code: def _send_mustsend(self, retries=3):
      if (
          self._cached_mustsend is not None
          and self._send_mustsend_remaining_tries > 0
      ):
          # If we don't succeed in sending the must-send, we will try again
          # next time.
          if self._send_internal(
              Message(MessageTypes.MUST_SEND, self._cached_mustsend),
              retries,
          ):
              self._cached_mustsend = None
              self._send_mustsend_remaining_tries = 0
              return True
          else:
              self._send_mustsend_remaining_tries -= 1
              if self._send_mustsend_remaining_tries == 0:
                  # Mark as "failed after try"
                  self._send_mustsend_remaining_tries = -1
              return False
  
imports: from __future__ import print_function
  import subprocess
  import fcntl
  import select
  import os
  import sys
  import platform
  from fcntl import F_SETFL
  from os import O_NONBLOCK
  from .sidecar_messages import Message, MessageTypes
  from ..debug import debug
  from metaflow.tracing import inject_tracing_vars
label_desc: Use image node_id 7 for calling the SidecarSubProcess obj's underlying member method code with example usage: obj._send_mustsend(retries) and returns: True, False