how could an erlang port's port info change to undefined?
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I get a erlang server, and many clients use tcp to connect to the server.

After some minutes, use erlang:ports() and erlang:port_info(), I can find some port's port_info is undefined, and the port is closed.I can't find anyway to remove it from ports.

So why did it happen, and how could I remove these undefined port from erlang ports?