nixos/shells: Avoid overriding the environment for other child shells

nixos/modules/config/shells-environment.nix

@@ -162,15 +162,24 @@ in
         /bin/sh
       '';
 
+    # For resetting environment with `. /etc/set-environment` when needed
+    # and discoverability (see motivation of #30418).
+    environment.etc."set-environment".source = config.system.build.setEnvironment;
+
     system.build.setEnvironment = pkgs.writeText "set-environment"
-       ''
-         ${exportedEnvVars}
+      ''
+        # DO NOT EDIT -- this file has been generated automatically.
+
+        # Prevent this file from being sourced by child shells.
+        export __NIXOS_SET_ENVIRONMENT_DONE=1
 
-         ${cfg.extraInit}
+        ${exportedEnvVars}
 
-         # ~/bin if it exists overrides other bin directories.
-         export PATH="$HOME/bin:$PATH"
-       '';
+        ${cfg.extraInit}
+
+        # ~/bin if it exists overrides other bin directories.
+        export PATH="$HOME/bin:$PATH"
+      '';
 
     system.activationScripts.binsh = stringAfter [ "stdio" ]
       ''

nixos/modules/programs/bash/bash.nix

@@ -126,7 +126,9 @@ in
     programs.bash = {
 
       shellInit = ''
-        ${config.system.build.setEnvironment.text}
+        if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]; then
+            . ${config.system.build.setEnvironment}
+        fi
 
         ${cfge.shellInit}
       '';
@@ -166,11 +168,11 @@ in
 
         # Read system-wide modifications.
         if test -f /etc/profile.local; then
-          . /etc/profile.local
+            . /etc/profile.local
         fi
 
         if [ -n "''${BASH_VERSION:-}" ]; then
-          . /etc/bashrc
+            . /etc/bashrc
         fi
       '';
 
@@ -191,12 +193,12 @@ in
 
         # We are not always an interactive shell.
         if [ -n "$PS1" ]; then
-          ${cfg.interactiveShellInit}
+            ${cfg.interactiveShellInit}
         fi
 
         # Read system-wide modifications.
         if test -f /etc/bashrc.local; then
-          . /etc/bashrc.local
+            . /etc/bashrc.local
         fi
       '';
 

nixos/modules/programs/fish.nix

@@ -27,7 +27,7 @@ in
         '';
         type = types.bool;
       };
-      
+
       vendor.config.enable = mkOption {
         type = types.bool;
         default = true;
@@ -43,7 +43,7 @@ in
           Whether fish should use completion files provided by other packages.
         '';
       };
-      
+
       vendor.functions.enable = mkOption {
         type = types.bool;
         default = true;
@@ -107,9 +107,11 @@ in
       # This happens before $__fish_datadir/config.fish sets fish_function_path, so it is currently
       # unset. We set it and then completely erase it, leaving its configuration to $__fish_datadir/config.fish
       set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $__fish_datadir/functions
-      
+
       # source the NixOS environment config
-      fenv source ${config.system.build.setEnvironment}
+      if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]
+          fenv source ${config.system.build.setEnvironment}
+      end
 
       # clear fish_function_path so that it will be correctly set when we return to $__fish_datadir/config.fish
       set -e fish_function_path
@@ -123,7 +125,7 @@ in
         set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
         fenv source /etc/fish/foreign-env/shellInit > /dev/null
         set -e fish_function_path[1]
-        
+
         ${cfg.shellInit}
 
         # and leave a note so we don't source this config section again from
@@ -137,7 +139,7 @@ in
         set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
         fenv source /etc/fish/foreign-env/loginShellInit > /dev/null
         set -e fish_function_path[1]
-        
+
         ${cfg.loginShellInit}
 
         # and leave a note so we don't source this config section again from
@@ -149,12 +151,11 @@ in
       status --is-interactive; and not set -q __fish_nixos_interactive_config_sourced
       and begin
         ${fishAliases}
-
 
         set fish_function_path ${pkgs.fish-foreign-env}/share/fish-foreign-env/functions $fish_function_path
         fenv source /etc/fish/foreign-env/interactiveShellInit > /dev/null
         set -e fish_function_path[1]
-        
+
         ${cfg.promptInit}
         ${cfg.interactiveShellInit}
 
@@ -170,7 +171,7 @@ in
       ++ optional cfg.vendor.config.enable "/share/fish/vendor_conf.d"
       ++ optional cfg.vendor.completions.enable "/share/fish/vendor_completions.d"
       ++ optional cfg.vendor.functions.enable "/share/fish/vendor_functions.d";
-    
+
     environment.systemPackages = [ pkgs.fish ];
 
     environment.shells = [

nixos/modules/programs/zsh/zsh.nix

@@ -70,7 +70,7 @@ in
       promptInit = mkOption {
         default = ''
           if [ "$TERM" != dumb ]; then
-            autoload -U promptinit && promptinit && prompt walters
+              autoload -U promptinit && promptinit && prompt walters
           fi
         '';
         description = ''
@@ -103,15 +103,17 @@ in
         if [ -n "$__ETC_ZSHENV_SOURCED" ]; then return; fi
         export __ETC_ZSHENV_SOURCED=1
 
-        ${config.system.build.setEnvironment.text}
+        if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]; then
+            . ${config.system.build.setEnvironment}
+        fi
 
         ${cfge.shellInit}
 
         ${cfg.shellInit}
 
         # Read system-wide modifications.
         if test -f /etc/zshenv.local; then
-          . /etc/zshenv.local
+            . /etc/zshenv.local
         fi
       '';
 
@@ -130,7 +132,7 @@ in
 
         # Read system-wide modifications.
         if test -f /etc/zprofile.local; then
-          . /etc/zprofile.local
+            . /etc/zprofile.local
         fi
       '';
 
@@ -156,7 +158,7 @@ in
 
         # Tell zsh how to find installed completions
         for p in ''${(z)NIX_PROFILES}; do
-          fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions)
+            fpath+=($p/share/zsh/site-functions $p/share/zsh/$ZSH_VERSION/functions $p/share/zsh/vendor-completions)
         done
 
         ${optionalString cfg.enableCompletion "autoload -U compinit && compinit"}
@@ -171,7 +173,7 @@ in
 
         # Read system-wide modifications.
         if test -f /etc/zshrc.local; then
-          . /etc/zshrc.local
+            . /etc/zshrc.local
         fi
       '';