delkov

class SocketLineReader:
    def __init__(self, socket):
        self.socket = socket
        self._buffer = b''

    def readline(self):
        pre, separator, post = self._buffer.partition(b'\n')
        if separator:
            self._buffer = post
            return pre + separator

        while True:
            data = self.socket.recv(1024)
            if not data:
                return None

            pre, separator, post = data.partition(b'\n')
            if not separator:
                self._buffer += data
            else:
                data = self._buffer + pre + separator
                self._buffer = post
                return data


import socket

sock = socket.socket()
sock.bind(('', 9090))
sock.listen(1)

conn, addr = sock.accept()

print('connected:', addr)

reader = SocketLineReader(conn)
while True:
    data = reader.readline()
    print(data)
    if not data:
        break
    conn.send(data.upper())

conn.close()

решение найдено, просто поиграйтесь с параметром --block:

cat -v /dev/pts/2 | parallel --j 3 --pipe --block 12M python3 process.py