nixos/activation: improve preSwitchChecks (#375961)

nixos/modules/system/activation/pre-switch-check.nix

@@ -1,21 +1,35 @@
-{ lib, pkgs, ... }:
+{
+  lib,
+  config,
+  pkgs,
+  ...
+}:
 let
-  preSwitchCheckScript =
-    set:
-    lib.concatLines (
-      lib.mapAttrsToList (name: text: ''
-        # pre-switch check ${name}
-        (
-          ${text}
-        )
-        if [[ $? != 0 ]]; then
-          echo "Pre-switch check '${name}' failed"
-          exit 1
-        fi
-      '') set
-    );
+  preSwitchCheckScript = lib.concatLines (
+    lib.mapAttrsToList (name: text: ''
+      # pre-switch check ${name}
+      if ! (
+        ${text}
+      ) >&2 ; then
+        echo "Pre-switch check '${name}' failed" >&2
+        exit 1
+      fi
+    '') config.system.preSwitchChecks
+  );
 in
 {
+  options.system.preSwitchChecksScript = lib.mkOption {
+    type = lib.types.pathInStore;
+    internal = true;
+    readOnly = true;
+    default = lib.getExe (
+      pkgs.writeShellApplication {
+        name = "pre-switch-checks";
+        text = preSwitchCheckScript;
+      }
+    );
+  };
+
   options.system.preSwitchChecks = lib.mkOption {
     default = { };
     example = lib.literalExpression ''
@@ -33,12 +47,5 @@ in
     '';
 
     type = lib.types.attrsOf lib.types.str;
-
-    apply =
-      set:
-      set
-      // {
-        script = pkgs.writeShellScript "pre-switch-checks" (preSwitchCheckScript set);
-      };
   };
 }

nixos/modules/system/activation/switchable-system.nix

@@ -61,7 +61,7 @@ in
           --subst-var-by coreutils "${pkgs.coreutils}" \
           --subst-var-by distroId ${lib.escapeShellArg config.system.nixos.distroId} \
           --subst-var-by installBootLoader ${lib.escapeShellArg config.system.build.installBootLoader} \
-          --subst-var-by preSwitchCheck ${lib.escapeShellArg config.system.preSwitchChecks.script} \
+          --subst-var-by preSwitchCheck ${lib.escapeShellArg config.system.preSwitchChecksScript} \
           --subst-var-by localeArchive "${config.i18n.glibcLocales}/lib/locale/locale-archive" \
           --subst-var-by perl "${perlWrapped}" \
           --subst-var-by shell "${pkgs.bash}/bin/sh" \
@@ -94,7 +94,7 @@ in
             --set TOPLEVEL ''${!toplevelVar} \
             --set DISTRO_ID ${lib.escapeShellArg config.system.nixos.distroId} \
             --set INSTALL_BOOTLOADER ${lib.escapeShellArg config.system.build.installBootLoader} \
-            --set PRE_SWITCH_CHECK ${lib.escapeShellArg config.system.preSwitchChecks.script} \
+            --set PRE_SWITCH_CHECK ${lib.escapeShellArg config.system.preSwitchChecksScript} \
             --set LOCALE_ARCHIVE ${config.i18n.glibcLocales}/lib/locale/locale-archive \
             --set SYSTEMD ${config.systemd.package}
         )

nixos/modules/system/activation/top-level.nix

@@ -343,7 +343,7 @@ in
       perl = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp ]);
       # End if legacy environment variables
 
-      preSwitchCheck = config.system.preSwitchChecks.script;
+      preSwitchCheck = config.system.preSwitchChecksScript;
 
       # Not actually used in the builder. `passedChecks` is just here to create
       # the build dependencies. Checks are similar to build dependencies in the

nixos/tests/switch-test.nix

@@ -611,6 +611,10 @@ in {
     other = {
       system.switch.enable = true;
       users.mutableUsers = true;
+      system.preSwitchChecks.succeeds = ''
+        echo this will succeed
+        true
+      '';
       specialisation.failingCheck.configuration.system.preSwitchChecks.failEveryTime = ''
         echo this will fail
         false