nixos/nix-channel: move stale-channel warning to preSwitchChecks (#527658)

nixos/modules/config/nix-channel.nix

@@ -11,7 +11,6 @@ let
   inherit (lib)
     mkIf
     mkOption
-    stringAfter
     types
     ;
 
@@ -98,8 +97,8 @@ in
       ''f /root/.nix-channels - - - - ${config.system.defaultChannel} nixos\n''
     ];
 
-    system.activationScripts.no-nix-channel = mkIf (!cfg.channel.enable) (
-      stringAfter [ "etc" "users" ] (builtins.readFile ./nix-channel/activation-check.sh)
+    system.preSwitchChecks.no-nix-channel = mkIf (!cfg.channel.enable) (
+      builtins.readFile ./nix-channel/pre-switch-check.sh
     );
   };
 }

nixos/modules/config/nix-channel/pre-switch-check.sh

@@ -1,4 +1,6 @@
-# shellcheck shell=bash
+warn() {
+    printf "\033[1;35mwarning:\033[0m %s\n" "$*" >&2
+}
 
 explainChannelWarning=0
 if [[ -e "/root/.nix-defexpr/channels" ]]; then
@@ -11,11 +13,13 @@ if [[ -e "/nix/var/nix/profiles/per-user/root/channels" ]]; then
 fi
 while IFS=: read -r _ _ _ _ _ home _ ; do
     if [[ -n  "$home" && -e "$home/.nix-defexpr/channels" ]]; then
-        warn "$home/.nix-defexpr/channels exists, but channels have been disabled." 1>&2
+        warn "$home/.nix-defexpr/channels exists, but channels have been disabled."
         explainChannelWarning=1
     fi
 done < <(getent passwd)
 if [[ $explainChannelWarning -eq 1 ]]; then
-    echo "Due to https://github.com/NixOS/nix/issues/9574, Nix may still use these channels when NIX_PATH is unset." 1>&2
-    echo "Delete the above directory or directories to prevent this." 1>&2
+    echo "Due to https://github.com/NixOS/nix/issues/9574, Nix may still use these channels when NIX_PATH is unset." >&2
+    echo "Delete the above directory or directories to prevent this." >&2
 fi
+# This check is informational only and must never block a switch.
+true

nixos/modules/config/nix-channel/test.nix

@@ -1,20 +0,0 @@
-# Run:
-#   nix-build -A nixosTests.nix-channel
-{ lib, testers }:
-let
-  inherit (lib) fileset;
-
-  runShellcheck = testers.shellcheck {
-    name = "activation-check";
-    src = fileset.toSource {
-      root = ./.;
-      fileset = fileset.unions [
-        ./activation-check.sh
-      ];
-    };
-  };
-
-in
-lib.recurseIntoAttrs {
-  inherit runShellcheck;
-}

nixos/tests/all-tests.nix

@@ -1122,7 +1122,6 @@ in
   nimdow = runTest ./nimdow.nix;
   nipap = runTest ./web-apps/nipap.nix;
   nitter = runTest ./nitter.nix;
-  nix-channel = pkgs.callPackage ../modules/config/nix-channel/test.nix { };
   nix-config = runTest ./nix-config.nix;
   nix-daemon-firewall = runTest ./nix-daemon-firewall.nix;
   nix-daemon-unprivileged = runTest ./nix-daemon-unprivileged.nix;