Assign slave status even if no master is available

The master validity check now checks if the master is down. This requires
that the slave status is assigned even if no master is available.

The failover precondition is also fulfilled as long as one valid promotion
candidate is found. Previously a slave that didn't use GTID replication
appeared to prevent failover.

server/modules/monitor/mariadbmon/cluster_discovery.cc

@@ -664,6 +664,16 @@ void MariaDBMonitor::assign_master_and_slave()
         reset_node_index_info();
         assign_slave_and_relay_master(m_master);
     }
+    else
+    {
+        for (MariaDBServer* s: m_servers)
+        {
+            if (s->has_status(SERVER_WAS_SLAVE))
+            {
+                s->set_status(SERVER_SLAVE);
+            }
+        }
+    }
 }
 
 /**
@@ -760,7 +770,7 @@ bool MariaDBMonitor::master_is_valid(std::string* reason_out)
     // The master server of the cluster needs to be re-calculated in the following four cases:
     bool rval = true;
     // 1) There is no master.
-    if (m_master == NULL)
+    if (m_master == NULL || m_master->is_down())
     {
         rval = false;
     }

server/modules/monitor/mariadbmon/cluster_manipulation.cc

@@ -1327,6 +1327,12 @@ bool MariaDBMonitor::failover_check(string* error_out)
         *error_out += separator + "No valid slaves to promote.";
         error = true;
     }
+    else
+    {
+        // There's at least one valid promotion candidate
+        error = false;
+    }
+
     return !error;
 }