MXS-3143: Route FOUND_ROWS to last used target

This will cause the query to be routed to the same server where a possible
SQL_CALC_FOUND_ROWS was executed.

Documentation/Routers/ReadWriteSplit.md

@@ -643,3 +643,16 @@ executed session command for the duration of the session. Applications that use
 long-running sessions might cause MariaDB MaxScale to consume a growing amount
 of memory unless the sessions are closed. This can be solved by adjusting the
 value of `max_sescmd_history`.
+
+### Routing to previous target
+
+In the following cases, a query is routed to the same server where the previous
+query was executed. If no previous target is found, the query is routed to the
+current master.
+
+* If a query uses the `FOUND_ROWS()` function, it will be routed to the server
+  where the last query was executed. This is done with the assumption that a
+  query with `SQL_CALC_FOUND_ROWS` was previously executed.
+
+* COM_STMT_FETCH_ROWS will always be routed to the same server where the
+  COM_STMT_EXECUTE was routed.

server/core/queryclassifier.cc

@@ -1107,6 +1107,21 @@ QueryClassifier::RouteInfo QueryClassifier::update_route_info(
 
         process_routing_hints(pBuffer->hint, &route_target);
 
+        if (route_target == TARGET_SLAVE)
+        {
+            const QC_FUNCTION_INFO* infos = nullptr;
+            size_t n_infos = 0;
+            qc_get_function_info(pBuffer, &infos, &n_infos);
+
+            for (size_t i = 0; i < n_infos; ++i)
+            {
+                if (strcasecmp(infos[i].name, "FOUND_ROWS") == 0)
+                {
+                    route_target = TARGET_LAST_USED;
+                }
+            }
+        }
+
         if (session_trx_is_ending(m_pSession)
             || qc_query_is_type(type_mask, QUERY_TYPE_BEGIN_TRX))
         {