MXS-1503: Respect max_slave_connections

The slave selection now again respects max_slave_connections. This means
that the amount of slave connections each session has will never grow
beyond the configured value.

server/modules/routing/readwritesplit/rwsplit_internal.hh

@@ -15,6 +15,7 @@
 #include "readwritesplit.hh"
 
 #include <string>
+#include <utility>
 
 #include <maxscale/query_classifier.h>
 #include <maxscale/protocol/mysql.h>
@@ -100,6 +101,17 @@ bool select_connect_backend_servers(RWSplit *inst, MXS_SESSION *session,
                                     int* expected_responses,
                                     connection_type type);
 SRWBackend get_root_master(const SRWBackendList& backends);
+
+/**
+ * Get total slave count and connected slave count
+ *
+ * @param backends List of backend servers
+ * @param master   Current master
+ *
+ * @return Total number of slaves and number of slaves we are connected to
+ */
+std::pair<int, int> get_slave_counts(SRWBackendList& backends, SRWBackend& master);
+
 /*
  * The following are implemented in rwsplit_tmp_table_multi.c
  */

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -389,6 +389,7 @@ SRWBackend get_hinted_backend(RWSplitSession *rses, char *name)
 SRWBackend get_slave_backend(RWSplitSession *rses, int max_rlag)
 {
     SRWBackend rval;
+    auto counts = get_slave_counts(rses->backends, rses->current_master);
 
     for (auto it = rses->backends.begin(); it != rses->backends.end(); it++)
     {
@@ -406,7 +407,7 @@ SRWBackend get_slave_backend(RWSplitSession *rses, int max_rlag)
                     rval = backend;
                 }
             }
-            else
+            else if (backend->in_use() || counts.second < rses->router->max_slave_count())
             {
                 if (!rses->rses_config.master_accept_reads && rval->is_master())
                 {