add support for 'required' tag value

Author: bonifaido

mapstructure.go

@@ -1230,9 +1230,19 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 		fieldName := field.Name
 
 		tagValue := field.Tag.Get(d.config.TagName)
-		tagValue = strings.SplitN(tagValue, ",", 2)[0]
-		if tagValue != "" {
-			fieldName = tagValue
+		tagFieldValue := strings.SplitN(tagValue, ",", 2)[0]
+		if tagFieldValue != "" {
+			fieldName = tagFieldValue
+		}
+
+		// If "required" is specified in the tag, we need make sure it is present.
+		required := false
+		tagParts := strings.Split(tagValue, ",")
+		for _, tag := range tagParts[1:] {
+			if tag == "required" {
+				required = true
+				break
+			}
 		}
 
 		rawMapKey := reflect.ValueOf(fieldName)
@@ -1255,6 +1265,10 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 			}
 
 			if !rawMapVal.IsValid() {
+				if required {
+					err := fmt.Errorf("'%s' is missing required key: %s", name, fieldName)
+					errors = appendErrors(errors, err)
+				}
 				// There was no matching key in the map for the value in
 				// the struct. Just ignore.
 				continue

mapstructure_test.go

@@ -160,6 +160,9 @@ type StructWithOmitEmpty struct {
 	OmitMapField       map[string]interface{} `mapstructure:"omittable-map,omitempty"`
 	NestedField        *Nested                `mapstructure:"visible-nested"`
 	OmitNestedField    *Nested                `mapstructure:"omittable-nested,omitempty"`
+type Required struct {
+	RequiredBar string `mapstructure:"bar,required"`
+	Value       string `mapstructure:"foo"`
 }
 
 type TypeConversionResult struct {
@@ -2230,6 +2233,17 @@ func TestDecode_StructTaggedWithOmitempty_KeepNonEmptyValues(t *testing.T) {
 
 	if !reflect.DeepEqual(actual, expected) {
 		t.Fatalf("Decode() expected: %#v, got: %#v", expected, actual)
+func TestRequired(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"foo": "bar",
+	}
+
+	var result Required
+	err := Decode(input, &result)
+	if err == nil {
+		t.Fatal("unexpected success decoding required field was missing")
 	}
 }