db/update/Walk: clear `Song::in_playlist`

Without clearing all `in_playlist` flags, the songs will never be
revealed again if they were hidden once by a CUE sheet, not even after
the CUE sheet gets deleted or modified.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1784

NEWS

@@ -3,6 +3,8 @@ ver 0.23.13 (not yet released)
   - curl: fix busy loop after connection failed
 * archive
   - zzip: fix crash bug
+* database
+  - simple: reveal hidden songs after deleting containing CUE
 * decoder
   - ffmpeg: reorder to a lower priority than "gme"
   - gme: require GME 0.6 or later

src/db/plugins/simple/Directory.cxx

@@ -126,6 +126,18 @@ Directory::LookupTargetSong(std::string_view _target) noexcept
 	return lr.directory->FindSong(lr.rest);
 }
 
+void
+Directory::ClearInPlaylist() noexcept
+{
+	assert(holding_db_lock());
+
+	for (auto &child : children)
+		child.ClearInPlaylist();
+
+	for (auto &song : songs)
+		song.in_playlist = false;
+}
+
 void
 Directory::PruneEmpty() noexcept
 {

src/db/plugins/simple/Directory.hxx

@@ -287,6 +287,14 @@ public:
 	 */
 	SongPtr RemoveSong(Song *song) noexcept;
 
+	/**
+	 * Recursively walk through the whole tree and set all
+	 * `Song::in_playlist` fields to `false`.
+	 *
+	 * Caller must lock the #db_mutex.
+	 */
+	void ClearInPlaylist() noexcept;
+
 	/**
 	 * Caller must lock the #db_mutex.
 	 */

src/db/update/Walk.cxx

@@ -531,6 +531,7 @@ UpdateWalk::Walk(Directory &root, const char *path, bool discard) noexcept
 
 	{
 		const ScopeDatabaseLock protect;
+		root.ClearInPlaylist();
 		PurgeDanglingFromPlaylists(root);
 	}