> I got a failure in newly added test_pidfd_open:

> I'm running kernel 5.3.7-x86_64-linode130 with Arch Linux.

> I think you must still be experiencing some sort of sandboxing. I don't know how else you would get an EPERM out of pidfd_open.

I believe Benjamin is correct. On a native install of Arch Linux with kernel 5.3.7 (using latest updates from the official repos), I received no failures in test_posix. 

[aeros:~/repos/benjaminp-cpython]$ ./python -m test test_posix     (asyncio-pidfd) 
0:00:00 load avg: 1.86 Run tests sequentially
0:00:00 load avg: 1.86 [1/1] test_posix

== Tests result: SUCCESS ==

1 test OK.

Total duration: 544 ms
Tests result: SUCCESS

To confirm there weren't intermittent failures, I also ran test_posix indefinitely, sending SIGINT after ~2500 iterations. No failures occurred:

[aeros:~/repos/benjaminp-cpython]$ ./python -m test test_pty -F    (asyncio-pidfd) 
...
0:01:31 load avg: 1.57 [2506] test_pty
0:01:31 load avg: 1.57 [2507] test_pty
^C

== Tests result: INTERRUPTED ==
Test suite interrupted by signal SIGINT.

2506 tests OK.

Total duration: 1 min 31 sec
Tests result: INTERRUPTED

It seems that the issue is likely specific to Chih-Hsuan Yen's environment.