When I run this code (on a UNIX system with temporary directory `/tmp/`):

```
from multiprocessing import connection
import threading
key=b"1"

def run():
    c=connection.Client("/tmp/xxx", authkey=key)
    c.send("data")

l=connection.Listener("/tmp/xxx", authkey=key)
threading.Thread(target=run).start()
c=l.accept()
print("receive", c.recv())
```

it prints out "receive data" normally.

However, in the special case that `key=b""` the doesn't print anything, and can only be interrupted with Ctrl+C.

`key=None` doesn't have that issue.

Note that this issue does happen when the client uses the key `b""` and the server uses the key `None`, but the program works normally if the reverse situation.

Python version: Python 3.9.3.

I tried debugging this and from what I can see it's because there's an if that checks if the authkey is not None in the Client constructor:

https://github.com/python/cpython/blob/v3.9.4/Lib/multiprocessing/connection.py#L512

```
    if authkey is not None:
        answer_challenge(c, authkey)
        deliver_challenge(c, authkey)
```

Whereas in the Listener, the check is different:

https://github.com/python/cpython/blob/v3.9.4/Lib/multiprocessing/connection.py#L469

```
        c = self._listener.accept()
        if self._authkey:
            deliver_challenge(c, self._authkey)
            answer_challenge(c, self._authkey)
        return c
```

If I change the Listener to:

```
        if self._authkey is not None:
            deliver_challenge(c, self._authkey)
            answer_challenge(c, self._authkey)
        return c
```

it works.


The docs say:

"""
If authkey is given and not None, it should be a byte string and will be used as the secret key for an HMAC-based authentication challenge. No authentication is done if authkey is None. AuthenticationError is raised if authentication fails. See Authentication keys.
"""


Now the question is, if None is OK because no auth will be done what about empty bytes? Can it be used as secret key? If empty bytes is not accepted shouldn't Listener/Client raise an exception in the constructor?

I had a look at the HMAC RFC and apparently empty bytes sequence can be used as secret key.

"The definition of HMAC requires a cryptographic hash function, which
we denote by H, and a secret key K. 

...

The authentication key K can be of any length up to B, the
block length of the hash function."
   
https://tools.ietf.org/html/rfc2104.html#section-2

Assuming that is the case, the fix would be to change the Listener to:

```
        if self._authkey is not None:
            deliver_challenge(c, self._authkey)
            answer_challenge(c, self._authkey)
        return c
```


I created a PR for that, if anyone can review it, I appreciate it.
https://github.com/python/cpython/pull/25845