Detect ID conflicts in namespace transformer

plugin/builtin/NamespaceTransformer.go

@@ -2,6 +2,8 @@
 package builtin
 
 import (
+	"fmt"
+
 	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 	"sigs.k8s.io/kustomize/v3/pkg/resid"
 	"sigs.k8s.io/kustomize/v3/pkg/resmap"
@@ -51,6 +53,11 @@ func (p *NamespaceTransformerPlugin) Transform(m resmap.ResMap) error {
 				return err
 			}
 		}
+
+		matches := m.GetMatchingResourcesByCurrentId(r.CurId().Equals)
+		if len(matches) != 1 {
+			return fmt.Errorf("namespace tranformation produces ID conflict: %#v", matches)
+		}
 	}
 	return nil
 }

plugin/builtin/namespacetransformer/NamespaceTransformer.go

@@ -5,6 +5,8 @@
 package main
 
 import (
+	"fmt"
+
 	"sigs.k8s.io/kustomize/v3/pkg/ifc"
 	"sigs.k8s.io/kustomize/v3/pkg/resid"
 	"sigs.k8s.io/kustomize/v3/pkg/resmap"
@@ -52,6 +54,11 @@ func (p *plugin) Transform(m resmap.ResMap) error {
 				return err
 			}
 		}
+
+		matches := m.GetMatchingResourcesByCurrentId(r.CurId().Equals)
+		if len(matches) != 1 {
+			return fmt.Errorf("namespace tranformation produces ID conflict: %#v", matches)
+		}
 	}
 	return nil
 }

plugin/builtin/namespacetransformer/NamespaceTransformer_test.go

@@ -4,9 +4,10 @@
 package main_test
 
 import (
+	"strings"
 	"testing"
 
-	"sigs.k8s.io/kustomize/v3/pkg/kusttest"
+	kusttest_test "sigs.k8s.io/kustomize/v3/pkg/kusttest"
 	plugins_test "sigs.k8s.io/kustomize/v3/pkg/plugins/test"
 )
 
@@ -238,3 +239,43 @@ fieldSpecs:
 
 	th.AssertActualEqualsExpected(rm, noChangeExpected)
 }
+
+func TestNamespaceTransformerObjectConflict(t *testing.T) {
+	tc := plugins_test.NewEnvForTest(t).Set()
+	defer tc.Reset()
+
+	tc.BuildGoPlugin(
+		"builtin", "", "NamespaceTransformer")
+
+	th := kusttest_test.NewKustTestPluginHarness(t, "/app")
+
+	err := th.ErrorFromLoadAndRunTransformer(`
+apiVersion: builtin
+kind: NamespaceTransformer
+metadata:
+  name: notImportantHere
+  namespace: test
+fieldSpecs:
+- path: metadata/namespace
+  create: true
+`, `
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+  namespace: foo
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: cm
+  namespace: bar
+`)
+
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "ID conflict") {
+		t.Fatalf("unexpected error: %s", err.Error())
+	}
+}