Comment 1 for bug 2027735

I was able to reproduce this. Thanks for reporting!

We start multiple regiond processes and each of them is running

    def _makeEndpoint(self):
        """Make the endpoint for the webapp."""

        socket_path = os.getenv(
            "MAAS_HTTP_SOCKET_PATH",
            get_maas_data_path("maas-regiond-webapp.sock"),
        )

        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        if os.path.exists(socket_path):
            os.unlink(socket_path)

        s.bind(socket_path)
        # Use a backlog of 50, which seems to be fairly common.
        s.listen(50)

        # Adopt this socket into Twisted's reactor setting the endpoint.
        endpoint = AdoptedStreamServerEndpoint(reactor, s.fileno(), s.family)
        endpoint.socket = s # Prevent garbage collection.
        return endpoint

unfortunately only one process is going to process the requests as there is no load balancing for unix sockets (ignore the race condition on os.path.exists, I noticed that when we hit the race an exception is thrown and it is silently ignored).
When we execute
        if os.path.exists(socket_path):
            os.unlink(socket_path)
we remove the connection of the worker that was using that unix socket.

Unless I missed something, the only fix is to create a single unix socket for each process and let nginx round robin the requests to the unix sockets.