Song: make the "parent" attribute mandatory

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

src/DirectorySave.cxx

@@ -145,7 +145,7 @@ directory_load(TextFile &file, Directory &directory, Error &error)
 				return false;
 
 			directory.AddSong(Song::NewFrom(std::move(*song),
-							&directory));
+							directory));
 			delete song;
 		} else if (StringStartsWith(line, PLAYLIST_META_BEGIN)) {
 			const char *name = line + sizeof(PLAYLIST_META_BEGIN) - 1;

src/Song.cxx

@@ -29,8 +29,8 @@
 #include <string.h>
 #include <stdlib.h>
 
-inline Song::Song(const char *_uri, size_t uri_length, Directory *_parent)
-	:parent(_parent), mtime(0), start_ms(0), end_ms(0)
+inline Song::Song(const char *_uri, size_t uri_length, Directory &_parent)
+	:parent(&_parent), mtime(0), start_ms(0), end_ms(0)
 {
 	memcpy(uri, _uri, uri_length + 1);
 }
@@ -40,7 +40,7 @@ inline Song::~Song()
 }
 
 static Song *
-song_alloc(const char *uri, Directory *parent)
+song_alloc(const char *uri, Directory &parent)
 {
 	size_t uri_length;
 
@@ -54,7 +54,7 @@ song_alloc(const char *uri, Directory *parent)
 }
 
 Song *
-Song::NewFrom(DetachedSong &&other, Directory *parent)
+Song::NewFrom(DetachedSong &&other, Directory &parent)
 {
 	Song *song = song_alloc(other.GetURI(), parent);
 	song->tag = std::move(other.WritableTag());
@@ -65,7 +65,7 @@ Song::NewFrom(DetachedSong &&other, Directory *parent)
 }
 
 Song *
-Song::NewFile(const char *path, Directory *parent)
+Song::NewFile(const char *path, Directory &parent)
 {
 	return song_alloc(path, parent);
 }
@@ -81,7 +81,7 @@ Song::GetURI() const
 {
 	assert(*uri);
 
-	if (parent == nullptr || parent->IsRoot())
+	if (parent->IsRoot())
 		return std::string(uri);
 	else {
 		const char *path = parent->GetPath();

src/Song.hxx

@@ -54,7 +54,7 @@ struct Song {
 	 * the current database plugin does not manage the parent
 	 * directory this way.
 	 */
-	Directory *parent;
+	Directory *const parent;
 
 	time_t mtime;
 
@@ -75,15 +75,15 @@ struct Song {
 	 */
 	char uri[sizeof(int)];
 
-	Song(const char *_uri, size_t uri_length, Directory *parent);
+	Song(const char *_uri, size_t uri_length, Directory &parent);
 	~Song();
 
 	gcc_malloc
-	static Song *NewFrom(DetachedSong &&other, Directory *parent);
+	static Song *NewFrom(DetachedSong &&other, Directory &parent);
 
 	/** allocate a new song with a local file name */
 	gcc_malloc
-	static Song *NewFile(const char *path_utf8, Directory *parent);
+	static Song *NewFile(const char *path_utf8, Directory &parent);
 
 	/**
 	 * allocate a new song structure with a local file name and attempt to
@@ -91,11 +91,7 @@ struct Song {
 	 * data, nullptr is returned.
 	 */
 	gcc_malloc
-	static Song *LoadFile(const char *path_utf8, Directory *parent);
-
-	static Song *LoadFile(const char *path_utf8, Directory &parent) {
-		return LoadFile(path_utf8, &parent);
-	}
+	static Song *LoadFile(const char *path_utf8, Directory &parent);
 
 	void Free();
 

src/SongUpdate.cxx

@@ -40,19 +40,18 @@
 #include <sys/stat.h>
 
 Song *
-Song::LoadFile(const char *path_utf8, Directory *parent)
+Song::LoadFile(const char *path_utf8, Directory &parent)
 {
 	Song *song;
 	bool ret;
 
-	assert((parent == nullptr) == PathTraitsUTF8::IsAbsolute(path_utf8));
 	assert(!uri_has_scheme(path_utf8));
 	assert(strchr(path_utf8, '\n') == nullptr);
 
 	song = NewFile(path_utf8, parent);
 
 	//in archive ?
-	if (parent != nullptr && parent->device == DEVICE_INARCHIVE) {
+	if (parent.device == DEVICE_INARCHIVE) {
 		ret = song->UpdateFileInArchive();
 	} else {
 		ret = song->UpdateFile();

src/UpdateContainer.cxx

@@ -102,7 +102,7 @@ update_container_file(Directory &directory,
 	unsigned int tnum = 0;
 	TagBuilder tag_builder;
 	while ((vtrack = plugin.container_scan(pathname.c_str(), ++tnum)) != nullptr) {
-		Song *song = Song::NewFile(vtrack, contdir);
+		Song *song = Song::NewFile(vtrack, *contdir);
 
 		// shouldn't be necessary but it's there..
 		song->mtime = st->st_mtime;

src/UpdateSong.cxx

@@ -69,7 +69,7 @@ update_song_file2(Directory &directory,
 	if (song == nullptr) {
 		FormatDebug(update_domain, "reading %s/%s",
 			    directory.GetPath(), name);
-		song = Song::LoadFile(name, &directory);
+		song = Song::LoadFile(name, directory);
 		if (song == nullptr) {
 			FormatDebug(update_domain,
 				    "ignoring unrecognized file %s/%s",