MXS-2606: Sort servers before loading users

By sorting the servers in descending order based on their role we make
sure that the users are loaded from a master if one is available.

server/modules/authenticator/MySQLAuth/dbusers.cc

@@ -21,6 +21,9 @@
 #include <netdb.h>
 #include <stdio.h>
 
+#include <algorithm>
+#include <vector>
+
 #include <maxscale/alloc.h>
 #include <maxscale/dcb.h>
 #include <maxscale/log.h>
@@ -1025,17 +1028,17 @@ bool query_and_process_users(const char* query, MYSQL* con, sqlite3* handle, SER
     return rval;
 }
 
-int get_users_from_server(MYSQL* con, SERVER_REF* server_ref, SERVICE* service, SERV_LISTENER* listener)
+int get_users_from_server(MYSQL* con, SERVER* server, SERVICE* service, SERV_LISTENER* listener)
 {
-    if (server_ref->server->version_string[0] == 0)
+    if (server->version_string[0] == 0)
     {
-        mxs_mysql_update_server_version(con, server_ref->server);
+        mxs_mysql_update_server_version(con, server);
     }
 
-    char* query = get_users_query(server_ref->server->version_string,
+    char* query = get_users_query(server->version_string,
-                                  server_ref->server->version,
+                                  server->version,
                                   service->enable_root,
-                                  roles_are_available(con, service, server_ref->server));
+                                  roles_are_available(con, service, server));
 
     MYSQL_AUTH* instance = (MYSQL_AUTH*)listener->auth_instance;
     sqlite3* handle = get_handle(instance);
@@ -1043,20 +1046,20 @@ int get_users_from_server(MYSQL* con, SERVER_REF* server_ref, SERVICE* service,
 
     bool rv = query_and_process_users(query, con, handle, service, &users);
 
-    if (!rv && have_mdev13453_problem(con, server_ref->server))
+    if (!rv && have_mdev13453_problem(con, server))
     {
         /**
          * Try to work around MDEV-13453 by using a query without CTEs. Masquerading as
          * a 10.1.10 server makes sure CTEs aren't used.
          */
         MXS_FREE(query);
-        query = get_users_query(server_ref->server->version_string, 100110, service->enable_root, true);
+        query = get_users_query(server->version_string, 100110, service->enable_root, true);
         rv = query_and_process_users(query, con, handle, service, &users);
     }
 
     if (!rv)
     {
-        MXS_ERROR("Failed to load users from server '%s': %s", server_ref->server->name, mysql_error(con));
+        MXS_ERROR("Failed to load users from server '%s': %s", server->name, mysql_error(con));
     }
 
     MXS_FREE(query);
@@ -1084,6 +1087,28 @@ int get_users_from_server(MYSQL* con, SERVER_REF* server_ref, SERVICE* service,
     return users;
 }
 
+// Sorts candidates servers so that masters are before slaves which are before only running servers
+static std::vector<SERVER*> get_candidates(SERVICE* service, bool skip_local)
+{
+    std::vector<SERVER*> candidates;
+
+    for (auto server = service->dbref; server; server = server->next)
+    {
+        if (SERVER_REF_IS_ACTIVE(server) && server_is_running(server->server)
+            && (!skip_local || !server_is_mxs_service(server->server)))
+        {
+            candidates.push_back(server->server);
+        }
+    }
+
+    std::sort(candidates.begin(), candidates.end(), [](SERVER* a, SERVER* b) {
+                  return (server_is_master(a) && !server_is_master(b))
+                  || (server_is_slave(a) && !server_is_slave(b) && !server_is_master(b));
+              });
+
+    return candidates;
+}
+
 /**
  * Load the user/passwd form mysql.user table into the service users' hashtable
  * environment.
@@ -1112,33 +1137,18 @@ static int get_users(SERV_LISTENER* listener, bool skip_local, SERVER** srv)
     sqlite3* handle = get_handle(instance);
     delete_mysql_users(handle);
 
-    SERVER_REF* server = service->dbref;
     int total_users = -1;
-    bool no_active_servers = true;
+    auto candidates = get_candidates(service, skip_local);
 
-    for (server = service->dbref; !maxscale_is_shutting_down() && server; server = server->next)
+    for (auto server : candidates)
     {
-        if (!SERVER_REF_IS_ACTIVE(server) || !server_is_active(server->server)
+        if (MYSQL* con = gw_mysql_init())
-            || (skip_local && server_is_mxs_service(server->server))
-            || !server_is_running(server->server))
         {
-            continue;
+            if (mxs_mysql_real_connect(con, server, service_user, dpwd) == NULL)
-        }
-
-        no_active_servers = false;
-
-        MYSQL* con = gw_mysql_init();
-        if (con)
             {
-            if (mxs_mysql_real_connect(con, server->server, service_user, dpwd) == NULL)
+                MXS_ERROR("Failure loading users data from backend [%s:%i] for service [%s]. "
-            {
+                          "MySQL error %i, %s", server->address, server->port, service->name,
-                MXS_ERROR("Failure loading users data from backend "
+                          mysql_errno(con), mysql_error(con));
-                          "[%s:%i] for service [%s]. MySQL error %i, %s",
-                          server->server->address,
-                          server->server->port,
-                          service->name,
-                          mysql_errno(con),
-                          mysql_error(con));
                 mysql_close(con);
             }
             else
@@ -1148,7 +1158,7 @@ static int get_users(SERV_LISTENER* listener, bool skip_local, SERVER** srv)
 
                 if (users > total_users)
                 {
-                    *srv = server->server;
+                    *srv = server;
                     total_users = users;
                 }
 
@@ -1164,12 +1174,12 @@ static int get_users(SERV_LISTENER* listener, bool skip_local, SERVER** srv)
 
     MXS_FREE(dpwd);
 
-    if (no_active_servers)
+    if (candidates.empty())
     {
         // This service has no servers or all servers are local MaxScale services
         total_users = 0;
     }
-    else if (server == NULL && total_users == -1)
+    else if (*srv == nullptr && total_users == -1)
     {
         MXS_ERROR("Unable to get user data from backend database for service [%s]."
                   " Failed to connect to any of the backend databases.",