db/update: convert to OO API

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

src/db/update/Queue.hxx

@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_UPDATE_QUEUE_HXX
+#define MPD_UPDATE_QUEUE_HXX
+
+#include "check.h"
+
+#include <string>
+#include <queue>
+#include <list>
+
+struct UpdateQueueItem {
+	std::string path_utf8;
+	unsigned id;
+	bool discard;
+
+	UpdateQueueItem():id(0) {}
+	UpdateQueueItem(const char *_path, bool _discard,
+			unsigned _id)
+		:path_utf8(_path), id(_id), discard(_discard) {}
+
+	bool IsDefined() const {
+		return id != 0;
+	}
+};
+
+class UpdateQueue {
+	static constexpr unsigned MAX_UPDATE_QUEUE_SIZE = 32;
+
+	std::queue<UpdateQueueItem, std::list<UpdateQueueItem>> update_queue;
+
+public:
+	bool Push(const char *path, bool discard, unsigned id);
+
+	UpdateQueueItem Pop();
+};
+
+#endif