lib/decode: fix hook to work with embedded squash struct

The decode hook is not call for the embedded squashed struct, so we need to recurse when we
find squash tags.

See https://github.com/mitchellh/mapstructure/issues/226

lib/decode/decode.go

@@ -73,12 +73,28 @@ func translationsForType(to reflect.Type) map[string]string {
 	translations := map[string]string{}
 	for i := 0; i < to.NumField(); i++ {
 		field := to.Field(i)
+		tags := fieldTags(field)
+		if tags.squash {
+			embedded := field.Type
+			if embedded.Kind() == reflect.Ptr {
+				embedded = embedded.Elem()
+			}
+			if embedded.Kind() != reflect.Struct {
+				// mapstructure will handle reporting this error
+				continue
+			}
+
+			for k, v := range translationsForType(embedded) {
+				translations[k] = v
+			}
+			continue
+		}
+
 		tag, ok := field.Tag.Lookup("alias")
 		if !ok {
 			continue
 		}
-
-		canonKey := strings.ToLower(canonicalFieldKey(field))
+		canonKey := strings.ToLower(tags.name)
 		for _, alias := range strings.Split(tag, ",") {
 			translations[strings.ToLower(alias)] = canonKey
 		}
@@ -86,19 +102,31 @@ func translationsForType(to reflect.Type) map[string]string {
 	return translations
 }
 
-func canonicalFieldKey(field reflect.StructField) string {
+func fieldTags(field reflect.StructField) mapstructureFieldTags {
 	tag, ok := field.Tag.Lookup("mapstructure")
 	if !ok {
-		return field.Name
+		return mapstructureFieldTags{name: field.Name}
 	}
-	parts := strings.SplitN(tag, ",", 2)
-	switch {
-	case len(parts) < 1:
-		return field.Name
-	case parts[0] == "":
-		return field.Name
+
+	tags := mapstructureFieldTags{name: field.Name}
+	parts := strings.Split(tag, ",")
+	if len(parts) == 0 {
+		return tags
 	}
-	return parts[0]
+	if parts[0] != "" {
+		tags.name = parts[0]
+	}
+	for _, part := range parts[1:] {
+		if part == "squash" {
+			tags.squash = true
+		}
+	}
+	return tags
+}
+
+type mapstructureFieldTags struct {
+	name   string
+	squash bool
 }
 
 // HookWeakDecodeFromSlice looks for []map[string]interface{} and []interface{}

lib/decode/decode_test.go

@@ -1,6 +1,7 @@
 package decode
 
 import (
+	"fmt"
 	"reflect"
 	"testing"
 
@@ -210,6 +211,17 @@ type translateExample struct {
 	FieldWithMapstructureTag     string `alias:"second" mapstructure:"field_with_mapstruct_tag"`
 	FieldWithMapstructureTagOmit string `mapstructure:"field_with_mapstruct_omit,omitempty" alias:"third"`
 	FieldWithEmptyTag            string `mapstructure:"" alias:"forth"`
+	EmbeddedStruct               `mapstructure:",squash"`
+	*PtrEmbeddedStruct           `mapstructure:",squash"`
+	BadField                     string `mapstructure:",squash"`
+}
+
+type EmbeddedStruct struct {
+	NextField string `alias:"next"`
+}
+
+type PtrEmbeddedStruct struct {
+	OtherNextField string `alias:"othernext"`
 }
 
 func TestTranslationsForType(t *testing.T) {
@@ -220,6 +232,8 @@ func TestTranslationsForType(t *testing.T) {
 		"second":    "field_with_mapstruct_tag",
 		"third":     "field_with_mapstruct_omit",
 		"forth":     "fieldwithemptytag",
+		"next":      "nextfield",
+		"othernext": "othernextfield",
 	}
 	require.Equal(t, expected, actual)
 }
@@ -389,3 +403,35 @@ service {
 	}
 	require.Equal(t, target, expected)
 }
+
+func TestFieldTags(t *testing.T) {
+	type testCase struct {
+		tags     string
+		expected mapstructureFieldTags
+	}
+
+	fn := func(t *testing.T, tc testCase) {
+		tag := fmt.Sprintf(`mapstructure:"%v"`, tc.tags)
+		field := reflect.StructField{
+			Tag:  reflect.StructTag(tag),
+			Name: "Original",
+		}
+		actual := fieldTags(field)
+		require.Equal(t, tc.expected, actual)
+	}
+
+	var testCases = []testCase{
+		{tags: "", expected: mapstructureFieldTags{name: "Original"}},
+		{tags: "just-a-name", expected: mapstructureFieldTags{name: "just-a-name"}},
+		{tags: "name,squash", expected: mapstructureFieldTags{name: "name", squash: true}},
+		{tags: ",squash", expected: mapstructureFieldTags{name: "Original", squash: true}},
+		{tags: ",omitempty,squash", expected: mapstructureFieldTags{name: "Original", squash: true}},
+		{tags: "named,omitempty,squash", expected: mapstructureFieldTags{name: "named", squash: true}},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.tags, func(t *testing.T) {
+			fn(t, tc)
+		})
+	}
+}