In the case of multiprocessing.synchronize() being missing, the
test_concurrent_futures test suite now skips only the tests that
require multiprocessing.synchronize().

Validate that multiprocessing.synchronize exists as part of
_check_system_limits(), allowing ProcessPoolExecutor to raise
NotImplementedError during __init__, rather than crashing with
ImportError during __init__ when creating a lock imported from
multiprocessing.synchronize.

Use _check_system_limits() to disable tests of
ProcessPoolExecutor on systems without multiprocessing.synchronize.

Running the test suite without multiprocessing.synchronize reveals
that Lib/compileall.py crashes when it uses a ProcessPoolExecutor.
Therefore, change Lib/compileall.py to call _check_system_limits()
before creating the ProcessPoolExecutor.

Note that both Lib/compileall.py and Lib/test/test_compileall.py
were attempting to sanity-check ProcessPoolExecutor by expecting
ImportError. In multiprocessing.resource_tracker, sem_unlink() is also absent
on platforms where POSIX semaphores aren't available. Avoid using
sem_unlink() if it, too, does not exist.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Fix a race condition in concurrent.futures._ThreadWakeup: access to
_ThreadWakeup is now protected with the shutdown lock.

Process.shutdown(wait=True) of concurrent.futures now closes
explicitly the result queue.

Roughly based on https://github.com/python/cpython/commit/904e34d4e6b6007986dcc585d5c553ee8ae06f95, but with a few substantial differences.

/cc @pitrou @brianquinlan

Remove daemon threads from :mod:`concurrent.futures` by adding
an internal `threading._register_atexit()`, which calls registered functions
prior to joining all non-daemon threads. This allows for compatibility
with subinterpreters, which don't support daemon threads.

As reported initially by @rad-pat in #6084, the following script causes a deadlock.

```
from concurrent.futures import ProcessPoolExecutor


class ObjectWithPickleError():
    """Triggers a RuntimeError when sending job to the workers"""

    def __reduce__(self):
        raise RuntimeError()


if __name__ == "__main__":
    e = ProcessPoolExecutor()
    f = e.submit(id, ObjectWithPickleError())
    e.shutdown(wait=False)
    f.result()  # Deadlock on get
```

This is caused by the fact that the main process is closing communication channels that might be necessary to the `queue_management_thread` later. To avoid this, this PR let the `queue_management_thread` manage all the closing.



https://bugs.python.org/issue39104



Automerge-Triggered-By: @pitrou