fix(GetForToken): added support for pointer to interface{}

reflect-based switch doesn't work when the value passed is *any:
the resulting indirection is typed as reflect.Interface and not
the actual underlying type.

* added more stringent checking on nil values (i.e. covers
  interface{}(nil)
* contributes: go-swagger/go-swagger#1898 (pointers to content of a
  x-... swagger extension)

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

Author: fredbi

pointer.go

@@ -110,16 +110,36 @@ func SetForToken(document any, decodedToken string, value any) (any, error) {
 	return document, setSingleImpl(document, value, decodedToken, swag.DefaultJSONNameProvider)
 }
 
+func isNil(input any) bool {
+	if input == nil {
+		return true
+	}
+
+	kind := reflect.TypeOf(input).Kind()
+	switch kind { //nolint:exhaustive
+	case reflect.Ptr, reflect.Map, reflect.Slice, reflect.Chan:
+		return reflect.ValueOf(input).IsNil()
+	default:
+		return false
+	}
+}
+
 func getSingleImpl(node any, decodedToken string, nameProvider *swag.NameProvider) (any, reflect.Kind, error) {
 	rValue := reflect.Indirect(reflect.ValueOf(node))
 	kind := rValue.Kind()
+	if isNil(node) {
+		return nil, kind, fmt.Errorf("nil value has not field %q", decodedToken)
+	}
 
-	if rValue.Type().Implements(jsonPointableType) {
-		r, err := node.(JSONPointable).JSONLookup(decodedToken)
+	switch typed := node.(type) {
+	case JSONPointable:
+		r, err := typed.JSONLookup(decodedToken)
 		if err != nil {
 			return nil, kind, err
 		}
 		return r, kind, nil
+	case *any: // case of a pointer to interface, that is not resolved by reflect.Indirect
+		return getSingleImpl(*typed, decodedToken, nameProvider)
 	}
 
 	switch kind { //nolint:exhaustive

pointer_test.go

@@ -140,19 +140,56 @@ func TestIsEmpty(t *testing.T) {
 func TestGetSingle(t *testing.T) {
 	const in = `/obj`
 
-	_, err := New(in)
-	require.NoError(t, err)
-	result, _, err := GetForToken(testDocumentJSON, "obj")
-	require.NoError(t, err)
-	assert.Len(t, result, TestNodeObjNBItems)
+	t.Run("should create a new JSON pointer", func(t *testing.T) {
+		_, err := New(in)
+		require.NoError(t, err)
+	})
 
-	result, _, err = GetForToken(testStructJSONDoc, "Obj")
-	require.Error(t, err)
-	assert.Nil(t, result)
+	t.Run(`should find token "obj" in JSON`, func(t *testing.T) {
+		result, _, err := GetForToken(testDocumentJSON, "obj")
+		require.NoError(t, err)
+		assert.Len(t, result, TestNodeObjNBItems)
+	})
 
-	result, _, err = GetForToken(testStructJSONDoc, "Obj2")
-	require.Error(t, err)
-	assert.Nil(t, result)
+	t.Run(`should find token "obj" in type alias interface`, func(t *testing.T) {
+		type alias interface{}
+		var in alias = testDocumentJSON
+		result, _, err := GetForToken(in, "obj")
+		require.NoError(t, err)
+		assert.Len(t, result, TestNodeObjNBItems)
+	})
+
+	t.Run(`should find token "obj" in pointer to interface`, func(t *testing.T) {
+		in := &testDocumentJSON
+		result, _, err := GetForToken(in, "obj")
+		require.NoError(t, err)
+		assert.Len(t, result, TestNodeObjNBItems)
+	})
+
+	t.Run(`should not find token "Obj" in struct`, func(t *testing.T) {
+		result, _, err := GetForToken(testStructJSONDoc, "Obj")
+		require.Error(t, err)
+		assert.Nil(t, result)
+	})
+
+	t.Run(`should not find token "Obj2" in struct`, func(t *testing.T) {
+		result, _, err := GetForToken(testStructJSONDoc, "Obj2")
+		require.Error(t, err)
+		assert.Nil(t, result)
+	})
+
+	t.Run(`should not find token in nil`, func(t *testing.T) {
+		result, _, err := GetForToken(nil, "obj")
+		require.Error(t, err)
+		assert.Nil(t, result)
+	})
+
+	t.Run(`should not find token in nil interface`, func(t *testing.T) {
+		var in interface{}
+		result, _, err := GetForToken(in, "obj")
+		require.Error(t, err)
+		assert.Nil(t, result)
+	})
 }
 
 type pointableImpl struct {