Fix nil pointer panic when calculating patches

provider/pkg/resources/patching.go

@@ -23,9 +23,26 @@ func CalcPatch(oldInputs resource.PropertyMap, newInputs resource.PropertyMap, s
 	createOnlyProps := codegen.NewStringSet(spec.CreateOnly...)
 	writeOnlyProps := codegen.NewStringSet(spec.WriteOnly...)
 	mustSendProps := writeOnlyProps.Subtract(createOnlyProps)
+
+	isDiffEmpty := diff == nil
+	if isDiffEmpty {
+		// If there's no diff and no write-only properties to send, we can return an empty patch.
+		if len(mustSendProps) == 0 {
+			return []jsonpatch.JsonPatchOperation{}, nil
+		}
+
+		// Init the diff with empty values so we can safely access it later
+		diff = &resource.ObjectDiff{
+			Adds:    make(resource.PropertyMap),
+			Deletes: make(resource.PropertyMap),
+			Sames:   make(resource.PropertyMap),
+			Updates: make(map[resource.PropertyKey]resource.ValueDiff),
+		}
+	}
+
 	for _, writeOnlyPropName := range mustSendProps.SortedValues() {
 		propKey := resource.PropertyKey(writeOnlyPropName)
-		if _, ok := diff.Sames[propKey]; ok {
+		if _, ok := diff.Sames[propKey]; ok || isDiffEmpty {
 			delete(diff.Sames, propKey)
 			diff.Adds[propKey] = newInputs[propKey]
 		}

provider/pkg/resources/patching_test.go

@@ -63,6 +63,28 @@ func TestCalcPatch(t *testing.T) {
 					}})
 				assert.Empty(t, patch)
 			})
+			t.Run("no diff with must send props", func(t *testing.T) {
+				expected := []jsonpatch.JsonPatchOperation{
+					{Operation: "add", Path: "/Prop1", Value: "1"},
+				}
+				patch := run(t, args{
+					oldInputs: resource.PropertyMap{
+						"prop1": resource.NewStringProperty("1"),
+						"prop2": resource.NewStringProperty("2"),
+					},
+					newInputs: resource.PropertyMap{
+						"prop1": resource.NewStringProperty("1"),
+						"prop2": resource.NewStringProperty("2"),
+					},
+					spec: metadata.CloudAPIResource{
+						Inputs: map[string]schema.PropertySpec{
+							"prop1": {TypeSpec: schema.TypeSpec{Type: "string"}},
+							"prop2": {TypeSpec: schema.TypeSpec{Type: "string"}},
+						},
+						WriteOnly: []string{"prop1"},
+					}})
+				assert.Equal(t, expected, patch)
+			})
 			t.Run("always sends write-only properties", func(t *testing.T) {
 				expected := []jsonpatch.JsonPatchOperation{
 					{Operation: "add", Path: "/Prop1", Value: "1"},