When running a process which changes UID/GID, some of the following processes will run as the user I change to per process.

In order to reproduce (see the attached reproducer):
1. Change the 'USERNAME' to an unprivileged user on your system.
2. Run the reproducer as a user with elevated privileges (e.g. root or some secondary user you have on your system). Mind you, I don't think the user you run as needs elevated privileges, but that's the user I ran as when I observed this behaviour.
3. The reproducer iterates over a list (It stems from a test function which was checking permissions on log files). Observe the print out, which prints the process' GID, UID and secondary groups before we're changing to the users GID, UID and secondary groups.
4. You should observe that at some point the process prints the user information of the user we want to change to not the one which initially started the script.

Example output when running locally as root:

('B', (0, 0, [0]))
('A', (0, 0, [0]))
('C', (0, 0, [0]))
('E', (0, 0, [0]))
('D', (0, 0, [0]))
('F', (1002, 1002, [10, 135, 1000, 1002]))
('H', (1002, 1002, [10, 135, 1000, 1002]))
('I', (1002, 1002, [10, 135, 1000, 1002]))
('J', (1002, 1002, [10, 135, 1000, 1002]))
('G', (1002, 1002, [10, 135, 1000, 1002]))
('K', (1002, 1002, [10, 135, 1000, 1002]))
('L', (1002, 1002, [10, 135, 1000, 1002]))
('M', (1002, 1002, [10, 135, 1000, 1002]))
('N', (1002, 1002, [10, 135, 1000, 1002]))

I would have expected `0` all the way through.

However, if I initialise the Pool with `maxtasksperchild=1` the isolation seems as expected.

I don't know whether this is a bug or I'm foolish to invoke multiprocessing like this. I've run out of time to investigate this further. It's certainly strange behaviour to me and I thought I better report it, since reproducing seems fairly deterministic.