title: libssh2 session cleanup without blocking?
body: <p>My app uses libssh2 to communicate over SSH, and generally works fine.  One problem I have is when the remote host dies unexpectedly -- the remote host in this case is an embedded device that can lose power at any time, so this isn't uncommon.</p>
  
  <p>When that happens, my app detects that the remote computer has stopped responding to pings, and tears down the local end of the SSH connection like this:</p>
  
  <pre><code>void SSHSession :: CleanupSession()
  {
     if (_uploadFileChannel)
     {
        libssh2_channel_free(_uploadFileChannel);
        _uploadFileChannel = NULL;
     }
  
     if (_sendCommandsChannel)
     {
        libssh2_channel_free(_sendCommandsChannel);
        _sendCommandsChannel = NULL;
     }
  
     if (_session)
     {
        libssh2_session_disconnect(_session, "bye bye");
        libssh2_session_free(_session);
        _session = NULL;
     }
  }
  </code></pre>
  
  <p>Pretty straightforward, but the problem is that the libssh2_channel_free() calls can block for a long time waiting for the remote end to respond to the "I'm going away now" message, which it will never do because it's powered off... but in the meantime, my app is frozen (blocked in the cleanup-routine), which isn't good.</p>
  
  <p>Is there any way (short of hacking libssh2) to avoid this?  I'd like to just tear down the local SSH data structures, and never block during this tear-down.  (I suppose I could simply leak the SSH session memory, or delegate it to a different thread, but those seem like ugly hacks rather than proper solutions)</p>