tmux: use idempotent new-session to avoid duplicate sessions

[lovesegfault]

Description

When both system-level (e.g., NixOS programs.tmux) and home-manager
tmux configs set newSession = true, tmux would create two sessions
on startup since both /etc/tmux.conf and ~/.config/tmux/tmux.conf
contain the new-session command.

Change new-session to new-session -A -s 0, which attaches to session
"0" if it exists, otherwise creates it. This makes the command
idempotent so multiple configs can safely enable newSession.

c.f. NixOS/nixpkgs#468337

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

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  nix-shell --pure tests -A run.all.

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[lovesegfault]

cc. @teto for review maybe?

[khaneliman]

Change seems to make sense to me, just not a tmux user anymore and not familiar enough with how this might affect existing workflows.

[teto]

I just occasionnaly use tmux. Is there a good reason to enable both the system and user modules ? having 2 differents sessions or a single one, I can see reasons for both behaviors.
In doubt, I would refrain from changing it.

Would be nice to have the feedback of a more experienced tmux user. Either way do you want to set yourself as the maintainer for this module ?

NB: Unrelated but the config chains up optionalString which tend to create empty lines that trigger my OCD in the generated config. The way to go IMO is https://github.com/NixOS/nixpkgs/blob/864b51ebb4d1143e83d05594ab8d7669f058d5d3/nixos/modules/services/networking/ssh/sshd.nix#L814

[lovesegfault]

Is there a good reason to enable both the system and user modules ?

It's not really about that, you may be using home-manager on a host with new-session on the /etc/tmux.conf which you cannot control. This change makes it work under any condition.

The NixOS change is just a nice-to-have for correctness.