Merge pull request #43682 from primeos/never-stop-system.slice

nixos/switch-to-configuration: Never stop system.slice

nixos/modules/system/activation/switch-to-configuration.pl

···
 
     if (-e $prevUnitFile && ($state->{state} eq "active" || $state->{state} eq "activating")) {
         if (! -e $newUnitFile || abs_path($newUnitFile) eq "/dev/null") {
+            # Ignore (i.e. never stop) these units:
+            if ($unit eq "system.slice") {
+                # TODO: This can be removed a few months after 18.09 is out
+                # (i.e. after everyone switched away from 18.03).
+                # Problem: Restarting (stopping) system.slice would not only
+                # stop X11 but also most system units/services. We obviously
+                # don't want this happening to users when they switch from 18.03
+                # to 18.09 or nixos-unstable.
+                # Reason: The following change in systemd:
+                # https://github.com/systemd/systemd/commit/d8e5a9338278d6602a0c552f01f298771a384798
+                # The commit adds system.slice to the perpetual units, which
+                # means removing the unit file and adding it to the source code.
+                # This is done so that system.slice can't be stopped anymore but
+                # in our case it ironically would cause this script to stop
+                # system.slice because the unit was removed (and an older
+                # systemd version is still running).
+                next;
+            }
             my $unitInfo = parseUnit($prevUnitFile);
             $unitsToStop{$unit} = 1 if boolIsTrue($unitInfo->{'X-StopOnRemoval'} // "yes");
         }