filter: added new plugin API for filters

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.

src/filter/null_filter_plugin.c

@@ -0,0 +1,93 @@
+/*
+ * 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.
+ */
+
+/** \file
+ *
+ * This filter plugin does nothing.  That is not quite useful, except
+ * for testing the filter core, or as a template for new filter
+ * plugins.
+ */
+
+#include "filter_plugin.h"
+#include "filter_internal.h"
+#include "filter_registry.h"
+
+#include <assert.h>
+
+struct null_filter {
+	struct filter filter;
+};
+
+static struct filter *
+null_filter_init(G_GNUC_UNUSED const struct config_param *param,
+		 G_GNUC_UNUSED GError **error_r)
+{
+	struct null_filter *filter = g_new(struct null_filter, 1);
+
+	filter_init(&filter->filter, &null_filter_plugin);
+	return &filter->filter;
+}
+
+static void
+null_filter_finish(struct filter *_filter)
+{
+	struct null_filter *filter = (struct null_filter *)_filter;
+
+	g_free(filter);
+}
+
+static const struct audio_format *
+null_filter_open(struct filter *_filter,
+		 const struct audio_format *audio_format,
+		 G_GNUC_UNUSED GError **error_r)
+{
+	struct null_filter *filter = (struct null_filter *)_filter;
+	(void)filter;
+
+	return audio_format;
+}
+
+static void
+null_filter_close(struct filter *_filter)
+{
+	struct null_filter *filter = (struct null_filter *)_filter;
+	(void)filter;
+}
+
+static const void *
+null_filter_filter(struct filter *_filter,
+		   const void *src, size_t src_size,
+		   size_t *dest_size_r, G_GNUC_UNUSED GError **error_r)
+{
+	struct null_filter *filter = (struct null_filter *)_filter;
+	(void)filter;
+
+	/* return the unmodified source buffer */
+	*dest_size_r = src_size;
+	return src;
+}
+
+const struct filter_plugin null_filter_plugin = {
+	.name = "null",
+	.init = null_filter_init,
+	.finish = null_filter_finish,
+	.open = null_filter_open,
+	.close = null_filter_close,
+	.filter = null_filter_filter,
+};