MXS-2546 Add DNS-resolving to topology detection

When matching hostnames between MaxScale server configuration and the
SHOW SLAVE STATUS-output, use DNS-resolution if a simple string comparison
doesn't find an answer. Results of the resolution are saved to avoid
repeating the operation for the same address.

server/modules/monitor/mariadbmon/cluster_discovery.cc

@@ -12,11 +12,13 @@
  */
 
 #include "mariadbmon.hh"
+
 #include <algorithm>
 #include <inttypes.h>
 #include <string>
 #include <queue>
 #include <maxbase/format.hh>
+#include <maxbase/host.hh>
 #include <maxscale/modutil.hh>
 #include <maxscale/mysql_utils.hh>
 
@@ -1002,3 +1004,35 @@ bool MariaDBMonitor::is_candidate_valid(MariaDBServer* cand, RequireRunning req_
     }
     return is_valid;
 };
+
+string MariaDBMonitor::DNSResolver::resolve_server(const string& host)
+{
+    auto now = mxb::Clock::now();
+    const auto MAX_AGE = mxb::Duration((double)5*60); // Refresh interval for cache entries.
+    auto recent_time = now - MAX_AGE;
+
+    string rval;
+    auto iter = m_mapping.find(host);
+
+    if (iter == m_mapping.end() || iter->second.timestamp < recent_time)
+    {
+        // Map did not have a record, or it was too old. In either case, generate a new one.
+        string addr;
+        string error_msg;
+        bool dns_success = mxb::name_lookup(host, &addr, &error_msg);
+        if (!dns_success)
+        {
+            MXB_ERROR("Could not resolve host '%s'. %s", host.c_str(), error_msg.c_str());
+        }
+        // If dns failed, addr will be empty. Add the element anyway to prevent repeated lookups.
+        MapElement newelem = {addr, now};
+        m_mapping[host] = newelem;
+        rval = addr;
+    }
+    else
+    {
+        // Return recent value.
+        rval = iter->second.address;
+    }
+    return rval;
+}

server/modules/monitor/mariadbmon/mariadbmon.cc

@@ -94,6 +94,8 @@ void MariaDBMonitor::reset_server_info()
     {
         m_servers.push_back(new MariaDBServer(mon_server, m_servers.size(), m_settings.shared));
     }
+
+    m_resolver = DNSResolver(); // Erases result cache.
 }
 
 void MariaDBMonitor::reset_node_index_info()
@@ -106,10 +108,9 @@ void MariaDBMonitor::reset_node_index_info()
 
 MariaDBServer* MariaDBMonitor::get_server(const EndPoint& search_ep)
 {
-    // TODO: Do this with a map lookup
-    // TODO: Add DNS check here.
     MariaDBServer* found = NULL;
-    for (MariaDBServer* server : m_servers)
+    // Phase 1: Direct string compare
+    for (auto server : m_servers)
     {
         EndPoint srv(server->m_server_base->server);
         if (srv == search_ep)
@@ -118,6 +119,28 @@ MariaDBServer* MariaDBMonitor::get_server(const EndPoint& search_ep)
             break;
         }
     }
+
+    if (!found)
+    {
+        // Phase 2: Was not found with simple string compare. Try DNS resolving for endpoints with
+        // matching ports.
+        string target_addr = m_resolver.resolve_server(search_ep.host());
+        if (!target_addr.empty())
+        {
+            for (auto server : m_servers)
+            {
+                if (server->m_server_base->server->port == search_ep.port())
+                {
+                    string server_addr = m_resolver.resolve_server(server->m_server_base->server->address);
+                    if (server_addr == target_addr)
+                    {
+                        found = server;
+                        break;
+                    }
+                }
+            }
+        }
+    }
     return found;
 }
 

server/modules/monitor/mariadbmon/mariadbmon.hh

@@ -169,6 +169,21 @@ private:
         bool result_waiting = false;        /* Guard variable for has_result */
     };
 
+    class DNSResolver
+    {
+    public:
+        std::string resolve_server(const std::string& host);
+
+    private:
+        struct MapElement
+        {
+            std::string    address;
+            mxb::TimePoint timestamp;
+        };
+
+        std::unordered_map<std::string, MapElement> m_mapping; // hostname -> address cache
+    };
+
     ManualCommand m_manual_cmd;     /* Communicates manual commands and results */
 
     // Server containers, mostly constant.
@@ -185,6 +200,8 @@ private:
     bool m_cluster_modified = false;        /* Has a cluster operation been performed this loop? Prevents
                                              * other operations during this tick. */
 
+    DNSResolver m_resolver;                 /* DNS-resolver with cache */
+
     /* Counter for temporary automatic cluster operation disabling. */
     int cluster_operation_disable_timer = 0;