archive_plugin: wrap method calls

Make archive_file a "real" struct, extended by all plugins.  Add the
plugin pointer to it.  Wrap all method calls in functions.

src/archive_plugin.c

@@ -0,0 +1,87 @@
+/*
+ * Copyright (C) 2003-2009 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.
+ */
+
+#include "archive_plugin.h"
+#include "archive_internal.h"
+
+#include <assert.h>
+
+struct archive_file *
+archive_file_open(const struct archive_plugin *plugin, const char *path)
+{
+	struct archive_file *file;
+
+	assert(plugin != NULL);
+	assert(plugin->open != NULL);
+	assert(path != NULL);
+
+	file = plugin->open(path);
+
+	if (file != NULL) {
+		assert(file->plugin != NULL);
+		assert(file->plugin->close != NULL);
+		assert(file->plugin->scan_reset != NULL);
+		assert(file->plugin->scan_next != NULL);
+		assert(file->plugin->open_stream != NULL);
+	}
+
+	return file;
+}
+
+void
+archive_file_close(struct archive_file *file)
+{
+	assert(file != NULL);
+	assert(file->plugin != NULL);
+	assert(file->plugin->close != NULL);
+
+	file->plugin->close(file);
+}
+
+void
+archive_file_scan_reset(struct archive_file *file)
+{
+	assert(file != NULL);
+	assert(file->plugin != NULL);
+	assert(file->plugin->scan_reset != NULL);
+	assert(file->plugin->scan_next != NULL);
+
+	file->plugin->scan_reset(file);
+}
+
+char *
+archive_file_scan_next(struct archive_file *file)
+{
+	assert(file != NULL);
+	assert(file->plugin != NULL);
+	assert(file->plugin->scan_next != NULL);
+
+	return file->plugin->scan_next(file);
+}
+
+bool
+archive_file_open_stream(struct archive_file *file, struct input_stream *is,
+			 const char *path, GError **error_r)
+{
+	assert(file != NULL);
+	assert(file->plugin != NULL);
+	assert(file->plugin->open_stream != NULL);
+
+	return file->plugin->open_stream(file, is, path, error_r);
+}