RuntimeError: cannot use unpack() on <cdata 'char *' NULL>

[sdimovv]

I believe I have encountered an edge case in pynng.

To reproduce:

1. Start a Rep0 server serving on ipc:///tmp/my_socket.ipc
2. Use nngcat:

nngcat --req0 --dial=${addr} --data=$(echo -n -e "\x00\x00\x00\x00") # addr=ipc:///tmp/my_socket.ipc

You should observe the following error:

Traceback (most recent call last):
  File "~/.pyenv/versions/3.7.12/lib/python3.7/threading.py", line 926, in _bootstrap_inner
    self.run()
  File "~/.pyenv/versions/3.7.12/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "~/path/to/dir/source_file.py", line 82, in listen
    request = self.rep_socket.recv()
  File "~/path/to/dir/.venv/lib/python3.7/site-packages/pynng/nng.py", line 455, in recv
    recvd = ffi.unpack(data[0], size_t[0])
RuntimeError: cannot use unpack() on <cdata 'char *' NULL>

I also tried serving the same request with nngcat like so:

nngcat --rep0 --bind-ipc=/tmp/my_socket.ipc --format=hex

Then, after running:

nngcat --req0 --dial=${addr} --data=$(echo -n -e "\x00\x00\x00\x00")

I get as output from the rep0 server:

""

If I change the req0 command to read the message body from file, like so:

nngcat --req0 --dial=${addr} --file=/tmp/my_msg_body --format=hex

It works with both nngcat and pynng, which leads me to believe that this is a bug.

Expected behaviour:

I would expect for pynng to either dismiss the request, return an error, or handle it like nngcat does.