Add a REST hook for post-validation canonicalize

This is a simple hook that only Endpoints uses for now.

pkg/api/rest/create.go

@@ -37,11 +37,15 @@ type RESTCreateStrategy interface {
 	NamespaceScoped() bool
 	// PrepareForCreate is invoked on create before validation to normalize
 	// the object.  For example: remove fields that are not to be persisted,
-	// sort order-insensitive list fields, etc.
+	// sort order-insensitive list fields, etc.  This should not remove fields
+	// whose presence would be considered a validation error.
 	PrepareForCreate(obj runtime.Object)
 	// Validate is invoked after default fields in the object have been filled in before
-	// the object is persisted.
+	// the object is persisted.  This method should not mutate the object.
 	Validate(ctx api.Context, obj runtime.Object) fielderrors.ValidationErrorList
+	// Canonicalize is invoked after validation has succeeded but before the
+	// object has been persisted.  This method may mutate the object.
+	Canonicalize(obj runtime.Object)
 }
 
 // BeforeCreate ensures that common operations for all resources are performed on creation. It only returns
@@ -77,6 +81,8 @@ func BeforeCreate(strategy RESTCreateStrategy, ctx api.Context, obj runtime.Obje
 		return errors.NewInvalid(kind, objectMeta.Name, errs)
 	}
 
+	strategy.Canonicalize(obj)
+
 	return nil
 }