[issue38692] add a pidfd child process watcher

[Chih-Hsuan Yen]

Chih-Hsuan Yen <yan12125 at gmail.com> added the comment:

I got a failure in newly added test_pidfd_open:

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FAIL: test_pidfd_open (test.test_posix.PosixTester)
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Traceback (most recent call last):
  File "/build/python-git/src/cpython/Lib/test/test_posix.py", line 1479, in test_pidfd_open
    self.assertEqual(cm.exception.errno, errno.EINVAL)
AssertionError: 1 != 22

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I'm running kernel 5.3.7-x86_64-linode130 with Arch Linux. At first I suspect that it's related to system call filters from systemd as tests are run in a systemd-nspawn container. However, there are still failures after patching systemd with https://github.com/systemd/systemd/commit/9e486265716963439fb0fd7f2a97abf109f24f75.

How about also skipping the test if pidfd_open returns EPERM?

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nosy: +yan12125

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<https://bugs.python.org/issue38692>
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