output: new option "tags" may be used to disable sending tags

Implements Mantis ticket 0003340.

NEWS

@@ -3,6 +3,8 @@ ver 0.18 (2012/??/??)
   - adplug: new decoder plugin using libadplug
   - opus: new decoder plugin for the Opus codec
   - vorbis: skip 16 bit quantisation, provide float samples
+* output:
+  - new option "tags" may be used to disable sending tags to output
 * improved decoder/output error reporting
 
 ver 0.17.2 (2012/??/??)

doc/user.xml

@@ -436,6 +436,18 @@ systemctl start mpd.socket</programlisting>
                 enabled.
               </entry>
             </row>
+            <row>
+              <entry>
+                <varname>tags</varname>
+                <parameter>yes|no</parameter>
+              </entry>
+              <entry>
+                If set to "no", then MPD will not send tags to this
+                output.  This is only useful for output plugins that
+                can receive tags, for example the
+                <varname>httpd</varname> output plugin.
+              </entry>
+            </row>
             <row>
               <entry>
                 <varname>always_on</varname>

src/output_init.c

@@ -165,6 +165,7 @@ ao_base_init(struct audio_output *ao,
 	}
 
 	ao->plugin = plugin;
+	ao->tags = config_get_block_bool(param, "tags", true);
 	ao->always_on = config_get_block_bool(param, "always_on", false);
 	ao->enabled = config_get_block_bool(param, "enabled", true);
 	ao->really_enabled = false;

src/output_internal.h

@@ -72,6 +72,13 @@ struct audio_output {
 	 */
 	struct mixer *mixer;
 
+	/**
+	 * Will this output receive tags from the decoder?  The
+	 * default is true, but it may be configured to false to
+	 * suppress sending tags to the output.
+	 */
+	bool tags;
+
 	/**
 	 * Shall this output always play something (i.e. silence),
 	 * even when playback is stopped?

src/output_thread.c

@@ -435,7 +435,7 @@ ao_play_chunk(struct audio_output *ao, const struct music_chunk *chunk)
 	assert(ao != NULL);
 	assert(ao->filter != NULL);
 
-	if (gcc_unlikely(chunk->tag != NULL)) {
+	if (ao->tags && gcc_unlikely(chunk->tag != NULL)) {
 		g_mutex_unlock(ao->mutex);
 		ao_plugin_send_tag(ao, chunk->tag);
 		g_mutex_lock(ao->mutex);