proto->current_command == MXS_COM_QUIT)
     {
         /** Close router session which causes closing of backends */
+        ss_info_dassert(session_valid_for_pool(dcb->session), "Session should qualify for pooling");
         dcb_close(dcb);
     }
 
@@ -1471,7 +1473,15 @@ static int gw_client_hangup_event(DCB *dcb)
             }
 
             // The client did not send a COM_QUIT packet
-            modutil_send_mysql_err_packet(dcb, 0, 0, 1927, "08S01", "Connection killed by MaxScale");
+            std::string errmsg{"Connection killed by MaxScale"};
+            std::string extra{session_get_close_reason(dcb->session)};
+
+            if (!extra.empty())
+            {
+                errmsg += ": " + extra;
+            }
+
+            modutil_send_mysql_err_packet(dcb, 0, 0, 1927, "08S01", errmsg.c_str());
         }
         dcb_close(dcb);
     }

server/modules/protocol/MySQL/mysql_common.cc

@@ -1704,6 +1704,7 @@ static bool kill_func(DCB *dcb, void *data)
         else
         {
             // DCB is not yet connected, send a hangup to forcibly close it
+            dcb->session->close_reason = SESSION_CLOSE_KILLED;
             poll_fake_hangup_event(dcb);
         }
     }