MXS-1951: Move worker selection into Listener

The worker to which the client DCB is assigned to is now chosen by the
Listener. This makes the protocol code simpler which is always a good
thing.

server/core/listener.cc

@@ -1024,7 +1024,10 @@ uint32_t Listener::poll_handler(MXB_POLL_DATA* data, MXB_WORKER* worker, uint32_
 
     while ((client_dcb = listener->accept_one_dcb()))
     {
+        auto worker = mxs::RoutingWorker::pick_worker();
+        worker->execute([listener, client_dcb]() {
                             listener->m_proto_func.accept(client_dcb);
+                        }, mxs::RoutingWorker::EXECUTE_AUTO);
     }
 
     return 1;

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

@@ -72,7 +72,6 @@ static void           mysql_client_auth_error_handling(DCB* dcb, int auth_val, i
 static int            gw_read_do_authentication(DCB* dcb, GWBUF* read_buffer, int nbytes_read);
 static int            gw_read_normal_data(DCB* dcb, GWBUF* read_buffer, int nbytes_read);
 static int            gw_read_finish_processing(DCB* dcb, GWBUF* read_buffer, uint64_t capabilities);
-static void           gw_process_one_new_client(DCB* client_dcb);
 static spec_com_res_t process_special_commands(DCB* client_dcb, GWBUF* read_buffer, int nbytes_read);
 static spec_com_res_t handle_query_kill(DCB* dcb,
                                         GWBUF* read_buffer,
@@ -1379,59 +1378,32 @@ return_1:
 }
 
 /**
- * @node Accept a new connection, using the DCB code for the basic work
+ * @node Accept and prepare a new client connection
  *
- * For as long as dcb_accept can return new client DCBs for new connections,
+ * @param dcb The client DCB that was created
- * continue to loop. The code will always give a failure return, since it
- * continues to try to create new connections until a failure occurs.
- *
- * @param listener - The Listener DCB that picks up new connection requests
- * @return 0 in success, 1 in failure
  *
+ * @return always 1
  */
-int gw_MySQLAccept(DCB* client_dcb)
+int gw_MySQLAccept(DCB* dcb)
 {
-    gw_process_one_new_client(client_dcb);
+    dcb->protocol = mysql_protocol_init(dcb, dcb->fd);
-    return 1;
+    MXS_ABORT_IF_NULL(dcb->protocol);
-}
 
-static void gw_process_one_new_client(DCB* client_dcb)
+    if (poll_add_dcb(dcb) == -1)
     {
-    /**
+        mysql_send_custom_error(dcb, 1, 0,
-     * The worker who owns the DCB is chosen here, before any epoll events for it can be processed.
-     * This guarantees that the first event for the DCB is processed only after the following
-     * task has been processed by the owning thread.
-     */
-    mxs::RoutingWorker* worker = mxs::RoutingWorker::pick_worker();
-
-    worker->execute([=]() {
-                        client_dcb->protocol = mysql_protocol_init(client_dcb, client_dcb->fd);
-                        MXS_ABORT_IF_NULL(client_dcb->protocol);
-
-                        /**
-                         * Set new descriptor to event set. At the same time,
-                         * change state to DCB_STATE_POLLING so that
-                         * thread which wakes up sees correct state.
-                         */
-                        if (poll_add_dcb(client_dcb) == -1)
-                        {
-                            /* Send a custom error as MySQL command reply */
-                            mysql_send_custom_error(client_dcb, 1, 0,
                                 "MaxScale encountered system limit while "
                                 "attempting to register on an epoll instance.");
 
-                            /** close client_dcb */
+        MXS_ERROR("Failed to add dcb %p for fd %d to epoll set.", dcb, dcb->fd);
-                            dcb_close(client_dcb);
+        dcb_close(dcb);
-
-                            /** Previous state is recovered in poll_add_dcb. */
-                            MXS_ERROR("Failed to add dcb %p for fd %d to epoll set.",
-                                      client_dcb, client_dcb->fd);
     }
     else
     {
-                            MySQLSendHandshake(client_dcb);
+        MySQLSendHandshake(dcb);
     }
-                    }, mxs::RoutingWorker::EXECUTE_AUTO);
+
+    return 1;
 }
 
 static int gw_error_client_event(DCB* dcb)