db/simple: prune CUE entries from database for non-existent songs

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1019

src/db/update/Walk.cxx

@@ -133,6 +133,28 @@ UpdateWalk::PurgeDeletedFromDirectory(Directory &directory) noexcept
 	}
 }
 
+void
+UpdateWalk::PurgeDanglingFromPlaylists(Directory &directory) noexcept
+{
+	/* recurse */
+	for (Directory &child : directory.children)
+		PurgeDanglingFromPlaylists(child);
+
+	if (!directory.IsPlaylist())
+		/* this check is only for virtual directories
+		   representing a playlist file */
+		return;
+
+	directory.ForEachSongSafe([&](Song &song){
+		if (!song.target.empty() &&
+		    !PathTraitsUTF8::IsAbsoluteOrHasScheme(song.target.c_str()) &&
+		    !directory.TargetExists(song.target)) {
+			editor.DeleteSong(directory, &song);
+			modified = true;
+		}
+	});
+}
+
 #ifndef _WIN32
 static bool
 update_directory_stat(Storage &storage, Directory &directory) noexcept
@@ -530,5 +552,10 @@ UpdateWalk::Walk(Directory &root, const char *path, bool discard) noexcept
 		UpdateDirectory(root, exclude_list, info);
 	}
 
+	{
+		const ScopeDatabaseLock protect;
+		PurgeDanglingFromPlaylists(root);
+	}
+
 	return modified;
 }

src/db/update/Walk.hxx

@@ -85,6 +85,12 @@ private:
 
 	void PurgeDeletedFromDirectory(Directory &directory) noexcept;
 
+	/**
+	 * Remove all virtual songs inside playlists whose "target"
+	 * field points to a non-existing song file.
+	 */
+	void PurgeDanglingFromPlaylists(Directory &directory) noexcept;
+
 	void UpdateSongFile2(Directory &directory,
 			     const char *name, const char *suffix,
 			     const StorageFileInfo &info) noexcept;