OSError: [Errno 9] Bad file descriptor on socket.shutdown(socket.SHUT_RDWR) #597

❓ I'm submitting a ...

🐞 Describe the bug. What is the current behavior?

Sometimes, sending a TCP FIN over a socket, errors out with OSError: [Errno 9] Bad file descriptor . The exception originates at https://github.com/cherrypy/cheroot/blob/57fc412/cheroot/server.py#L1480 and is re-raised at https://github.com/cherrypy/cheroot/blob/57fc412/cheroot/server.py#L1484-L1485 since the corresponding syscall return code ( errno.EBADF / 9 ) is not listed as allowed to be ignored. We may need to reconsider what's ignored there.
I noticed this in one of the recent CI runs, and it does not appear to be reoccurring, but I'm still filing this issue to log that this has happened.

❓ What is the motivation / use case for changing the behavior?

Clean runtime operation.

💡 To Reproduce

Not clear.

💡 Expected behavior

No exception logged .

📋 Details

This is what's visible in the CI log:

>           assert c_msg in raw_testing_server.error_log.ignored_msgs, (
                'Found error in the error log: '
                "message = '{c_msg}', level = '{c_level}'\n"
                '{c_traceback}'.format(**locals()),
E           AssertionError: ("Found error in the error log: message = 'Error in HTTPServer.serve', level = '40'
E             Traceback (most recent call last)...e_kernel_socket
E                 shutdown(socket.SHUT_RDWR)  # actually send a TCP FIN
E             OSError: [Errno 9] Bad file descriptor
E             ",)
E           assert 'Error in HTTPServer.serve' in []
E            +  where [] = <cheroot.test.test_conn.ErrorLogMonitor object at 0x10906e4f0>.ignored_msgs
E            +    where <cheroot.test.test_conn.ErrorLogMonitor object at 0x10906e4f0> = <cheroot.wsgi.Server object at 0x10906ef10>.error_log

It's not clear why exactly this is happening but googling suggests that the underlying syscall is what returns [Errno 9] Bad file descriptor . https://man7.org/linux/man-pages/man2/shutdown.2.html includes the following:

ERRORS

EBADF sockfd is not a valid file descriptor.

Which may mean that the underlying file descriptor for the socket in question may have been closed prior to this call somehow. It's interesting that this error is separate from

ENOTCONN
The specified socket is not connected.

and ENOTCONN is listed as needed to be ignored at