Fix a panic when doing variable replacement.

This fixes an issue where Kustomize expects a []string, but gets a slice
of something else instead, which causing a runtime panic.

This fixes 2241

api/internal/accumulator/refvartransformer.go

@@ -40,7 +40,11 @@ func (rv *refVarTransformer) replaceVars(in interface{}) (interface{}, error) {
 	case []interface{}:
 		var xs []interface{}
 		for _, a := range in.([]interface{}) {
-			xs = append(xs, expansion2.Expand(a.(string), rv.mappingFunc))
+			x, ok := a.(string)
+			if !ok {
+				return nil, fmt.Errorf("expected array of strings, found %v", in)
+			}
+			xs = append(xs, expansion2.Expand(x, rv.mappingFunc))
 		}
 		return xs, nil
 	case map[string]interface{}:
@@ -49,7 +53,7 @@ func (rv *refVarTransformer) replaceVars(in interface{}) (interface{}, error) {
 		for k, v := range inMap {
 			s, ok := v.(string)
 			if !ok {
-				// This field not contain a $(VAR) since it is not
+				// This field can not contain a $(VAR) since it is not
 				// of string type. For instance .spec.replicas: 3 in
 				// a Deployment object
 				xs[k] = v
@@ -64,7 +68,7 @@ func (rv *refVarTransformer) replaceVars(in interface{}) (interface{}, error) {
 	case interface{}:
 		s, ok := in.(string)
 		if !ok {
-			// This field not contain a $(VAR) since it is not of string type.
+			// This field can not contain a $(VAR) since it is not of string type.
 			return in, nil
 		}
 		// This field can potentially contain a $(VAR) since it is

api/internal/accumulator/refvartransformer_test.go

@@ -29,6 +29,7 @@ func TestRefVarTransformer(t *testing.T) {
 		description string
 		given       given
 		expected    expected
+		errMessage  string
 	}{
 		{
 			description: "var replacement in map[string]",
@@ -111,6 +112,27 @@ func TestRefVarTransformer(t *testing.T) {
 				unused: []string{"BAR"},
 			},
 		},
+		{
+			description: "var replacement panic in map[string]",
+			given: given{
+				varMap: map[string]interface{}{},
+				fs: []types.FieldSpec{
+					{Gvk: resid.Gvk{Version: "v1", Kind: "ConfigMap"}, Path: "data/slice"},
+				},
+				res: resmaptest_test.NewRmBuilder(
+					t, resource.NewFactory(kunstruct.NewKunstructuredFactoryImpl())).
+					Add(map[string]interface{}{
+						"apiVersion": "v1",
+						"kind":       "ConfigMap",
+						"metadata": map[string]interface{}{
+							"name": "cm1",
+						},
+						"data": map[string]interface{}{
+							"slice": []interface{}{5}, // noticeably *not* a []string
+						}}).ResMap(),
+			},
+			errMessage: "expected array of strings, found [5]",
+		},
 	}
 
 	for _, tc := range testCases {
@@ -122,6 +144,13 @@ func TestRefVarTransformer(t *testing.T) {
 			err := tr.Transform(tc.given.res)
 
 			// assert
+			if tc.errMessage != "" {
+				if err == nil {
+					t.Fatalf("missing expected error %v", tc.errMessage)
+				} else if err.Error() != tc.errMessage {
+					t.Fatalf("actual error doesn't match expected error: \nACTUAL: %v\nEXPECTED: %v", err.Error(), tc.errMessage)
+				}
+			} else {
 				if err != nil {
 					t.Errorf("unexpected error: %v", err)
 				}
@@ -131,7 +160,7 @@ func TestRefVarTransformer(t *testing.T) {
 					err = e.ErrorIfNotEqualLists(a)
 					t.Fatalf("actual doesn't match expected: \nACTUAL:\n%v\nEXPECTED:\n%v\nERR: %v", a, e, err)
 				}
-
+			}
 		})
 	}
 }