nixos: reduce pam files rebuilds on updates

Before whenever environment variables changed, pam files had to be
rebuild.

This is expensive since each file needs its own sandbox set up.

nixos/modules/config/system-environment.nix

@@ -65,42 +65,40 @@ in
   };
 
   config = {
+    environment.etc."pam/environment".text = let
+      suffixedVariables =
+        flip mapAttrs cfg.profileRelativeSessionVariables (envVar: suffixes:
+          flip concatMap cfg.profiles (profile:
+            map (suffix: "${profile}${suffix}") suffixes
+          )
+        );
 
-    system.build.pamEnvironment =
-      let
-        suffixedVariables =
-          flip mapAttrs cfg.profileRelativeSessionVariables (envVar: suffixes:
-            flip concatMap cfg.profiles (profile:
-              map (suffix: "${profile}${suffix}") suffixes
-            )
-          );
+      # We're trying to use the same syntax for PAM variables and env variables.
+      # That means we need to map the env variables that people might use to their
+      # equivalent PAM variable.
+      replaceEnvVars = replaceStrings ["$HOME" "$USER"] ["@{HOME}" "@{PAM_USER}"];
 
-        # We're trying to use the same syntax for PAM variables and env variables.
-        # That means we need to map the env variables that people might use to their
-        # equivalent PAM variable.
-        replaceEnvVars = replaceStrings ["$HOME" "$USER"] ["@{HOME}" "@{PAM_USER}"];
+      pamVariable = n: v:
+        ''${n}   DEFAULT="${concatStringsSep ":" (map replaceEnvVars (toList v))}"'';
 
-        pamVariable = n: v:
-          ''${n}   DEFAULT="${concatStringsSep ":" (map replaceEnvVars (toList v))}"'';
-
-        pamVariables =
-          concatStringsSep "\n"
-          (mapAttrsToList pamVariable
-          (zipAttrsWith (n: concatLists)
-            [
-              # Make sure security wrappers are prioritized without polluting
-              # shell environments with an extra entry. Sessions which depend on
-              # pam for its environment will otherwise have eg. broken sudo. In
-              # particular Gnome Shell sometimes fails to source a proper
-              # environment from a shell.
-              { PATH = [ config.security.wrapperDir ]; }
-
-              (mapAttrs (n: toList) cfg.sessionVariables)
-              suffixedVariables
-            ]));
-      in
-        pkgs.writeText "pam-environment" "${pamVariables}\n";
+      pamVariables =
+        concatStringsSep "\n"
+        (mapAttrsToList pamVariable
+        (zipAttrsWith (n: concatLists)
+          [
+            # Make sure security wrappers are prioritized without polluting
+            # shell environments with an extra entry. Sessions which depend on
+            # pam for its environment will otherwise have eg. broken sudo. In
+            # particular Gnome Shell sometimes fails to source a proper
+            # environment from a shell.
+            { PATH = [ config.security.wrapperDir ]; }
 
+            (mapAttrs (n: toList) cfg.sessionVariables)
+            suffixedVariables
+          ]));
+    in ''
+      ${pamVariables}
+    '';
   };
 
 }

nixos/modules/security/pam.nix

@@ -475,7 +475,7 @@ let
 
           # Session management.
           ${optionalString cfg.setEnvironment ''
-            session required pam_env.so conffile=${config.system.build.pamEnvironment} readenv=0
+            session required pam_env.so conffile=/etc/pam/environment readenv=0
           ''}
           session required pam_unix.so
           ${optionalString cfg.setLoginUid

nixos/modules/services/wayland/cage.nix

@@ -82,7 +82,7 @@ in {
       auth    required pam_unix.so nullok
       account required pam_unix.so
       session required pam_unix.so
-      session required pam_env.so conffile=${config.system.build.pamEnvironment} readenv=0
+      session required pam_env.so conffile=/etc/pam/environment readenv=0
       session required ${pkgs.systemd}/lib/security/pam_systemd.so
     '';
 

nixos/modules/services/x11/display-managers/gdm.nix

@@ -309,7 +309,7 @@ in
         password required       pam_deny.so
 
         session  required       pam_succeed_if.so audit quiet_success user = gdm
-        session  required       pam_env.so conffile=${config.system.build.pamEnvironment} readenv=0
+        session  required       pam_env.so conffile=/etc/pam/environment readenv=0
         session  optional       ${pkgs.systemd}/lib/security/pam_systemd.so
         session  optional       pam_keyinit.so force revoke
         session  optional       pam_permit.so

nixos/modules/services/x11/display-managers/lightdm.nix

@@ -284,7 +284,7 @@ in
         password required       pam_deny.so
 
         session  required       pam_succeed_if.so audit quiet_success user = lightdm
-        session  required       pam_env.so conffile=${config.system.build.pamEnvironment} readenv=0
+        session  required       pam_env.so conffile=/etc/pam/environment readenv=0
         session  optional       ${pkgs.systemd}/lib/security/pam_systemd.so
         session  optional       pam_keyinit.so force revoke
         session  optional       pam_permit.so

nixos/modules/services/x11/display-managers/sddm.nix

@@ -229,7 +229,7 @@ in
         password required       pam_deny.so
 
         session  required       pam_succeed_if.so audit quiet_success user = sddm
-        session  required       pam_env.so conffile=${config.system.build.pamEnvironment} readenv=0
+        session  required       pam_env.so conffile=/etc/pam/environment readenv=0
         session  optional       ${pkgs.systemd}/lib/security/pam_systemd.so
         session  optional       pam_keyinit.so force revoke
         session  optional       pam_permit.so