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Connection._send_bytes() has a comment about broken pipes:
 def _send_bytes(self, buf):
 # For wire compatibility with 3.2 and lower
 n = len(buf)
 self._send(struct.pack("=i", len(buf)))
 # The condition is necessary to avoid "broken pipe" errors
 # when sending a 0-length buffer if the other end closed the pipe.
 if n > 0:
 self._send(buf)
But the OSError(32, "Broken pipe") occurs on sending the buffer size (a chunk of 4 bytes: self._send(struct.pack("=i", len(buf)))), not on sending the buffer content.
See also maybe the (closed) issue #9205: Parent process hanging in multiprocessing if children terminate unexpectedly