state_file: save only if something has changed

If nothing has changed since the last save, don't save the state
file.  Saving will spin up the hard drive, which is undesirable on
hosts where MPD is idling in background.

NEWS

@@ -36,6 +36,7 @@ ver 0.16 (20??/??/??)
 * renamed option "--stdout" to "--stderr"
 * removed options --create-db and --no-create-db
 * automatically update the database with Linux inotify
+* state_file: save only if something has changed
 * obey $(sysconfdir) for default mpd.conf location
 
 

src/output_command.c

@@ -31,6 +31,8 @@
 #include "mixer_control.h"
 #include "idle.h"
 
+extern unsigned audio_output_state_version;
+
 bool
 audio_output_enable_index(unsigned idx)
 {
@@ -44,6 +46,8 @@ audio_output_enable_index(unsigned idx)
 	ao->enabled = true;
 	idle_add(IDLE_OUTPUT);
 
+	++audio_output_state_version;
+
 	return true;
 }
 
@@ -67,5 +71,7 @@ audio_output_disable_index(unsigned idx)
 		idle_add(IDLE_MIXER);
 	}
 
+	++audio_output_state_version;
+
 	return true;
 }

src/output_state.c

@@ -34,6 +34,8 @@
 
 #define AUDIO_DEVICE_STATE "audio_device_state:"
 
+unsigned audio_output_state_version;
+
 void
 audio_output_state_save(FILE *fp)
 {
@@ -80,3 +82,9 @@ audio_output_state_read(const char *line)
 	ao->enabled = false;
 	return true;
 }
+
+unsigned
+audio_output_state_get_version(void)
+{
+	return audio_output_state_version;
+}

src/output_state.h

@@ -34,4 +34,12 @@ audio_output_state_read(const char *line);
 void
 audio_output_state_save(FILE *fp);
 
+/**
+ * Generates a version number for the current state of the audio
+ * outputs.  This is used by timer_save_state_file() to determine
+ * whether the state has changed and the state file should be saved.
+ */
+unsigned
+audio_output_state_get_version(void);
+
 #endif

src/playlist_state.c

@@ -200,3 +200,21 @@ playlist_state_restore(const char *line, FILE *fp, struct playlist *playlist)
 
 	return true;
 }
+
+unsigned
+playlist_state_get_hash(const struct playlist *playlist)
+{
+	return playlist->queue.version ^
+		(getPlayerElapsedTime() << 8) ^
+		(playlist->current >= 0
+		 ? (queue_order_to_position(&playlist->queue,
+					    playlist->current) << 16)
+		 : 0) ^
+		((int)getPlayerCrossFade() << 20) ^
+		(getPlayerState() << 24) ^
+		(playlist->queue.random << 27) ^
+		(playlist->queue.repeat << 28) ^
+		(playlist->queue.single << 29) ^
+		(playlist->queue.consume << 30) ^
+		(playlist->queue.random << 31);
+}

src/playlist_state.h

@@ -36,4 +36,13 @@ playlist_state_save(FILE *fp, const struct playlist *playlist);
 bool
 playlist_state_restore(const char *line, FILE *fp, struct playlist *playlist);
 
+/**
+ * Generates a hash number for the current state of the playlist and
+ * the playback options.  This is used by timer_save_state_file() to
+ * determine whether the state has changed and the state file should
+ * be saved.
+ */
+unsigned
+playlist_state_get_hash(const struct playlist *playlist);
+
 #endif

src/state_file.c

@@ -36,6 +36,13 @@ static char *state_file_path;
 /** the GLib source id for the save timer */
 static guint save_state_source_id;
 
+/**
+ * These version numbers determine whether we need to save the state
+ * file.  If nothing has changed, we won't let the hard drive spin up.
+ */
+static unsigned prev_volume_version, prev_output_version,
+	prev_playlist_version;
+
 static void
 state_file_write(void)
 {
@@ -57,6 +64,10 @@ state_file_write(void)
 	playlist_state_save(fp, &g_playlist);
 
 	while(fclose(fp) && errno == EINTR) /* nothing */;
+
+	prev_volume_version = sw_volume_state_get_hash();
+	prev_output_version = audio_output_state_get_version();
+	prev_playlist_version = playlist_state_get_hash(&g_playlist);
 }
 
 static void
@@ -88,6 +99,10 @@ state_file_read(void)
 	}
 
 	while(fclose(fp) && errno == EINTR) /* nothing */;
+
+	prev_volume_version = sw_volume_state_get_hash();
+	prev_output_version = audio_output_state_get_version();
+	prev_playlist_version = playlist_state_get_hash(&g_playlist);
 }
 
 /**
@@ -97,6 +112,13 @@ state_file_read(void)
 static gboolean
 timer_save_state_file(G_GNUC_UNUSED gpointer data)
 {
+	if (prev_volume_version == sw_volume_state_get_hash() &&
+	    prev_output_version == audio_output_state_get_version() &&
+	    prev_playlist_version == playlist_state_get_hash(&g_playlist))
+		/* nothing has changed - don't save the state file,
+		   don't spin up the hard disk */
+		return true;
+
 	state_file_write();
 	return true;
 }

src/volume.c

@@ -122,3 +122,9 @@ void save_sw_volume_state(FILE *fp)
 {
 	fprintf(fp, SW_VOLUME_STATE "%u\n", volume_software_set);
 }
+
+unsigned
+sw_volume_state_get_hash(void)
+{
+	return volume_software_set;
+}

src/volume.h

@@ -36,4 +36,13 @@ read_sw_volume_state(const char *line);
 
 void save_sw_volume_state(FILE *fp);
 
+/**
+ * Generates a hash number for the current state of the software
+ * volume control.  This is used by timer_save_state_file() to
+ * determine whether the state has changed and the state file should
+ * be saved.
+ */
+unsigned
+sw_volume_state_get_hash(void);
+
 #endif