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I wasn't sure how posix_spawn() handles file descriptors of standard streams with O_CLOEXEC flag set.
I wrote a test. Result: _posixsubprocess and os.posix_spawn() have the same behavior! If fd 2 (stderr) is marked with O_CLOEXEC, the fd 2 is closed in the child process, as expected. (There is no black magic to keep it open.)
$ cat x.py 
import subprocess, sys, os
args = [sys.executable, 'y.py']
os.set_inheritable(2, False)
subprocess.run(args, close_fds=False, restore_signals=False)
$ cat y.py 
import os
def fd_valid(fd):
 try:
 os.fstat(fd)
 return True
 except OSError:
 return False
for fd in (0, 1, 2):
 print("fd %s valid? %s" % (fd, fd_valid(fd)))
$ python3 x.py # unpatched
fd 0 valid? True
fd 1 valid? True
fd 2 valid? False
$ ./python x.py # patched, use posix_spawn()
fd 0 valid? True
fd 1 valid? True
fd 2 valid? False