nixos/switch-to-configuration: restart changed socket units

Previously, socket units wouldn't be restarted if they were
changed. To restart the socket, the service the socket is attached
to needs to be stopped first before the socket can be restarted.

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

@@ -214,7 +214,17 @@ while (my ($unit, $state) = each %{$activePrev}) {
                 # Reload the changed mount unit to force a remount.
                 $unitsToReload{$unit} = 1;
                 recordUnit($reloadListFile, $unit);
-            } elsif ($unit =~ /\.socket$/ || $unit =~ /\.path$/ || $unit =~ /\.slice$/) {
+            } elsif ($unit =~ /\.socket$/) {
+                my $unitInfo = parseUnit($newUnitFile);
+                # If a socket unit has been changed, the corresponding
+                # service unit has to be stopped before the socket can
+                # be restarted. The service will be started again on demand.
+                my $serviceUnit = $unitInfo->{'Unit'} // "$baseName.service";
+                $unitsToStop{$serviceUnit} = 1;
+                $unitsToStop{$unit} = 1;
+                $unitsToStart{$unit} = 1;
+                recordUnit($startListFile, $unit);
+            } elsif ($unit =~ /\.path$/ || $unit =~ /\.slice$/) {
                 # FIXME: do something?
             } else {
                 my $unitInfo = parseUnit($newUnitFile);