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> Simply list operations such as remove() and append() are
> thread safe,

OK, my apologies... I did not know.

I did some more tests. On Linux, I found lots of:

  File "./subprocess.py", line 428, in call
    return Popen(*args, **kwargs).wait()
  File "./subprocess.py", line 1023, in wait
    pid, sts = os.waitpid(self.pid, 0)
OSError: [Errno 10] No child processes

The try...except solution does not handle this (since we are
in the "posix" section).

In my opinion, the call to _cleanup() in Popen.__init__() is
useless (it just checks if older processes have stopped when
a new one is started. I cannot see why it could be
mandatory) and it is the root of this bug.

I commented it out (line 506) and retried my tests on Linux
& Windows plateforms: I had no exception at all.