Add master extraction via REST API

The master server of a monitor can now be extracted with the
mon_get_external_master function. It uses the REST API to find a master
server in the set of servers monitored by a monitor.

include/maxscale/monitor.hh

@@ -338,4 +338,18 @@ MXS_MONITOR_API MonitorApi<MonitorInstance>::s_api =
     &MonitorApi<MonitorInstance>::diagnostics_json,
 };
 
+/**
+ * Get the master server of the monitor pointed by @c url
+ *
+ * The URL must be in `http://hostname-of-maxscale:<rest-api-port>/v1/monitors/<name-of-monitor>`
+ * format and must point to a single monitor resource.
+ *
+ * TODO: Move the base URL and credentials into a global parameter
+ *
+ * @param url URL to a REST API endpoint for a monitor
+ *
+ * @return The host and port of the monitor or an empty string and 0 if an error occurred
+ */
+std::pair<std::string, int> mon_get_external_master(const std::string& url);
+
 }

server/core/monitor.cc

@@ -2866,4 +2866,59 @@ void MonitorInstance::run_one_tick()
     store_server_journal(m_monitor, m_master);
 }
 
+static bool remote_server_is_master(const std::string& url, std::string* host, int* port)
+{
+    bool rval = false;
+    auto res = mxs::http::get(url);
+    json_t* state = mxs_json_pointer(res.body.get(), "data/attributes/state");
+    json_t* json_host = mxs_json_pointer(res.body.get(), "data/attributes/parameters/address");
+    json_t* json_port = mxs_json_pointer(res.body.get(), "data/attributes/parameters/port");
+
+    if (json_is_string(json_host) && json_is_integer(json_port) && json_is_string(state))
+    {
+        const std::set<std::string> master_states{ "Master, Running", "Master, Synced, Running" };
+
+        if (master_states.count(json_string_value(state)))
+        {
+            *host = json_string_value(json_host);
+            *port = json_integer_value(json_port);
+            rval = true;
+        }
+    }
+
+    return rval;
+}
+
+std::pair<std::string, int> mon_get_external_master(const std::string& url)
+{
+    std::string host;
+    int port = 0;
+
+    auto res = mxs::http::get(url);
+    json_t* remote = mxs_json_pointer(res.body.get(), "data/relationships/servers/links/self");
+    json_t* arr = mxs_json_pointer(res.body.get(), "data/relationships/servers/data");
+
+    if (json_is_string(remote) && json_is_array(arr))
+    {
+        std::string remote_host = json_string_value(remote);
+        size_t i;
+        json_t* value;
+
+        json_array_foreach(arr, i, value)
+        {
+            json_t* id = mxs_json_pointer(value, "id");
+
+            if (json_is_string(id))
+            {
+                if (remote_server_is_master(remote_host + json_string_value(id), &host, &port))
+                {
+                    break;
+                }
+            }
+        }
+    }
+
+    return {host, port};
+}
+
 }