clapper: Prevent concurrent cache cleanups

We do not want to create situations where multiple threads are running
cache cleanup at once (since they all think last one was run long ago).
This can be prevented with a combination of global mutex and a "trylock",
so we can simply avoid both mutex locking and starting another cleanup.

src/lib/clapper/gst/clapper-enhancer-director.c

@@ -60,6 +60,8 @@ typedef struct
   GError **error;
 } ClapperEnhancerDirectorData;
 
+static GMutex cleanup_lock;
+
 static gpointer
 clapper_enhancer_director_extract_in_thread (ClapperEnhancerDirectorData *data)
 {
@@ -356,6 +358,11 @@ _cache_cleanup_func (ClapperEnhancerDirector *self)
   const gchar *data;
   gint64 since_cleanup, epoch_now, epoch_last = 0;
 
+  if (!g_mutex_trylock (&cleanup_lock)) {
+    GST_LOG_OBJECT (self, "Cache cleanup is already running");
+    return G_SOURCE_REMOVE;
+  }
+
   date = g_date_time_new_now_utc ();
   epoch_now = g_date_time_to_unix (date);
   g_date_time_unref (date);
@@ -418,6 +425,7 @@ _cache_cleanup_func (ClapperEnhancerDirector *self)
         CLAPPER_TIME_FORMAT " ago", CLAPPER_TIME_ARGS (since_cleanup));
   }
 
+  g_mutex_unlock (&cleanup_lock);
   g_free (filename);
 
   return G_SOURCE_REMOVE;