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Debugging Remote REPL SSH Tunnels

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Constantine Vetoshev edited this page Aug 18, 2021 · 3 revisions

Debugging Remote REPL SSH Tunnels

Sometimes, remote REPL sessions fail to start. This wiki page collects some ideas about how to go about debugging this problem.

Use a netcat client instead of Emacs

One way to understand what may have gone wrong is to get Emacs out of the picture and understand what happens with the SSH tunnel. To do this, use netcat (nc) to send commands from the local machine (client) to the Snail server on the cluster. (You may have to install netcat from your OS package manager.)

  1. Clean up all stray julia instances on the remote host.

  2. Shut down all SSH connections from your local machine to the remote host.

  3. Copy JuliaSnail.jl, Project.toml, and Manifest.toml from the Snail package (or directly from GitHub) to some directory on your remote host.

  4. Start the ssh tunnel: ssh -t -L 10069:localhost:10099 remotehost /path/to/julia/binary -L /path/to/JuliaSnail.jl/on/remote/host

  5. When the Julia REPL pops up, run JuliaSnail.start(10099) and wait for the prompt.

  6. From your local machine, run this:

echo '(reqid = "abcd1234", ns = [:Main], code = "println(\"hello world\")")' | nc localhost 10069

This should print hello world to your Julia REPL and error out with a message like IOError: stream is closed or unusable.

If the Snail server fails to start on the remote host with an error like EADDRINUSE, then replace 10099 with a different port number in both the SSH tunnel call and the JuliaSnail.start call. If that still does not work, something prevents you from opening up server sockets on the remote host, and you should ask your system administration staff for an explanation.

If the Snail server starts but there is no output in the Julia REPL from the nc call, your tunnel is not being set up correctly. Maybe it’s your local machine, maybe it’s a firewall, maybe it’s configuration, and maybe it’s something else.

If everything works without Emacs, but does not work inside Emacs, then maybe your Snail installation is broken. Blow it away completely (delete from disk), reinstall from MELPA or GitHub, and restart Emacs.

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