Use references instead of copies of SRWBackend

As the DCB passed as the clientReply parameter is guaranteed to match one
of the DCBs in the RWBackends. By using a reference, the need to copy a
shared_ptr is removed (along with the atomic operation that it implies)
thus reducing the overhead in the clientReply and the functions it uses.

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -124,6 +124,30 @@ SRWBackend get_backend_from_dcb(RWSplitSession *rses, DCB *dcb)
     return SRWBackend();
 }
 
+static SRWBackend emptyref;
+
+static SRWBackend& get_backend_ref_from_dcb(RWSplitSession *rses, DCB *dcb)
+{
+    ss_dassert(dcb->dcb_role == DCB_ROLE_BACKEND_HANDLER);
+    CHK_DCB(dcb);
+    CHK_CLIENT_RSES(rses);
+
+    for (SRWBackendList::iterator it = rses->backends.begin();
+         it != rses->backends.end(); it++)
+    {
+        SRWBackend& backend = *it;
+
+        if (backend->dcb() == dcb)
+        {
+            return backend;
+        }
+    }
+
+    /** We should always have a valid backend reference */
+    ss_dassert(false);
+    return emptyref;
+}
+
 /**
  * @brief Process router options
  *
@@ -562,22 +586,13 @@ static inline bool is_result_set(GWBUF *buffer)
  *
  * @return True if the complete response has been received
  */
-bool reply_is_complete(SRWBackend backend, GWBUF *buffer)
+bool reply_is_complete(SRWBackend& backend, GWBUF *buffer)
 {
-    if (GWBUF_IS_COLLECTED_RESULT(buffer))
+    if (backend->get_reply_state() == REPLY_STATE_START &&
+        (!is_result_set(buffer) || GWBUF_IS_COLLECTED_RESULT(buffer)))
     {
-        // This branch should only be taken with a PS response
+        if (GWBUF_IS_COLLECTED_RESULT(buffer) ||
-        ss_dassert(backend->get_reply_state() == REPLY_STATE_START);
+            backend->current_command() == MXS_COM_STMT_PREPARE ||
-        ss_dassert(backend->current_command() == MXS_COM_STMT_PREPARE ||
-                   backend->current_command() == MXS_COM_QUERY);
-
-        // This is a complete result of a request
-        LOG_RS(backend, REPLY_STATE_DONE);
-        backend->set_reply_state(REPLY_STATE_DONE);
-    }
-    else if (backend->get_reply_state() == REPLY_STATE_START && !is_result_set(buffer))
-    {
-        if (backend->current_command() == MXS_COM_STMT_PREPARE ||
             !is_ok(buffer) || !more_results_exist(buffer))
         {
             /** Not a result set, we have the complete response */
@@ -1190,16 +1205,10 @@ static void clientReply(MXS_ROUTER *instance,
                GWBUF_IS_COLLECTED_RESULT(writebuf));
     RWSplitSession *rses = (RWSplitSession *)router_session;
     DCB *client_dcb = backend_dcb->session->client_dcb;
-
     CHK_CLIENT_RSES(rses);
+    ss_dassert(!rses->rses_closed);
 
-    if (rses->rses_closed)
+    SRWBackend& backend = get_backend_ref_from_dcb(rses, backend_dcb);
-    {
-        gwbuf_free(writebuf);
-        return;
-    }
-
-    SRWBackend backend = get_backend_from_dcb(rses, backend_dcb);
 
     if (backend->get_reply_state() == REPLY_STATE_DONE)
     {
@@ -1253,32 +1262,23 @@ static void clientReply(MXS_ROUTER *instance,
 
     bool queue_routed = false;
 
-    if (rses->expected_responses == 0)
+    if (rses->expected_responses == 0 && rses->query_queue)
     {
-        for (SRWBackendList::iterator it = rses->backends.begin();
+        queue_routed = true;
-             it != rses->backends.end(); it++)
-        {
-            ss_dassert((*it)->get_reply_state() == REPLY_STATE_DONE || (*it)->is_closed());
-        }
-
-        queue_routed = rses->query_queue != NULL;
         route_stored_query(rses);
     }
-    else
-    {
-        ss_dassert(rses->expected_responses > 0);
-    }
 
-    if (writebuf && client_dcb)
+    if (writebuf)
     {
+        ss_dassert(client_dcb);
         /** Write reply to client DCB */
         MXS_SESSION_ROUTE_REPLY(backend_dcb->session, writebuf);
     }
     /** Check pending session commands */
     else if (!queue_routed && backend->session_command_count())
     {
-        MXS_INFO("Backend %s processed reply and starts to execute active cursor.",
+        MXS_DEBUG("Backend %s processed reply and starts to execute active cursor.",
-                 backend->uri());
+                  backend->uri());
 
         if (backend->execute_session_command())
         {

server/modules/routing/readwritesplit/rwsplit_internal.hh

@@ -47,7 +47,7 @@ bool route_single_stmt(RWSplit *inst, RWSplitSession *rses,
                        GWBUF *querybuf);
 void closed_session_reply(GWBUF *querybuf);
 void print_error_packet(RWSplitSession *rses, GWBUF *buf, DCB *dcb);
-void check_session_command_reply(GWBUF *writebuf, SRWBackend bref);
+void check_session_command_reply(GWBUF *buffer, SRWBackend& backend);
 bool execute_sescmd_in_backend(SRWBackend& backend_ref);
 bool handle_target_is_all(route_target_t route_target,
                           RWSplit *inst, RWSplitSession *rses,

server/modules/routing/readwritesplit/rwsplit_mysql.cc

@@ -291,7 +291,7 @@ void closed_session_reply(GWBUF *querybuf)
  * @param buffer  Query buffer containing reply data
  * @param backend Router session data for a backend server
  */
-void check_session_command_reply(GWBUF *buffer, SRWBackend backend)
+void check_session_command_reply(GWBUF *buffer, SRWBackend& backend)
 {
     if (MYSQL_IS_ERROR_PACKET(((uint8_t *)GWBUF_DATA(buffer))))
     {