prevent multiple concurrent plugin scans.

libs/ardour/ardour/plugin_manager.h

@@ -144,6 +144,8 @@ class LIBARDOUR_API PluginManager : public boost::noncopyable {
 	std::string get_ladspa_category (uint32_t id);
 	std::vector<uint32_t> ladspa_plugin_whitelist;
 
+	Glib::Threads::Mutex _lock;
+
 	static PluginManager* _instance; // singleton
 	PluginManager ();
 };

libs/ardour/plugin_manager.cc

@@ -225,6 +225,12 @@ PluginManager::~PluginManager()
 void
 PluginManager::refresh (bool cache_only)
 {
+	Glib::Threads::Mutex::Lock lm (_lock, Glib::Threads::TRY_LOCK);
+
+	if (!lm.locked()) {
+		return;
+	}
+
 	DEBUG_TRACE (DEBUG::PluginManager, "PluginManager::refresh\n");
 	_cancel_scan = false;