[API] Add flag to explicitly preserve null values in merge

api/kptfile/v1/types.go

@@ -162,6 +162,11 @@ type Upstream struct {
 
 	// UpdateStrategy declares how a package will be updated from upstream.
 	UpdateStrategy UpdateStrategyType `yaml:"updateStrategy,omitempty" json:"updateStrategy,omitempty"`
+
+	// PreserveExplicitNull, when true, keeps a field explicitly nulled in the
+	// local package during resource-merge instead of deleting it. Only applies
+	// to the resource-merge update strategy.
+	PreserveExplicitNull bool `yaml:"preserveExplicitNull,omitempty" json:"preserveExplicitNull,omitempty"`
 }
 
 // Git is the user-specified locator for a package on Git.

documentation/content/en/reference/schema/kptfile/kptfile.json

@@ -525,6 +525,11 @@
         },
         "updateStrategy": {
           "$ref": "#/definitions/UpdateStrategyType"
+        },
+        "preserveExplicitNull": {
+          "description": "PreserveExplicitNull, when true, keeps a field explicitly nulled in the local package during resource-merge instead of deleting it. Only applies to the resource-merge update strategy.",
+          "type": "boolean",
+          "x-go-name": "PreserveExplicitNull"
         }
       },
       "x-go-package": "github.com/kptdev/kpt/pkg/api/kptfile/v1"

documentation/content/en/reference/schema/kptfile/kptfile.yaml

@@ -456,6 +456,10 @@ definitions:
                 $ref: '#/definitions/OriginType'
             updateStrategy:
                 $ref: '#/definitions/UpdateStrategyType'
+            preserveExplicitNull:
+                description: PreserveExplicitNull, when true, keeps a field explicitly nulled in the local package during resource-merge instead of deleting it. Only applies to the resource-merge update strategy.
+                type: boolean
+                x-go-name: PreserveExplicitNull
         title: Upstream is a user-specified upstream locator for a package.
         type: object
         x-go-package: github.com/kptdev/kpt/pkg/api/kptfile/v1