Freezegun behaving inconsistently with multiprocessing Pools

[RobbieFernandez]

freezegun 0.3.15 and django 1.11.29 with Python 3.7.5 on Ubuntu 18.04.4

freezegun doesn't seem to be correctly patching the datetime functions when it is being run inside a Pool. This can be reproduced with this small script:

from multiprocessing import Pool

import freezegun

def test_patched_time(d):
    import freezegun 
    from datetime import datetime
    from django.utils import timezone
    with freezegun.freeze_time(d):
        print(d, '|', datetime.utcnow(), '|', timezone.now())

test_data = ['2020-01-01', '2019-02-01', '2018-01-01', '2017-01-01', '2016-01-01', '2015-01-01', '2014-01-01', '2013-01-01', '2012-01-01']  


print("Out of freezegun context")
p = Pool(8)
p.map(test_patched_time, test_data)
p.close()
p.join()

print("\nIn freezegun context")
with freezegun.freeze_time(None]):
    p = Pool(8)
    p.map(test_patched_time, test_data)
    p.close()
    p.join()

Which produces this output:

Out of freezegun context
2020-01-01 | 2020-07-08 08:30:12.246679 | 2020-01-01 00:00:00+00:00
2012-01-01 | 2020-07-08 08:30:12.293028 | 2012-01-01 00:00:00+00:00
2016-01-01 | 2020-07-08 08:30:12.293913 | 2016-01-01 00:00:00+00:00
2015-01-01 | 2020-07-08 08:30:12.294940 | 2015-01-01 00:00:00+00:00
2017-01-01 | 2020-07-08 08:30:12.301806 | 2017-01-01 00:00:00+00:00
2019-02-01 | 2020-07-08 08:30:12.307442 | 2019-02-01 00:00:00+00:00
2018-01-01 | 2020-07-08 08:30:12.314767 | 2018-01-01 00:00:00+00:00
2014-01-01 | 2020-07-08 08:30:12.315739 | 2014-01-01 00:00:00+00:00
2013-01-01 | 2020-07-08 08:30:12.322053 | 2013-01-01 00:00:00+00:00

In freezegun context
2020-01-01 | 2020-01-01 00:00:00 | 2020-01-01 00:00:00+00:00
2018-01-01 | 2018-01-01 00:00:00 | 2018-01-01 00:00:00+00:00
2015-01-01 | 2015-01-01 00:00:00 | 2015-01-01 00:00:00+00:00
2014-01-01 | 2014-01-01 00:00:00 | 2014-01-01 00:00:00+00:00
2013-01-01 | 2013-01-01 00:00:00 | 2013-01-01 00:00:00+00:00
2012-01-01 | 2012-01-01 00:00:00 | 2012-01-01 00:00:00+00:00
2016-01-01 | 2016-01-01 00:00:00 | 2016-01-01 00:00:00+00:00
2017-01-01 | 2017-01-01 00:00:00 | 2017-01-01 00:00:00+00:00
2019-02-01 | 2019-02-01 00:00:00 | 2019-02-01 00:00:00+00:00

The first column is the date string being passed to freeze_time, the middle column is the output of datetime.utcnow and the third column is the output of django.utils.timezone.now

When the Pool is created outside of a freezegun context, datetime.utcnow is always returning the current time. If I wrap the whole thing in a freezegun context (with None passed to freeze_time) then utcnow returns the patched time. django.utils.timezone.now is working in either case. This is causing issues when django tests are run in parallel, which uses multiprocessing.Pool.