UDP 只读得下半条消息，剩下的能留到下次吗？

"""Real loopback UDP. A short receive buffer does not defer a datagram's tail."""
from contextlib import closing
import errno
import os
import socket


def main():
    checks = 0

    def check(condition, message):
        nonlocal checks
        if not condition:
            raise AssertionError(message)
        checks += 1

    with closing(socket.socket(socket.AF_INET, socket.SOCK_DGRAM)) as receiver:
        with closing(socket.socket(socket.AF_INET, socket.SOCK_DGRAM)) as sender:
            receiver.bind(("127.0.0.1", 0))
            sender.bind(("127.0.0.1", 0))
            receiver.settimeout(3)
            sender.settimeout(3)
            address = receiver.getsockname()
            payload = b"ABCDEFGHIJ"
            check(sender.sendto(payload, address) == len(payload), "first datagram accepted by sendto")
            try:
                first, peer = receiver.recvfrom(4)
            except OSError as error:
                code = getattr(error, "winerror", None) or error.errno
                check(code in (10040, errno.EMSGSIZE), "only message-too-large is expected")
                first_result = f"message_too_large:{code}"
            else:
                check(first == b"ABCD", "truncated-return platforms expose only the prefix")
                first_result = "truncated_prefix:ABCD"
            if os.name == "nt":
                check(first_result == "message_too_large:10040", "Windows recvfrom reports WSAEMSGSIZE")
            else:
                check(first_result == "truncated_prefix:ABCD", "POSIX run must show the observed prefix behavior")

            # Send NEXT only after the first receive has completed.
            sender.sendto(b"NEXT", address)
            following, peer = receiver.recvfrom(32)
            check(following == b"NEXT", "tail EFGHIJ must not appear in a later receive")
            check(peer == sender.getsockname(), "received datagram must come from our sender")

            sender.sendto(payload, address)
            complete, _ = receiver.recvfrom(32)
            check(complete == payload, "a sufficient buffer retains the whole datagram")
            check(sender.sendto(b"", address) == 0, "zero-length UDP payload is valid")
            empty, _ = receiver.recvfrom(32)
            check(empty == b"", "empty payload arrives as a datagram")
            sender.sendto(b"AFTER", address)
            after, _ = receiver.recvfrom(32)
            check(after == b"AFTER", "empty datagram is not a stream EOF")
            print(f"short_buffer={first_result}")
            print(f"next_datagram={following!r}; complete_datagram={complete!r}")
            print(f"empty_datagram={empty!r}; after_empty={after!r}")
    print(f"PASS checks={checks}; sockets_closed=True")


if __name__ == "__main__":
    main()