Improve master routing failure error logging

The error logging is now more detailed and tells why the connection is
being closed. This should help the user figure out what is happening when
write fails and the connection is closed.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -1854,6 +1854,60 @@ static int routeQuery(ROUTER *instance, void *router_session, GWBUF *querybuf)
     return rval;
 }
 
+/**
+ * @brief Log master write failure
+ *
+ * @param rses Router session
+ */
+static void log_master_routing_failure(ROUTER_CLIENT_SES *rses, DCB *master_dcb,
+                                       DCB *curr_master_dcb)
+{
+    char errmsg[MAX_SERVER_NAME_LEN * 2 + 100]; // Extra space for error message
+
+    if (master_dcb && curr_master_dcb)
+    {
+        /** We found a master but it's not the same connection */
+        ss_dassert(master_dcb != curr_master_dcb);
+        if (master_dcb->server != curr_master_dcb->server)
+        {
+            sprintf(errmsg, "Master server changed from '%s' to '%s'",
+                    master_dcb->server->unique_name,
+                    curr_master_dcb->server->unique_name);
+        }
+        else
+        {
+            ss_dassert(false); // Currently we don't reconnect to the master
+            sprintf(errmsg, "Connection to master '%s' was recreated",
+                    curr_master_dcb->server->unique_name);
+        }
+    }
+    else if (master_dcb)
+    {
+        /** We have an original master connection but we couldn't find it */
+        sprintf(errmsg, "The connection to master server '%s' is not available",
+                master_dcb->server->unique_name);
+    }
+    else
+    {
+        /** We never had a master connection, the session must be in read-only mode */
+        if (rses->rses_config.rw_master_failure_mode != RW_FAIL_INSTANTLY)
+        {
+            sprintf(errmsg, "Session is in read-only mode because it was created "
+                    "when no master was available");
+        }
+        else
+        {
+            ss_dassert(false); // A session should always have a master reference
+            sprintf(errmsg, "Was supposed to route to master but couldn't "
+                    "find master in a suitable state");
+        }
+    }
+
+    MXS_WARNING("[%s] Write query received from %s@%s. %s. Closing client connection.",
+                rses->router->service->name, rses->client_dcb->user,
+                rses->client_dcb->remote, errmsg);
+}
+
 /**
  * Routing function. Find out query type, backend type, and target DCB(s).
  * Then route query to found target(s).
@@ -2277,26 +2331,14 @@ static bool route_single_stmt(ROUTER_INSTANCE *inst, ROUTER_CLIENT_SES *rses,
         }
         else
         {
-            if (succp && master_dcb != curr_master_dcb)
+            /** The original master is not available, we can't route the write */
-            {
-                MXS_INFO("Was supposed to route to master but master has changed.");
-            }
-            else
-            {
-                MXS_INFO("Was supposed to route to master but couldn't find master"
-                         " in a suitable state.");
-            }
-
             if (rses->rses_config.rw_master_failure_mode == RW_ERROR_ON_WRITE)
             {
-                /** Old master is no longer available */
                 succp = send_readonly_error(rses->client_dcb);
             }
             else
             {
-                MXS_WARNING("[%s] Write query received from %s@%s when no master is "
+                log_master_routing_failure(rses, master_dcb, curr_master_dcb);
-                            "available, closing client connection.", inst->service->name,
-                            rses->client_dcb->user, rses->client_dcb->remote);
                 succp = false;
             }