pkg/cli/admin/upgrade: Use PATCH instead of POST for spec updates

ClusterVersion's spec can evolve over time, growing additional
properties like capabilities [1].  With the old POST/Update(...)
approach, that could lead to older oc (who didn't know about the new
properties) naively clearing them when they washed the in-cluster
resource through their local ClusterVersion Go type.  With this
commit, we transition to PATCH, so we can touch just
spec.desiredUpdate, regardless of what else is going on in spec.

[1]: openshift/api@5b82635

Author: wking

pkg/cli/admin/upgrade/upgrade.go

@@ -3,6 +3,7 @@ package upgrade
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -321,10 +322,12 @@ func (o *Options) Run() error {
 			fmt.Fprintf(o.ErrOut, "warning: --allow-upgrade-with-warnings is bypassing: %s", err)
 		}
 
-		cv.Spec.DesiredUpdate = update
-
-		_, err := o.Client.ConfigV1().ClusterVersions().Update(context.TODO(), cv, metav1.UpdateOptions{})
+		updateJSON, err := json.Marshal(update)
 		if err != nil {
+			return fmt.Errorf("marshal ClusterVersion patch: %v", err)
+		}
+		patch := []byte(fmt.Sprintf(`{"spec":{"desiredUpdate": %s}}`, updateJSON))
+		if _, err := o.Client.ConfigV1().ClusterVersions().Patch(context.TODO(), cv.Name, types.MergePatchType, patch, metav1.PatchOptions{}); err != nil {
 			return fmt.Errorf("Unable to upgrade: %v", err)
 		}