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.