MXS-1503: Split get_target_backend into smaller pieces

Split the code into separate functions. This should make it easier to
comprehend.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -353,59 +353,30 @@ static inline bool rpl_lag_is_ok(SRWBackend& backend, int max_rlag)
         backend->server()->rlag <= max_rlag);
 }
 
-/**
+SRWBackend get_hinted_backend(RWSplitSession *rses, char *name)
- * Provide the router with a reference to a suitable backend
- *
- * @param rses     Pointer to router client session
- * @param btype    Backend type
- * @param name     Name of the backend which is primarily searched. May be NULL.
- * @param max_rlag Maximum replication lag
- * @param target   The target backend
- *
- * @return True if a backend was found
- */
-SRWBackend get_target_backend(RWSplitSession *rses, backend_type_t btype,
-                              char *name, int max_rlag)
 {
-    CHK_CLIENT_RSES(rses);
-
-    /** Check whether using rses->target_node as target SLAVE */
-    if (rses->target_node && session_trx_is_read_only(rses->client_dcb->session))
-    {
-        MXS_DEBUG("In READ ONLY transaction, using server '%s'",
-                  rses->target_node->name());
-        return rses->target_node;
-    }
-
-    if (name) /*< Choose backend by name from a hint */
-    {
-        ss_dassert(btype != BE_MASTER); /*< Master dominates and no name should be passed with it */
-
-        for (SRWBackendList::iterator it = rses->backends.begin();
-             it != rses->backends.end(); it++)
-        {
-            SRWBackend& backend = *it;
-
-            /** The server must be a valid slave, relay server, or master */
-
-            if (backend->in_use() &&
-                (strcasecmp(name, backend->name()) == 0) &&
-                (backend->is_slave() ||
-                 backend->is_relay() ||
-                 backend->is_master()))
-            {
-                return backend;
-            }
-        }
-
-        /** No server found, use a normal slave for it */
-        btype = BE_SLAVE;
-    }
-
     SRWBackend rval;
 
-    if (btype == BE_SLAVE)
+    for (auto it = rses->backends.begin(); it != rses->backends.end(); it++)
     {
+        auto& backend = *it;
+
+        /** The server must be a valid slave, relay server, or master */
+        if (backend->in_use() && strcasecmp(name, backend->name()) == 0 &&
+            (backend->is_slave() || backend->is_relay() || backend->is_master()))
+        {
+            rval = backend;
+            break;
+        }
+    }
+
+    return rval;
+}
+
+SRWBackend get_slave_backend(RWSplitSession *rses, int max_rlag)
+{
+    SRWBackend rval;
+
     for (auto it = rses->backends.begin(); it != rses->backends.end(); it++)
     {
         auto& backend = *it;
@@ -438,13 +409,13 @@ SRWBackend get_target_backend(RWSplitSession *rses, backend_type_t btype,
             }
         }
     }
-    }
+
-    /**
+    return rval;
-     * If target was originally master only then the execution jumps
+}
-     * directly here.
+
-     */
+SRWBackend get_master_backend(RWSplitSession *rses)
-    else if (btype == BE_MASTER)
+{
-    {
+    SRWBackend rval;
     /** get root master from available servers */
     SRWBackend master = get_root_master(rses->backends);
 
@@ -468,6 +439,49 @@ SRWBackend get_target_backend(RWSplitSession *rses, backend_type_t btype,
                       master->name());
         }
     }
+
+    return rval;
+}
+
+/**
+ * Provide the router with a reference to a suitable backend
+ *
+ * @param rses     Pointer to router client session
+ * @param btype    Backend type
+ * @param name     Name of the backend which is primarily searched. May be NULL.
+ * @param max_rlag Maximum replication lag
+ * @param target   The target backend
+ *
+ * @return True if a backend was found
+ */
+SRWBackend get_target_backend(RWSplitSession *rses, backend_type_t btype,
+                              char *name, int max_rlag)
+{
+    CHK_CLIENT_RSES(rses);
+
+    /** Check whether using rses->target_node as target SLAVE */
+    if (rses->target_node && session_trx_is_read_only(rses->client_dcb->session))
+    {
+        MXS_DEBUG("In READ ONLY transaction, using server '%s'", rses->target_node->name());
+        return rses->target_node;
+    }
+
+    SRWBackend rval;
+
+    if (name) /*< Choose backend by name from a hint */
+    {
+        ss_dassert(btype != BE_MASTER);
+        btype = BE_SLAVE;
+        rval = get_hinted_backend(rses, name);
+    }
+
+    else if (btype == BE_SLAVE)
+    {
+        rval = get_slave_backend(rses, max_rlag);
+    }
+    else if (btype == BE_MASTER)
+    {
+        rval = get_master_backend(rses);
     }
 
     return rval;