daemon/update: disable systemd unit before overwriting

When overwriting a systemd unit with new content, we need to account for
the case where the new unit content has a different `[Install]` section.
If it does, then simply overwriting will leak the previous enablement
symlinks and become node state. That's OK most of the time, but this can
cause real issues as we've seen with the combination of openshift#3967 which does
exactly that (changing `[Install]` sections) and openshift#4213 which assumed
that those symlinks were cleaned up. More details on that cocktail in:

https://issues.redhat.com/browse/OCPBUGS-33694?focusedId=24917003#comment-24917003

Fix this by always checking if the unit is currently enabled, and if so,
running `systemctl disable` *before* overwriting its contents. The unit
will then be re-enabled (or not) based on the MachineConfig.

pkg/daemon/file_writers.go

@@ -297,6 +297,18 @@ func writeUnit(u ign3types.Unit, systemdRoot string, isCoreOSVariant bool) error
 				return err
 			}
 		}
+		// If the unit is currently enabled, disable it before overwriting since we might be
+		// changing its WantedBy= or RequiredBy= directive (see OCPBUGS-33694). Later code will
+		// re-enable the new unit as directed by the MachineConfig.
+		cmd := exec.Command("systemctl", "is-enabled", u.Name)
+		out, _ := cmd.CombinedOutput()
+		if cmd.ProcessState.ExitCode() == 0 && strings.TrimSpace(string(out)) == "enabled" {
+			klog.V(2).Infof("Disabling systemd unit %s before re-writing it", u.Name)
+			disable_out, err := exec.Command("systemctl", "disable", u.Name).CombinedOutput()
+			if err != nil {
+				return fmt.Errorf("disabling %s failed: %w (output: %s)", u.Name, err, string(disable_out))
+			}
+		}
 		if err := writeFileAtomicallyWithDefaults(fpath, []byte(*u.Contents)); err != nil {
 			return fmt.Errorf("failed to write systemd unit %q: %w", u.Name, err)
 		}