mixer: protect the mixer struct with a mutex

In some rare cases, there was a race condition between the output
thread and the main thread: when you disable/enable an output device
in the main thread, this caused a crash in the output thread.  Protect
the whole mixer struct with a GMutex to prevent that.

src/mixer_api.c

@@ -19,8 +19,6 @@
 
 #include "mixer_api.h"
 
-#include <glib.h>
-
 #undef G_LOG_DOMAIN
 #define G_LOG_DOMAIN "mixer"
 
@@ -28,4 +26,5 @@ void
 mixer_init(struct mixer *mixer, const struct mixer_plugin *plugin)
 {
 	mixer->plugin = plugin;
+	mixer->mutex = g_mutex_new();
 }

src/mixer_api.h

@@ -23,8 +23,16 @@
 #include "mixer_plugin.h"
 #include "mixer_list.h"
 
+#include <glib.h>
+
 struct mixer {
 	const struct mixer_plugin *plugin;
+
+	/**
+	 * This mutex protects all of the mixer struct, including its
+	 * implementation, so plugins don't have to deal with that.
+	 */
+	GMutex *mutex;
 };
 
 void

src/mixer_control.c

@@ -62,6 +62,9 @@ mixer_free(struct mixer *mixer)
 		return;
 	}
 	assert(mixer->plugin != NULL);
+	assert(mixer->mutex != NULL);
+
+	g_mutex_free(mixer->mutex);
 
 	mixer->plugin->finish(mixer);
 }
@@ -69,11 +72,18 @@ mixer_free(struct mixer *mixer)
 bool
 mixer_open(struct mixer *mixer)
 {
+	bool success;
+
 	if (!mixer) {
 		return false;
 	}
 	assert(mixer->plugin != NULL);
-	return mixer->plugin->open(mixer);
+
+	g_mutex_lock(mixer->mutex);
+	success = mixer->plugin->open(mixer);
+	g_mutex_unlock(mixer->mutex);
+
+	return success;
 }
 
 void
@@ -83,17 +93,32 @@ mixer_close(struct mixer *mixer)
 		return;
 	}
 	assert(mixer->plugin != NULL);
+
+	g_mutex_lock(mixer->mutex);
 	mixer->plugin->close(mixer);
+	g_mutex_unlock(mixer->mutex);
 }
 
 int
 mixer_get_volume(struct mixer *mixer)
 {
-	return mixer->plugin->get_volume(mixer);
+	int volume;
+
+	g_mutex_lock(mixer->mutex);
+	volume = mixer->plugin->get_volume(mixer);
+	g_mutex_unlock(mixer->mutex);
+
+	return volume;
 }
 
 bool
 mixer_set_volume(struct mixer *mixer, unsigned volume)
 {
-	return mixer->plugin->set_volume(mixer, volume);
+	bool success;
+
+	g_mutex_lock(mixer->mutex);
+	success = mixer->plugin->set_volume(mixer, volume);
+	g_mutex_unlock(mixer->mutex);
+
+	return success;
 }