song: add function song_replace_uri()

src/song.c

@@ -61,6 +61,18 @@ song_file_new(const char *path, struct directory *parent)
 	return song_alloc(path, parent);
 }
 
+struct song *
+song_replace_uri(struct song *old_song, const char *uri)
+{
+	struct song *new_song = song_alloc(uri, old_song->parent);
+	new_song->tag = old_song->tag;
+	new_song->mtime = old_song->mtime;
+	new_song->start_ms = old_song->start_ms;
+	new_song->end_ms = old_song->end_ms;
+	g_free(old_song);
+	return new_song;
+}
+
 void
 song_free(struct song *song)
 {

src/song.h

@@ -74,6 +74,15 @@ song_file_new(const char *path, struct directory *parent);
 struct song *
 song_file_load(const char *path, struct directory *parent);
 
+/**
+ * Replaces the URI of a song object.  The given song object is
+ * destroyed, and a newly allocated one is returned.  It does not
+ * update the reference within the parent directory; the caller is
+ * responsible for doing that.
+ */
+struct song *
+song_replace_uri(struct song *song, const char *uri);
+
 void
 song_free(struct song *song);