Set field manager for kubectl diff --server-side.

Bonus: check diff only dry-runs without persisting.

Co-authored-by: Takahiro HATORI <tahatori@zlab.co.jp>

Kubernetes-commit: bacc2c49605f608d3f919109e0c37ebf6e1d7036

pkg/cmd/diff/diff.go

@@ -57,7 +57,7 @@ var (
 		KUBECTL_EXTERNAL_DIFF environment variable can be used to select your own
 		diff command. By default, the "diff" command available in your path will be
 		run with "-u" (unified diff) and "-N" (treat absent files as empty) options.
-		
+
 		Exit status:
 		 0
 		No differences were found.
@@ -92,6 +92,7 @@ type DiffOptions struct {
 	FilenameOptions resource.FilenameOptions
 
 	ServerSideApply bool
+	FieldManager    string
 	ForceConflicts  bool
 
 	OpenAPISchema    openapi.Resources
@@ -296,6 +297,7 @@ type InfoObject struct {
 	OpenAPI         openapi.Resources
 	Force           bool
 	ServerSideApply bool
+	FieldManager    string
 	ForceConflicts  bool
 	genericclioptions.IOStreams
 }
@@ -316,8 +318,9 @@ func (obj InfoObject) Merged() (runtime.Object, error) {
 			return nil, err
 		}
 		options := metav1.PatchOptions{
-			Force:  &obj.ForceConflicts,
-			DryRun: []string{metav1.DryRunAll},
+			Force:        &obj.ForceConflicts,
+			FieldManager: obj.FieldManager,
+			DryRun:       []string{metav1.DryRunAll},
 		}
 		return resource.NewHelper(obj.Info.Client, obj.Info.Mapping).Patch(
 			obj.Info.Namespace,
@@ -441,6 +444,7 @@ func (o *DiffOptions) Complete(f cmdutil.Factory, cmd *cobra.Command) error {
 	}
 
 	o.ServerSideApply = cmdutil.GetServerSideApplyFlag(cmd)
+	o.FieldManager = cmdutil.GetFieldManagerFlag(cmd)
 	o.ForceConflicts = cmdutil.GetForceConflictsFlag(cmd)
 	if o.ForceConflicts && !o.ServerSideApply {
 		return fmt.Errorf("--force-conflicts only works with --server-side")
@@ -529,6 +533,7 @@ func (o *DiffOptions) Run() error {
 				OpenAPI:         o.OpenAPISchema,
 				Force:           force,
 				ServerSideApply: o.ServerSideApply,
+				FieldManager:    o.FieldManager,
 				ForceConflicts:  o.ForceConflicts,
 				IOStreams:       o.Diff.IOStreams,
 			}