Pass appropriate empty Value to hooks

Signed-off-by: Yuri Shkuro <[email protected]>

mapstructure.go

@@ -442,22 +442,33 @@ func (d *Decoder) Decode(input interface{}) error {
 	return err
 }
 
-// Decodes an unknown data type into a specific reflection value.
-func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value) error {
-	var inputVal reflect.Value
-	if input != nil {
-		inputVal = reflect.ValueOf(input)
-
-		// We need to check here if input is a typed nil. Typed nils won't
-		// match the "input == nil" below so we check that here.
-		if inputVal.Kind() == reflect.Ptr && inputVal.IsNil() {
-			input = nil
-		}
+// A comparison input == nil will fail if input is a typed nil.
+// This function converts a typed nil to an actual, untyped nil.
+func toRealNil(input interface{}) interface{} {
+	if input == nil {
+		return nil
 	}
+	val := reflect.ValueOf(input)
+	k := val.Kind()
+	if (k == reflect.Ptr ||
+		k == reflect.Interface ||
+		k == reflect.Map ||
+		k == reflect.Slice ||
+		k == reflect.Array) && val.IsNil() {
+		return nil
+	}
+	return input
+}
 
-	decodeNil := d.config.DecodeNil && d.config.DecodeHook != nil
-
-	if input == nil {
+// Decodes an unknown data type into a specific reflection value.
+func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value) error {
+	var (
+		inputVal   = reflect.ValueOf(input)
+		outputKind = getKind(outVal)
+		decodeNil  = d.config.DecodeNil && d.cachedDecodeHook != nil
+	)
+	input = toRealNil(input)
+	if input == nil || !inputVal.IsValid() {
 		// If the data is nil, then we don't set anything, unless ZeroFields is set
 		// to true.
 		if d.config.ZeroFields {
@@ -467,40 +478,37 @@ func (d *Decoder) decode(name string, input interface{}, outVal reflect.Value) e
 				d.config.Metadata.Keys = append(d.config.Metadata.Keys, name)
 			}
 		}
-
 		if !decodeNil {
 			return nil
 		}
 	}
 
-	if !inputVal.IsValid() {
-		if !decodeNil {
-			// If the input value is invalid, then we just set the value
-			// to be the zero value.
-			outVal.Set(reflect.Zero(outVal.Type()))
-			if d.config.Metadata != nil && name != "" {
-				d.config.Metadata.Keys = append(d.config.Metadata.Keys, name)
-			}
-			return nil
-		}
-
-		// If we get here, we have an untyped nil so the type of the input is assumed.
-		// We do this because all subsequent code requires a valid value for inputVal.
-		var mapVal map[string]interface{}
-		inputVal = reflect.MakeMap(reflect.TypeOf(mapVal))
-	}
-
 	if d.cachedDecodeHook != nil {
 		// We have a DecodeHook, so let's pre-process the input.
+		if !inputVal.IsValid() {
+			// Hooks need a valid inputVal, so reset it to zero value of outVal type.
+			switch outputKind {
+			case reflect.Struct, reflect.Map:
+				var mapVal map[string]interface{}
+				inputVal = reflect.ValueOf(mapVal)
+			case reflect.Slice, reflect.Array:
+				var sliceVal []interface{}
+				inputVal = reflect.ValueOf(sliceVal)
+			default:
+				inputVal = reflect.Zero(outVal.Type())
+			}
+		}
 		var err error
 		input, err = d.cachedDecodeHook(inputVal, outVal)
 		if err != nil {
 			return fmt.Errorf("error decoding '%s': %w", name, err)
 		}
 	}
+	if toRealNil(input) == nil {
+		return nil
+	}
 
 	var err error
-	outputKind := getKind(outVal)
 	addMetaKey := true
 	switch outputKind {
 	case reflect.Bool: