Continue name resolution fixing, add unit test

name_lookup() now returns all results given by getnameinfo(). When searching
for a server, finding one matching address in the lookup-results is enough for
a match.

Also, added a test for name_lookup(). The test is minimal on its own, as hardcoded
test cases are not generally valid.

server/modules/monitor/mariadbmon/cluster_discovery.cc

@@ -1005,34 +1005,32 @@ bool MariaDBMonitor::is_candidate_valid(MariaDBServer* cand, RequireRunning req_
     return is_valid;
 };
 
-string MariaDBMonitor::DNSResolver::resolve_server(const string& host)
+MariaDBMonitor::DNSResolver::StringSet 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;
+    DNSResolver::StringSet rval;
 
-    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;
+        DNSResolver::StringSet addresses;
         string error_msg;
-        bool dns_success = mxb::name_lookup(host, &addr, &error_msg);
+        bool dns_success = mxb::name_lookup(host, &addresses, &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;
+        // If dns failed, the array will be empty. Add/replace the element anyway to prevent repeated lookups.
+        m_mapping[host] = {addresses, now};
+        rval = std::move(addresses);
     }
     else
     {
         // Return recent value.
-        rval = iter->second.address;
+        rval = iter->second.addresses;
     }
     return rval;
 }

server/modules/monitor/mariadbmon/mariadbmon.cc

@@ -124,21 +124,27 @@ MariaDBServer* MariaDBMonitor::get_server(const EndPoint& search_ep)
     {
         // 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())
+        DNSResolver::StringSet target_addresses = m_resolver.resolve_server(search_ep.host());
+        if (!target_addresses.empty())
         {
             for (auto server : m_servers)
             {
-                if (server->m_server_base->server->port == search_ep.port())
+                SERVER* srv = server->m_server_base->server;
+                if (srv->port == search_ep.port())
                 {
-                    string server_addr = m_resolver.resolve_server(server->m_server_base->server->address);
-                    if (server_addr == target_addr)
+                    auto server_addresses = m_resolver.resolve_server(srv->address);
+                    // The number of elements in the arrays is rarely over 1.
+                    for (auto& address : server_addresses)
                     {
-                        found = server;
-                        break;
+                        if (target_addresses.count(address) > 0)
+                        {
+                            found = server;
+                            goto breakout;
+                        }
                     }
                 }
             }
+            breakout:;
         }
     }
     return found;

server/modules/monitor/mariadbmon/mariadbmon.hh

@@ -172,12 +172,13 @@ private:
     class DNSResolver
     {
     public:
-        std::string resolve_server(const std::string& host);
+        using StringSet = std::unordered_set<std::string>;
+        StringSet resolve_server(const std::string& host);
 
     private:
         struct MapElement
         {
-            std::string    address;
+            StringSet   addresses; // A hostname can map to multiple addresses
             mxb::TimePoint timestamp;
         };