nix-channel: do not set empty nix-path when disabling channels

An empty nix-path in nix.conf will disable NIX_PATH environment variable
entirely, which is not necessarily implied by users who want to disable
nix channels. NIX_PATH also has some usages in tools like nixos-rebuild
or just as user aliases.

That change is surprising and debatable, and also caused breakages in
nixpkgs-review and user configs.

See:
- https://github.com/NixOS/nixpkgs/pull/242098/files#r1269891427
- Mic92/nixpkgs-review#343
- NixOS/nix#10998

Co-authored-by: oxalica <[email protected]>
(cherry picked from commit 1e6acab)

nixos/modules/config/nix-channel.nix

@@ -12,6 +12,7 @@ let
     mkDefault
     mkIf
     mkOption
+    stringAfter
     types
     ;
 
@@ -94,10 +95,29 @@ in
       NIX_PATH = cfg.nixPath;
     };
 
-    nix.settings.nix-path = mkIf (! cfg.channel.enable) (mkDefault "");
-
     systemd.tmpfiles.rules = lib.mkIf cfg.channel.enable [
       ''f /root/.nix-channels - - - - ${config.system.defaultChannel} nixos\n''
     ];
+
+    system.activationScripts.no-nix-channel = mkIf (!cfg.channel.enable)
+      (stringAfter [ "etc" "users" ] ''
+        if [ -e "/root/.nix-defexpr/channels" ]; then
+            echo "WARNING: /root/.nix-defexpr/channels exists, but channels have been disabled." 1>&2
+            echo "Due to https://github.com/NixOS/nix/issues/9574, Nix may still use these channels when NIX_PATH is unset." 1>&2
+            echo "Delete the above directory to prevent this." 1>&2
+        fi
+        if [ -e "/nix/var/nix/profiles/per-user/root/channels" ]; then
+            echo "WARNING: /nix/var/nix/profiles/per-user/root/channels exists, but channels have been disabled." 1>&2
+            echo "Due to https://github.com/NixOS/nix/issues/9574, Nix may still use these channels when NIX_PATH is unset." 1>&2
+            echo "Delete the above directory to prevent this." 1>&2
+        fi
+        getent passwd | while IFS=: read -r _ _ _ _ _ home _ ; do
+            if [ -n  "$home" -a -e "$home/.nix-defexpr/channels" ]; then
+                echo "WARNING: $home/.nix-defexpr/channels exists, but channels have been disabled." 1>&2
+                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 to prevent this." 1>&2
+            fi
+        done
+      '');
   };
 }