Fix split brain detection

Galeramon will now only use the larger cluster in case a split brain
situation occurs. If the clusters are of equal size, the one whose UUID
compares less will be used. This will guarantee that all MaxScales that
see the same picture will end up using the same cluster.

server/modules/monitor/galeramon/galeramon.cc

@@ -328,6 +328,31 @@ void GaleraMonitor::update_server_status(MonitorServer* monitored_server)
     {
         monitored_server->mon_report_query_error();
     }
+
+    calculate_cluster();
+}
+
+void GaleraMonitor::calculate_cluster()
+{
+    std::unordered_map<std::string, int> clusters;
+
+    for (const auto& a : m_info)
+    {
+        clusters[a.second.cluster_uuid]++;
+    }
+
+    auto it = std::max_element(
+        clusters.begin(), clusters.end(),
+        [](const typename decltype(clusters)::value_type& a,
+           const typename decltype(clusters)::value_type& b) {
+            return a.second == b.second ? a.first < b.first : a.second < b.second;
+        });
+
+    if (it != clusters.end())
+    {
+        m_cluster_uuid = it->first;
+        m_cluster_size = it->second;
+    }
 }
 
 void GaleraMonitor::pre_tick()
@@ -813,22 +838,9 @@ static int compare_node_priority(const void* a, const void* b)
  */
 void GaleraMonitor::set_galera_cluster()
 {
-    int cluster_size = 0;
-    std::string cluster_uuid;
-
     for (auto it = m_info.begin(); it != m_info.end(); it++)
     {
-        if (it->second.joined && it->second.cluster_size > cluster_size)
-        {
-            // Use the UUID of the largest cluster
-            cluster_size = it->second.cluster_size;
-            cluster_uuid = it->second.cluster_uuid;
-        }
-    }
-
-    for (auto it = m_info.begin(); it != m_info.end(); it++)
-    {
-        if (it->second.joined)
+        if (it->second.joined && it->second.cluster_uuid == m_cluster_uuid)
         {
             it->first->set_pending_status(SERVER_JOINED);
         }

server/modules/monitor/galeramon/galeramon.hh

@@ -79,4 +79,5 @@ private:
     mxs::MonitorServer* get_candidate_master();
     void                set_galera_cluster();
     void                update_sst_donor_nodes(int is_cluster);
+    void                calculate_cluster();
 };