nixos/environment: make {sessionV,v}ariables items nullable (#354186)

nixos/modules/config/shells-environment.nix

@@ -34,14 +34,18 @@ in
       description = ''
         A set of environment variables used in the global environment.
         These variables will be set on shell initialisation (e.g. in /etc/profile).
+
         The value of each variable can be either a string or a list of
         strings.  The latter is concatenated, interspersed with colon
         characters.
+
+        Setting a variable to `null` does nothing. You can override a
+        variable set by another module to `null` to unset it.
       '';
-      type = with lib.types; attrsOf (oneOf [ (listOf (oneOf [ int str path ])) int str path ]);
+      type = with lib.types; attrsOf (nullOr (oneOf [ (listOf (oneOf [ int str path ])) int str path ]));
       apply = let
         toStr = v: if lib.isPath v then "${v}" else toString v;
-      in lib.mapAttrs (n: v: if lib.isList v then lib.concatMapStringsSep ":" toStr v else toStr v);
+      in attrs: lib.mapAttrs (n: v: if lib.isList v then lib.concatMapStringsSep ":" toStr v else toStr v) (lib.filterAttrs (n: v: v != null) attrs);
     };
 
     environment.profiles = lib.mkOption {

nixos/modules/config/system-environment.nix

@@ -21,6 +21,9 @@ in
         list of strings. The latter is concatenated, interspersed with
         colon characters.
 
+        Setting a variable to `null` does nothing. You can override a
+        variable set by another module to `null` to unset it.
+
         Note, due to limitations in the PAM format values may not
         contain the `"` character.
 

nixos/tests/env.nix

@@ -4,7 +4,7 @@ import ./make-test-python.nix ({ pkgs, ...} : {
     maintainers = [ nequissimus ];
   };
 
-  nodes.machine = { pkgs, ... }:
+  nodes.machine = { pkgs, lib, ... }: lib.mkMerge [
     {
       boot.kernelPackages = pkgs.linuxPackages;
       environment.etc.plainFile.text = ''
@@ -17,8 +17,15 @@ import ./make-test-python.nix ({ pkgs, ...} : {
       environment.sessionVariables = {
         TERMINFO_DIRS = "/run/current-system/sw/share/terminfo";
         NIXCON = "awesome";
+        SHOULD_NOT_BE_SET = "oops";
       };
-    };
+    }
+    {
+      environment.sessionVariables = {
+        SHOULD_NOT_BE_SET = lib.mkForce null;
+      };
+    }
+  ];
 
   testScript = ''
     machine.succeed('[ -L "/etc/plainFile" ]')
@@ -32,5 +39,6 @@ import ./make-test-python.nix ({ pkgs, ...} : {
         "echo ''${TERMINFO_DIRS}"
     )
     assert "awesome" in machine.succeed("echo ''${NIXCON}")
+    machine.fail("printenv SHOULD_NOT_BE_SET")
   '';
 })