add mutex to prevent concurrent session state saves.

Why wasn't this done 10 years ago?

libs/ardour/ardour/session.h

@@ -1126,6 +1126,7 @@ class LIBARDOUR_API Session : public PBD::StatefulDestructible, public PBD::Scop
 	friend class    StateProtector;
 	gint            _suspend_save; /* atomic */
 	volatile bool   _save_queued;
+	Glib::Threads::Mutex save_state_lock;
 
 	int      load_options (const XMLNode&);
 	int      load_state (std::string snapshot_name);

libs/ardour/session_state.cc

@@ -665,6 +665,10 @@ Session::save_state (string snapshot_name, bool pending, bool switch_to_snapshot
 	XMLTree tree;
 	std::string xml_path(_session_dir->root_path());
 
+	/* prevent concurrent saves from different threads */
+
+	Glib::Threads::Mutex::Lock lm (save_state_lock);
+
 	if (!_writable || (_state_of_the_state & CannotSave)) {
 		return 1;
 	}