"fileno()" for Motor's socket wrapper is not implemented for asyncio:

https://github.com/mongodb/motor/blob/bf9c961dba0dc4a7f1b26bcb9f72014631237534/motor/frameworks/asyncio/__init__.py#L265-L265

I believe this means a socket is always replaced when it's used after more than a second of idleness, since _closed() tries to call "select" on the Motor socket wrapper and closes it if "select" throws any exception, in this case an AssertionError.