[SSHD-1307] Make Nio2Session.shutdownOutputStream() asynchronous

[tomaswolf]

Nio2Session.shutdownOutputStream() can be invoked while there are still writes in progress. The output is shut down in TCP/IP port forwarding to propagate an SSH_MSG_CHANNEL_EOF from the channel to whatever is on the other side of the forwarded port; see SSHD-1055. However, writing through the socket is asynchronous, so a channel may get the following sequence of events:

1. receive last SSH_MSG_CHANNEL_DATA
2. Nio2Session.writeBuffer() enqueues the write request
3. SSH_MSG_CHANNEL_DATA handling completes
4. receive SSH_MSG_CHANNEL_EOF
5. call Nio2Session.shutdownOutputStream()

If (5) shuts down the output immediately but the write request from (2) has not been written yet, that write attempt will fail with a ClosedChannelException.

The output stream on the socket should not be shut down immediately but only once already pending writes have been done. This is already the case with the MINA and Netty transport back-ends, which only schedule a shutdown request on their write queue.

Implement the same for the NIO2 transport. A write future with a null buffer signifies a shutdown request; true write requests always have a non-null buffer. Make shutdownOutputStream() only enqueue such a shutdown request on the session's write queue; the output will be shut down once startWriting() pops this shutdown request from the write queue.

[AlexanderAmador]

I am interested in acquiring this fix in a release. I saw a comment that your next release may be 2.10.0. Do you have an ETA on that release ?

[tomaswolf]

Answer is on SSHD-1307, where you had asked the same.