MXS-1848 Introduce a specific monitor type

Instead of using void there's now a MXS_SPECIFIC_MONITOR struct
from which monitor specific types can be derived. This change
does not bring about other benefits than a bit of clarity but
this is the first step in clearing up the monitor API.

server/modules/monitor/galeramon/galeramon.cc

@@ -30,7 +30,8 @@ static void monitorMain(void *);
 /** Log a warning when a bad 'wsrep_local_index' is found */
 static bool warn_erange_on_local_index = true;
 
-static void *startMonitor(MXS_MONITOR *, const MXS_CONFIG_PARAMETER *params);
+static MXS_SPECIFIC_MONITOR *startMonitor(MXS_MONITOR *,
+                                          const MXS_CONFIG_PARAMETER *params);
 static void stopMonitor(MXS_MONITOR *);
 static void diagnostics(DCB *, const MXS_MONITOR *);
 static json_t* diagnostics_json(const MXS_MONITOR *);
@@ -119,7 +120,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
  *
  * @return A handle to use when interacting with the monitor
  */
-static void *
+static MXS_SPECIFIC_MONITOR *
 startMonitor(MXS_MONITOR *mon, const MXS_CONFIG_PARAMETER *params)
 {
     GALERA_MONITOR *handle = static_cast<GALERA_MONITOR*>(mon->handle);

server/modules/monitor/galeramon/galeramon.h

@@ -41,8 +41,6 @@
 #include <maxscale/config.h>
 #include <maxscale/hashtable.h>
 
-MXS_BEGIN_DECLS
-
 /**
  *  Galera Variables and server reference for each
  *  monitored node that could be part of cluster.
@@ -76,7 +74,7 @@ typedef struct galera_cluster_info
 /**
  * The handle for an instance of a Galera Monitor module
  */
-typedef struct
+struct GALERA_MONITOR : public MXS_SPECIFIC_MONITOR
 {
     THREAD thread; /**< Monitor thread */
     int shutdown; /**< Flag to shutdown the monitor thread */
@@ -96,8 +94,6 @@ typedef struct
     HASHTABLE *galera_nodes_info; /**< Contains Galera Cluster variables of all nodes */
     GALERA_CLUSTER_INFO cluster_info; /**< Contains Galera cluster info */
     MXS_MONITOR* monitor;
-} GALERA_MONITOR;
-
-MXS_END_DECLS
+};
 
 #endif