The proactor event loop of asyncio returns b'' on recv_into() if the socket/pipe is closed:

    def recv_into(self, conn, buf, flags=0):
        ...
        try:
            ...
        except BrokenPipeError:
            return self._result(b'')
        ...

But a socket recv_into() must always return a number: the number of written bytes. Example with socket.socket.recv_into() method:
https://docs.python.org/3/library/socket.html#socket.socket.recv_into

IMHO zero must be returned here. This bug may be the root cause of bpo-38912 bug.

Attached PR fix the issue.