How to detect when ssh connection (over a QProcess) has finished?

Question

I am running an ssh tunnel from an application using a QProcess:

QProcess* process = new QProcess();
process->start("ssh", QStringList()<<"-L"<<"27017:localhost:27017"<<"example.com");

So far it works great, the only problem being that there is no way for me to see when the port has actually been created.

When I run the command on a shell, it takes about 10 seconds to connect to the remote host after which the forwarded port is ready for usage. How do I detect it from my application?

EDIT:

As suggested by vahancho, I used the fact that post-connection there is some output on the terminal that can be used to detect that the connection has succeeded. However, there is a line which is run instantly after launch Pseudo-terminal will not be allocated because stdin is not a terminal, which probably would give a false alarm. The correct output is available in the second signal, emitted a bit later (which is a true indicator of the port having being opened). To get rid of the first message, I am now running ssh using ssh -t -t to force an stdin allocation.

So, the only question left is, can anyone help me without any concerns in this approach?

Answer 1

So, the only question left is, can anyone help me without any concerns in this approach?

This is not a stable and robust solution, unfortunately. It is similarly a broken concept to handling git outputs rather than using an actual library. The main problem is that these softwares do not have any guarantee for output compatibility, rightfully.

Just imagine that what happens if they have an unclear text, a typo, et all, unnoticed. They inherently need to fix the output respectively, and all the applications relying on the output would abruptly break.

This is also the reason behind working on dedicated libraries giving access to the functionality for reuse rather than working with the user facing output directly. In case of git, this means the libgit2 library, for instance.

Qt does not have an ssh mechanism in place by default like you can have such libraries in python, e.g. paramiko.

I would suggest to establish a way in your code by using libssh or libssh2 as you also noted yourself in the comment. I can understand the inconvenience that is not a truly Qt'ish way as of now, but at this point Qt cannot provide anything more robust without third-party.

That being said, it would be nice to see a similar add-on library in the Qt Project for the future, but this may not be happen any soon. If you write your software with proper design in mind, you will be able to switch to such a library withour major issues once someone stands up to maintain such an additional library to Qt or elsewhere.

Answer 2

I had the same problem, but in my case ssh do not output anything - so I couldn't just wait for output. I'm also using ssh to setupt tunnel, so I used QTcpSocket:

program = "ssh";
arguments << m_host << "-N" << "-L" << QString("3306:%1:3306").arg(m_host);
connect(tunnelProcess, &QProcess::started, this, &Database::waitForTunnel);
tunnelProcess->start(program, arguments);

waitForTunnel() slot:

QTcpSocket sock;
sock.connectToHost("127.0.0.1", 3306);
if(sock.waitForConnected(100000))
{
     sock.disconnectFromHost();
     openDatabaseConnection();
}
else
    qDebug() << "timeout";

I hope this will help future people finding this question ;)