My correspondent on the new authentication-plugin feature reports that
their plugin is not reliably receiving the final PLUGIN_AUTH_SUCCESS
message on Windows. I _think_ this is because the whole userauth layer
is being dismissed, leading to sk_close() of the Socket talking to the
plugin, before the data has actually been written to the outgoing
pipe.
This should fix it: track the Socket's backlog, and immediately after
sending that message, wait until we receive a notification that the
backlog has decreased to size 0. That stops us from terminating the
userauth layer until the message has left our process.
1d75ad4c93