concurrent.futures.ProcessPoolExecutor raises during shutdown

Bug report

Bug description:

from concurrent.futures import ProcessPoolExecutor as Pool

def nope():
    pass

def mk_done(pool):
    def done(_fut):
        pool.shutdown(wait=True)
    return done

if __name__ == "__main__":
    pool = Pool()
    future = pool.submit(nope)
    future.add_done_callback(mk_done(pool))

ProcessPoolExecutor invokes future done callbacks from its manager thread. When one of those future callbacks invokes ProcessPoolExecutor.shutdown(), shut down fails with a RuntimeError: cannot join current thread because the implementation joins the manager thread without protecting against self-joins.

The relevant lines Lib/concurrent/futures/process.py:845-846 currently read:

if self._executor_manager_thread is not None and wait:
            self._executor_manager_thread.join()

but should probably be updated to:

t = self._executor_manager_thread
if t is not None and wait and threading.current_thread() != t:
            self._executor_manager_thread.join()

Invoking shutdown(False) avoids the problem but also makes it impossible to wait for pool completion. A more general solution would be to deprecate the wait parameter for shutdown, never waiting in that method, and add a separate method, say wait_for_shutdown, that uses a threading.Event to wake any waiting threads. Alas, correctly setting that Event is going to be a bit involved.

While I tested on 3.12, the bug is also present in 3.13 and current.

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Start by running the provided ProcessPoolExecutor reproduction, then inspect Lib/concurrent/futures/process.py around lines 845-846 and the shutdown entry point. Add coverage for shutdown(wait=True) from a future callback and verify that the callback completes without a self-join error while pool completion remains waitable.