import asyncio
called_on_fut_callback0 = False
pad = lambda: ...
def evil_call_soon(*args, **kwargs):
    global called_on_fut_callback0
    if called_on_fut_callback0:
        # Called when handling fut->fut_callbacks[0]
        # and mutates the length fut->fut_callbacks.
        fut.remove_done_callback(int)
        fut.remove_done_callback(pad)
    else:
        called_on_fut_callback0 = True
fake_event_loop = lambda: ...
fake_event_loop.call_soon = evil_call_soon
fake_event_loop.get_debug = lambda: False  # suppress traceback
fut = asyncio.Future(loop=fake_event_loop)
fut.add_done_callback(str)  # sets fut->fut_callback0
fut.add_done_callback(int)  # sets fut->fut_callbacks[0]
fut.add_done_callback(pad)  # sets fut->fut_callbacks[1]
fut.add_done_callback(pad)  # sets fut->fut_callbacks[2]
fut.set_result("boom")