decoder_control: protect command, state with a mutex

Replace decoder_control.notify with decoder_control.mutex and
decoder_control.cond.  Lock the mutex on all accesses to
decoder_control.command and decoder_control.state.

src/decoder_control.h

@@ -22,13 +22,16 @@
 
 #include "decoder_command.h"
 #include "audio_format.h"
-#include "notify.h"
+
+#include <glib.h>
 
 #include <assert.h>
 
 #define DECODE_TYPE_FILE	0
 #define DECODE_TYPE_URL		1
 
+struct notify;
+
 enum decoder_state {
 	DECODE_STATE_STOP = 0,
 	DECODE_STATE_START,
@@ -48,14 +51,25 @@ struct decoder_control {
 	    thread isn't running */
 	GThread *thread;
 
-	struct notify notify;
+	/**
+	 * This lock protects #state and #command.
+	 */
+	GMutex *mutex;
+
+	/**
+	 * Trigger this object after you have modified #command.  This
+	 * is also used by the decoder thread to notify the caller
+	 * when it has finished a command.
+	 */
+	GCond *cond;
+
+	enum decoder_state state;
+	enum decoder_command command;
 
-	volatile enum decoder_state state;
-	volatile enum decoder_command command;
 	bool quit;
 	bool seek_error;
 	bool seekable;
-	volatile double seek_where;
+	double seek_where;
 
 	/** the format of the song file */
 	struct audio_format in_audio_format;
@@ -80,6 +94,46 @@ void dc_init(void);
 
 void dc_deinit(void);
 
+/**
+ * Locks the #decoder_control object.
+ */
+static inline void
+decoder_lock(void)
+{
+	g_mutex_lock(dc.mutex);
+}
+
+/**
+ * Unlocks the #decoder_control object.
+ */
+static inline void
+decoder_unlock(void)
+{
+	g_mutex_unlock(dc.mutex);
+}
+
+/**
+ * Waits for a signal on the #decoder_control object.  This function
+ * is only valid in the decoder thread.  The object must be locked
+ * prior to calling this function.
+ */
+static inline void
+decoder_wait(void)
+{
+	g_cond_wait(dc.cond, dc.mutex);
+}
+
+/**
+ * Signals the #decoder_control object.  This function is only valid
+ * in the player thread.  The object should be locked prior to calling
+ * this function.
+ */
+static inline void
+decoder_signal(void)
+{
+	g_cond_signal(dc.cond);
+}
+
 static inline bool decoder_is_idle(void)
 {
 	return (dc.state == DECODE_STATE_STOP ||
@@ -100,6 +154,39 @@ static inline bool decoder_has_failed(void)
 	return dc.state == DECODE_STATE_ERROR;
 }
 
+static inline bool decoder_lock_is_idle(void)
+{
+	bool ret;
+
+	decoder_lock();
+	ret = decoder_is_idle();
+	decoder_unlock();
+
+	return ret;
+}
+
+static inline bool decoder_lock_is_starting(void)
+{
+	bool ret;
+
+	decoder_lock();
+	ret = decoder_is_starting();
+	decoder_unlock();
+
+	return ret;
+}
+
+static inline bool decoder_lock_has_failed(void)
+{
+	bool ret;
+
+	decoder_lock();
+	ret = decoder_has_failed();
+	decoder_unlock();
+
+	return ret;
+}
+
 static inline struct song *
 decoder_current_song(void)
 {