Allow exec and function transformers to delete resources

plugin/someteam.example.com/v1/starlarkmixer/StarlarkMixer_test.go

@@ -4,6 +4,7 @@
 package starlarkmixer_test
 
 import (
+	"strings"
 	"testing"
 
 	kusttest_test "sigs.k8s.io/kustomize/api/testutils/kusttest"
@@ -29,6 +30,11 @@ metadata:
   name: some-cm
 data:
   foo: bar
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: delete-me
 `)
 
 	th.AssertActualEqualsExpected(rm, `
@@ -59,3 +65,46 @@ metadata:
   name: net-new
 `)
 }
+
+func TestStarlarkMixerPlugin_duplicate_rejection(t *testing.T) {
+	th := kusttest_test.MakeEnhancedHarness(t)
+	defer th.Reset()
+
+	th.RunTransformerAndCheckError(`
+apiVersion: someteam.example.com/v1
+kind: StarlarkMixer
+metadata:
+  name: mixer-instance
+  annotations:
+    config.kubernetes.io/function: |
+      starlark:
+        path: mixer.star
+`,
+		`apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: some-cm
+data:
+  foo: bar
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: some-cm-copy
+data:
+  foo: bar
+`,
+		func(t *testing.T, err error) {
+			if err == nil {
+				t.Fatal("expected error")
+			}
+			expectedErr := "plugin api: someteam.example.com/v1, " +
+				"kind: StarlarkMixer, " +
+				"name: mixer-instance generated duplicate " +
+				"resource: {\"version\":\"v1\",\"kind\":\"ConfigMap\",\"name\":\"some-cm-copy\"}"
+			if !strings.Contains(err.Error(), expectedErr) {
+				t.Fatalf("unexpected error: %s", err.Error())
+			}
+		},
+	)
+}