Context

receive_message (sync path) is missing the OP_COMPRESSED minimum-length guard that exists in the async PyMongoProtocol.process_header.

PyMongoProtocol.process_header has the correct guard:

if op_code == 2012:
    if length <= 25:
        raise ProtocolError(
            f"Message length ({length!r}) not longer than standard OP_COMPRESSED message header size (25)"
        )

Definition of done

Add the same guard to receive_message before entering the op_code == 2012 branch:

if op_code == 2012:
    if length <= 25:
        raise ProtocolError(
            f"Message length ({length!r}) not longer than standard OP_COMPRESSED message header size (25)"
        )
    ...

Pitfalls

NA