Shared party playlist

At a party, everyone's phone adds songs to one shared playlist, and other parts of the app (a screen showing the queue, a DJ bot) want to hear about every new song.

Implement a thread-safe Playlist() with:

• add(song) -> None: append one song.
• add_many(songs) -> None: append several songs as one block: no song from another thread may land in the middle of the block.
• songs() -> list: a copy of the playlist so far, in order.
• subscribe(listener) -> None: from now on, call listener(song) once for every song added (for add_many, once per song, in order, after the whole block is in).

Listeners are other people's code, and they do things like:

• read the playlist: lambda song: print(len(p.songs())),
• add a song themselves: the DJ bot answers "Finale" by adding "Encore",
• hand work to another thread and wait for it, and that thread calls p.songs().

p = Playlist()
p.subscribe(lambda s: p.add("Encore") if s == "Finale" else None)
p.add("Opener")
p.add_many(["Slow one", "Finale"])
p.songs()   # ["Opener", "Slow one", "Finale", "Encore"]

None of these may hang, and concurrent adds must never lose a song.

There are two deadlocks waiting here:

1. add_many calling add (or a listener calling add/songs) while the same thread already holds a plain Lock: a thread waiting for itself.
2. A listener that waits for another thread which needs the lock. Even an RLock doesn't save you: the lock is held by the thread that's waiting.

Constraints: don't sleep or busy-wait, and keep one lock per playlist.