ensure that all tmux session terminate when SSH session ends

Question

First off to anticipate confusion: this is indeed asking the opposite of your run of the mill tmux question.

How can one ensure that if the SSH tunnel dies, no tmux session is leaked at all. When running multiple shells over the same SSH session, terminating the session and the tmux instance cleanly in one terminal will still cause tmux instances in other terminals to leak. This can cause all kinds of trouble if e. g. one forgot to terminate a resource consuming process that was lingering hidden in some backgrounded pane. Not cool.

I’m using tmux almost exclusively as a terminal emulator and have little use if any for its “detach” feature. It’s not uncommon that I have multiple tmux instances in terminals accessing the same machine over a single SSH session. If I want to run a background process, I ask the shell to disown it or run it in systemd. Unfortunately, many systems I’m working with day to day haven’t upgraded to systemd yet and are unlikely to do that in the near future so KillUserProcesses is not an option.

Ideally I’d just invoke tmux with some command line switch that prevents it from running in the background so I can alias that to tmux.

What SSH tunnels?

One OpenSSH connection serving multiple sessions.

What tmux session?

Separate tmux instances, one per SSH session.

What leaks?

Leaks of tmux instances. As in: SSH into a machine. Start another shell session over the same connection. Start a tmux instance in each SSH session. You now have two tmux instances. Stop tmux in one session (<C-d>); then stop the SSH connection (again <C-d>): The other SSH session is closed but the tmux instance running inside it is leaked with all it child processes.

Please describe: (1) what setup you have now,

Stock OpenSSH, stock tmux, some shell. Mostly reverse SSH connections that I cannot reestablish to at will, but that shouldn’t matter.

and (2) what scenario you're trying to prevent.

Processes staying alive in the other tmux session.

Note that those are different boxes being SSH’d into, so the goal is to prevent even a single tmux instance from remaining alive after the connection dies. Also, since I use split panes and backgrounded windows heavily, I really wish for all the child processes to be collected.

Answer 1

When logging with new ssh session you can use something like tmux attach -d || tmux new to attach any existing tmux session or create a new session if no tmux sessions exist. While this won't prevent tmux session from running in background if you ssh connection drops, it will prevent these abandoned sessions from accumulating.

Answer 2

I believe your problem can be avoided entirely by using Mosh.

Mosh supports intermittent connectivity, and only leaks mosh-server instances if you don't disconnect properly (such as forcefully closing the client while disconnected from the server).