tmux: use idempotent new-session to avoid duplicate sessions

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.

modules/programs/tmux.nix

@@ -57,7 +57,11 @@ let
       # We need to set default-shell before calling new-session
       set  -g default-shell "${cfg.shell}"
     ''}
-    ${optionalString cfg.newSession "new-session"}
+    ${optionalString cfg.newSession ''
+      # Use -A to make new-session idempotent: attach if session "0" exists,
+      # otherwise create it. This prevents duplicate sessions when multiple
+      # configs (e.g., system and user) both enable newSession.
+      new-session -A -s 0''}
 
     ${optionalString cfg.reverseSplit ''
       bind -N "Split the pane into two, left and right" v split-window -h

tests/modules/programs/tmux/emacs-with-plugins.conf

@@ -3,7 +3,10 @@ set  -g default-terminal "screen"
 set  -g base-index      0
 setw -g pane-base-index 0
 
-new-session
+# Use -A to make new-session idempotent: attach if session "0" exists,
+# otherwise create it. This prevents duplicate sessions when multiple
+# configs (e.g., system and user) both enable newSession.
+new-session -A -s 0
 
 bind -N "Split the pane into two, left and right" v split-window -h
 bind -N "Split the pane into two, top and bottom" s split-window -v

tests/modules/programs/tmux/vi-all-true.conf

@@ -3,7 +3,10 @@ set  -g default-terminal "screen"
 set  -g base-index      0
 setw -g pane-base-index 0
 
-new-session
+# Use -A to make new-session idempotent: attach if session "0" exists,
+# otherwise create it. This prevents duplicate sessions when multiple
+# configs (e.g., system and user) both enable newSession.
+new-session -A -s 0
 
 bind -N "Split the pane into two, left and right" v split-window -h
 bind -N "Split the pane into two, top and bottom" s split-window -v