Graceful shutdown of virtual client doesn't flush remaing packets to real client

[aramperes]

In some cases, graceful shutdown by the virtual client socket will abort sending data back to the real client, which causes incomplete TCP exchanges with the real client. It only seems to happen on random occasions and with large HTTP requests; but I will need to investigate some more.

[aramperes]

So this seems to be because I'm running onetun behind ingress-nginx, which keeps a persistent connection to onetun.

After an HTTP request completes:

• The peer closes the connection (FIN)
• The virtual client's state changes to ESTABLISHED->CLOSE-WAIT
• The virtual interface ACKs the FIN

... but the virtual interface and client connection stay alive. On subsequent requests from ingress-nginx, onetun reuses the same virtual interface without reopening the connection and sends the data without doing the handshake. The peer then returns RST.

[aramperes]

Handling the CLOSE-WAIT state by closing the connection once the client is done sending data seems like a good fix. The only potential improvement would be to send FIN instead of RST in those cases.