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"""
CancelRequest message

The frontend sends a CancelRequest message to cancel the processing of the
current operation. The cancel request must be sent across a new connection to
the server. The server will process this request and then close the connection.

The cancel request might or might not have any effect. If the cancellation is
effective, the current command will terminate early and return an error message.
If the cancellation fails (e.g. the server has finished processing the command),
then there will be no visible result at all.
"""

from __future__ import print_function, division, absolute_import, annotations

from struct import pack

from ..message import BulkFrontendMessage


class CancelRequest(BulkFrontendMessage):
    message_id = None

    def __init__(self, backend_pid, backend_key):
        BulkFrontendMessage.__init__(self)
        self._backend_pid = backend_pid   # The process ID of the target backend
        self._backend_key = backend_key   # The secret key of the target backend

    def read_bytes(self):
        bytes_ = pack('!3I', 80877102, self._backend_pid, self._backend_key)
        return bytes_
