The traceback:

    Traceback (most recent call last):
      File "Lib/logging/handlers.py", line 917, in emit
        self.socket.sendto(msg, self.address)
    TypeError: getsockaddrarg: AF_INET address must be tuple, not bytes

The attached patch reproduces the issue with a test case and fixes it.

The following change looks good to me:

-        if isinstance(address, str):
+        if isinstance(address, (str, bytes)):
             self.unixsocket = True
             self._connect_unixsocket(address)

But I don't understand the testcase. Is an address starting with a NULL character a special address? How does it the bytes address case?

+        # override the definition in the base class
+        self.address = '\x00python_logging_test'

"syslog logging handler fails with address in unix abstract namespace"

What is the unix abstract namespace?

A unix abstract socket address is defined here:
    http://man7.org/linux/man-pages/man7/unix.7.html
as:

"Traditionally, UNIX domain sockets can be either unnamed, or bound to
a filesystem pathname (marked as being of type socket).  Linux also
supports an abstract namespace which is independent of the
filesystem."

and:

"abstract: an abstract socket address is distinguished (from a
pathname socket) by the fact that sun_path[0] is a null byte ('\0').
..."

It is also documented in the Python socket module documentation at
    https://docs.python.org/3/library/socket.html
and tested in the TestLinuxAbstractNamespace test of
Lib/test/test_socket.py.

Ok. And what is the link between abstract namespace and the fact that the syslog handler gets bytes? Is it tested by your change?

The SysLogHandlerTest class instantiates a 'server_class' that eventually calls the
server_bind() method of the TCPServer class of the socketserver module.

This server_bind() method executes the statements:

    self.socket.bind(self.server_address)
    self.server_address = self.socket.getsockname()

and when self.server_address is a string and contains a null byte, then
getsockname() returns a bytes object.

So finally, self.server in SysLogHandlerTest is a bytes object in this case,
hence the needed to fix Lib/logging/handlers.py.

Xavier de Gaye added the comment:
> when self.server_address is a string and contains a null byte, then
> getsockname() returns a bytes object.

Ah? It sounds strange to me that the type of getsockname() depends on
the NULL byte. I would also expect a Unicode name for the abstract
namespace. What do you think?

makesockaddr() in socketmodule.c calls PyBytes_FromStringAndSize() when the
first byte is a null byte.

My opinion is not worth much in this matter :). The socket documentation does
say that AF_UNIX addresses are "represented as a string, using the file system
encoding", so the implementation looks wrong. However it is possible that
PyUnicode_DecodeFSDefault() fails for some file system encodings when the
encoded address contains null bytes ?

> However it is possible that
PyUnicode_DecodeFSDefault() fails for some file system encodings when the
encoded address contains null bytes ?

No, it's not possible. Undecode bytes are escaped as surrogate characters using the surrogateescape error handler. See the PEP 393.

Then it seems that makesockaddr() should be fixed instead of the logging
handler.

> Then it seems that makesockaddr() should be fixed instead of the logging handler.

Yes. That's why I didn't understand the issue at the beginning :-) Would you like to work on fixing the _socket module?

It may be worth to keep your syslog unit test. It's up to you.

I will give it a try.

Here is the patch.

As a  reference, issue #8373 with changeset 1f23bb74f4bc changed the AF_UNIX
socket addresses encoding from UTF-8 to the file system encoding and the
'surrogateescape' error handler (PEP 383).

This patch corrects the non-ascii quote character in the Misc/NEWS diff.

In makesockaddr(), the current implementation does not do any decoding of AF_UNIX addresses in the abstract namespace in the struct sockaddr_un to bytes direction, i.e. system to python direction, but does encode a string or bytes object to struct sockaddr_un in the reverse direction, in getsockaddrarg(). This is not correct.

So the patch does more than fixing the type of the addresses as it also fixes this in a non backward compatible way. Maybe this should be indicated in Misc/NEWS.