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Note that there is no need to manually close p2cread at all, and other two descriptors need to be closed only if they are the result of dup above. This is because if a descriptor is created by subprocess.py (a pipe or for /dev/null), it's non-inheritable, and if a descriptor is specified by the user, is not in pass_fds and is > 2, it will be closed by _close_open_fds. So my original idea of fixing this issue was to add an async-signal-safe version of _Py_dup and use it instead of dup above, and then just remove these three checks.

Note also that p2cwrite and its three friends are never required to be manually closed since they are always non-inheritable pipe ends created by subprocess.py.