Song: add function song_equals()

decoder_is_current_song() now recognizes different instances of the
same physical song.

src/Song.cxx

@@ -84,6 +84,33 @@ song_free(struct song *song)
 	g_free(song);
 }
 
+gcc_pure
+static inline bool
+directory_equals(const struct directory &a, const struct directory &b)
+{
+	return strcmp(a.path, b.path) == 0;
+}
+
+gcc_pure
+static inline bool
+directory_is_same(const struct directory *a, const struct directory *b)
+{
+	return a == b ||
+		(a != nullptr && b != nullptr &&
+		 directory_equals(*a, *b));
+
+}
+
+bool
+song_equals(const struct song *a, const struct song *b)
+{
+	assert(a != nullptr);
+	assert(b != nullptr);
+
+	return directory_is_same(a->parent, b->parent) &&
+		strcmp(a->uri, b->uri) == 0;
+}
+
 char *
 song_get_uri(const struct song *song)
 {

src/decoder_control.c

@@ -20,6 +20,7 @@
 #include "config.h"
 #include "decoder_control.h"
 #include "pipe.h"
+#include "song.h"
 
 #include <assert.h>
 
@@ -111,7 +112,7 @@ decoder_is_current_song(const struct decoder_control *dc,
 
 	case DECODE_STATE_START:
 	case DECODE_STATE_DECODE:
-		return dc->song == song;
+		return song_equals(dc->song, song);
 	}
 
 	assert(false);

src/song.h

@@ -21,6 +21,7 @@
 #define MPD_SONG_H
 
 #include "util/list.h"
+#include "gcc.h"
 
 #include <stddef.h>
 #include <stdbool.h>
@@ -100,6 +101,13 @@ song_is_file(const struct song *song)
 	return song_in_database(song) || song->uri[0] == '/';
 }
 
+/**
+ * Returns true if both objects refer to the same physical song.
+ */
+gcc_pure
+bool
+song_equals(const struct song *a, const struct song *b);
+
 bool
 song_file_update(struct song *song);