Tests rustls's is_handshaking() method.
cargo run
This uses complete_io for looping over reading/writing until the handshake is complete, and as this shows, a client will believe it has stopped the handshaking process even though it needs to write TLS bytes. Also, the server is expecting the bytes.
The code is pretty self explanatory in terms of how this test works, but basically it feels incorrect that the client thinks the handshake is complete when the server does not.
Comment/uncomment the Client Config or Server Config lines to change whether client auth is required. At the time of this commit, lines 92 and 93 can be swapped for the server side, and line 111 for the client.