Testcases hangs : /usr/local/lib/python3.7/threading.py", line 1335, in _after_fork

When py3.7 test is executed for nova-powervm, it hangs and gets timed_out. the same testcases executes fine in py3.6 env.

below exception occurs and doesnt exit from the tests.

Exception ignored in: <function _after_fork at 0x7fe220046e18>
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/threading.py", line 1335, in _after_fork
    assert len(_active) == 1
AssertionError:
Exception ignored in: <function _after_fork at 0x7f40f5346e18>
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/threading.py", line 1335, in _after_fork
    assert len(_active) == 1
AssertionError:
Exception ignored in: <function _after_fork at 0x7f2795418e18>
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/threading.py", line 1335, in _after_fork
    assert len(_active) == 1
AssertionError:
Exception ignored in: <function _after_fork at 0x7ff2e7d26e18>
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/threading.py", line 1335, in _after_fork
    assert len(_active) == 1
AssertionError:
Exception ignored in: <function _after_fork at 0x7f17debb7e18>
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/threading.py", line 1335, in _after_fork
    assert len(_active) == 1
AssertionError:

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def _after_fork():
    """
    Cleanup threading module state that should not exist after a fork.
    """
    # Reset _active_limbo_lock, in case we forked while the lock was held
    # by another (non-forked) thread. http://bugs.python.org/issue874900
    global _active_limbo_lock, _main_thread
    _active_limbo_lock = _allocate_lock()

    # fork() only copied the current thread; clear references to others.
    new_active = {}
    current = current_thread()
    _main_thread = current
    with _active_limbo_lock:
        # Dangling thread instances must still have their locks reset,
        # because someone may join() them.
        threads = set(_enumerate())
        threads.update(_dangling)
        for thread in threads:
            # Any lock/condition variable may be currently locked or in an
            # invalid state, so we reinitialize them.
            if thread is current:
                # There is only one active thread. We reset the ident to
                # its new value since it can have changed.
                thread._reset_internal_locks(True)
                ident = get_ident()
                thread._ident = ident
                new_active[ident] = thread
            else:
                # All the others are already stopped.
                thread._reset_internal_locks(False)
                thread._stop()

        _limbo.clear()
        _active.clear()
        _active.update(new_active)
        assert len(_active) == 1