[3.6] bpo-34658: Fix rare subprocess prexec_fn fork error. (GH-9255) by gpshead · Pull Request #9292 · python/cpython
bpo-34658: Fix a rare interpreter unhandled exception state SystemError only
seen when using subprocess with a preexec_fn while an after_parent handler has
been registered with os.register_at_fork and the fork system call fails.
https://bugs.python.org/issue34658
Cherry picked from commit a20b6ad with manual fixups for it to make sense on 3.6.