db_plugin: introducing a plugin API for the song database

First draft, not really pluggable currently - hard-coded to use the
"simple" plugin, and calls several of its internal functions.

The API is very simple currently, all searches are still performed
over the root "directory" object.  Future changes to the API will move
those search implementations into the plugin, to allow more efficient
implementations, or implementations that don't have the whole tree in
memory all the time.

src/db_plugin.h

@@ -0,0 +1,111 @@
+/*
+ * Copyright (C) 2003-2011 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.
+ */
+
+/** \file
+ *
+ * This header declares the db_plugin class.  It describes a
+ * plugin API for databases of song metadata.
+ */
+
+#ifndef MPD_DB_PLUGIN_H
+#define MPD_DB_PLUGIN_H
+
+#include <glib.h>
+#include <assert.h>
+#include <stdbool.h>
+
+struct config_param;
+
+struct db {
+	const struct db_plugin *plugin;
+};
+
+struct db_plugin {
+	const char *name;
+
+	/**
+         * Allocates and configures a database.
+	 */
+	struct db *(*init)(const struct config_param *param, GError **error_r);
+
+	/**
+	 * Free instance data.
+         */
+	void (*finish)(struct db *db);
+
+	/**
+         * Open the database.  Read it into memory if applicable.
+	 */
+	bool (*open)(struct db *db, GError **error_r);
+
+	/**
+         * Close the database, free allocated memory.
+	 */
+	void (*close)(struct db *db);
+};
+
+G_GNUC_MALLOC
+static inline struct db *
+db_plugin_new(const struct db_plugin *plugin, const struct config_param *param,
+	      GError **error_r)
+{
+	assert(plugin != NULL);
+	assert(plugin->init != NULL);
+	assert(plugin->finish != NULL);
+	assert(error_r == NULL || *error_r == NULL);
+
+	struct db *db = plugin->init(param, error_r);
+	assert(db == NULL || db->plugin == plugin);
+	assert(db != NULL || error_r == NULL || *error_r != NULL);
+
+	return db;
+}
+
+static inline void
+db_plugin_free(struct db *db)
+{
+	assert(db != NULL);
+	assert(db->plugin != NULL);
+	assert(db->plugin->finish != NULL);
+
+	db->plugin->finish(db);
+}
+
+static inline bool
+db_plugin_open(struct db *db, GError **error_r)
+{
+	assert(db != NULL);
+	assert(db->plugin != NULL);
+
+	return db->plugin->open != NULL
+		? db->plugin->open(db, error_r)
+		: true;
+}
+
+static inline void
+db_plugin_close(struct db *db)
+{
+	assert(db != NULL);
+	assert(db->plugin != NULL);
+
+	if (db->plugin->close != NULL)
+		db->plugin->close(db);
+}
+
+#endif