Added configurable multistatement behavior

Whether all queries should be routed to the master after a multistatement
query is executed can now be controlled with the `strict_multi_stmt` option.

When the option is disabled queries executed after a multistatement query will
be routed normally.

Documentation/About/Limitations.md

@@ -32,6 +32,12 @@ In master-slave replication cluster also read-only queries are routed to master
 
 * statement includes a stored procedure, or an UDF call
 
+If the client enables and executes multi-statements, they will be routed to
+the master. All future queries will also be routed to the master to guarantee
+a consistent session state after the multi-statement query. This behavior can
+be controlled with the `strict_multi_stmt` router option. For more information,
+read the [ReadWriteSplit](../Routers/ReadWriteSplit.md) router documentation.
+
 ### Limitations in client session handling
 
 Some of the queries that client sends are routed to all backends instead of sending them just to one of server. These queries include `USE <db name>` and `SET autocommit=0` among many others. Readwritesplit sends a copy of these queries to each backend server and forwards the master's reply to the client. Below is a list of MySQL commands which are classified as session commands :

Documentation/Routers/ReadWriteSplit.md

@@ -143,6 +143,22 @@ disable_sescmd_history=true
 master_accept_reads=true
 ```
 
+### `strict_multi_stmt`
+
+When a client executes a multistatement query, all queries after that will be routed to
+the master to guarantee a consistent session state. This behavior can be controlled with
+the **`strict_multi_stmt`** router option. This option is enabled by default.
+
+If set to false, queries are routed normally after a multistatement query. **Warning**, this
+can cause false data to be read from the slaves if the multistatement query modifies
+the session state. Only disable the strict mode if you know that no changes to the session
+state will be made inside the multistatement queries.
+
+```
+# Disable strict multistatement mode
+strict_multi_stmt=false
+```
+
 ## Routing hints
 
 The readwritesplit router supports routing hints. For a detailed guide on hint syntax and functionality, please read [this](../Reference/Hint-Syntax.md) document.

server/modules/include/readwritesplit.h

@@ -252,6 +252,8 @@ typedef struct rwsplit_config_st
     int rw_max_sescmd_history_size;
     bool rw_disable_sescmd_hist;
     bool rw_master_reads; /*< Use master for reads */
+    bool rw_strict_multi_stmt; /*< Force non-multistatement queries to be routed
+                                * to the master after a multistatement query. */
 } rwsplit_config_t;
 
 #if defined(PREP_STMT_CACHING)

server/modules/routing/readwritesplit/readwritesplit.c

@@ -697,6 +697,9 @@ createInstance(SERVICE *service, char **options)
 	router->bitmask = 0;
 	router->bitvalue = 0;
 
+    /** Enable strict multistatement handling by default */
+    router->rwsplit_config.rw_strict_multi_stmt = true;
+
         /** Call this before refreshInstance */
 	if (options)
 	{
@@ -1384,7 +1387,7 @@ static route_target_t get_route_target(ROUTER_CLIENT_SES *rses,
     target_t use_sql_variables_in = rses->rses_config.rw_use_sql_variables_in;
     route_target_t target = TARGET_UNDEFINED;
 
-    if (rses->forced_node == rses->rses_master_ref)
+    if (rses->rses_config.rw_strict_multi_stmt && rses->forced_node == rses->rses_master_ref)
     {
         target = TARGET_MASTER;
     }
@@ -4640,6 +4643,10 @@ static void rwsplit_process_router_options(
 			{
 			    router->rwsplit_config.rw_master_reads = config_truth_value(value);
 			}
+			else if(strcmp(options[i],"strict_multi_stmt") == 0)
+			{
+			    router->rwsplit_config.rw_strict_multi_stmt = config_truth_value(value);
+			}
                 }
         } /*< for */
 }