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/Service.hxx

@@ -0,0 +1,89 @@
+/*
+ * 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_SERVICE_HXX
+#define MPD_UPDATE_SERVICE_HXX
+
+#include "check.h"
+#include "Queue.hxx"
+#include "Walk.hxx"
+#include "event/DeferredMonitor.hxx"
+#include "thread/Thread.hxx"
+
+/**
+ * This class manages the update queue and runs the update thread.
+ */
+class UpdateService final : DeferredMonitor {
+	enum Progress {
+		UPDATE_PROGRESS_IDLE = 0,
+		UPDATE_PROGRESS_RUNNING = 1,
+		UPDATE_PROGRESS_DONE = 2
+	};
+
+	Progress progress;
+
+	bool modified;
+
+	Thread update_thread;
+
+	static const unsigned update_task_id_max = 1 << 15;
+
+	unsigned update_task_id;
+
+	UpdateQueue queue;
+
+	UpdateQueueItem next;
+
+	UpdateWalk walk;
+
+public:
+	UpdateService(EventLoop &_loop);
+
+	/**
+	 * Returns a non-zero job id when we are currently updating
+	 * the database.
+	 */
+	unsigned GetId() const {
+		return next.id;
+	}
+
+	/**
+	 * Add this path to the database update queue.
+	 *
+	 * @param path a path to update; if an empty string,
+	 * the whole music directory is updated
+	 * @return the job id, or 0 on error
+	 */
+	gcc_nonnull_all
+	unsigned Enqueue(const char *path, bool discard);
+
+private:
+	/* virtual methods from class DeferredMonitor */
+	virtual void RunDeferred() override;
+
+	/* the update thread */
+	void Task();
+	static void Task(void *ctx);
+
+	void StartThread(UpdateQueueItem &&i);
+
+	unsigned GenerateId();
+};
+
+#endif