From 2af0544f8e032bc7a363e897fbd9c6ed4884a1d6 Mon Sep 17 00:00:00 2001
From: Gregory Oschwald <goschwald@maxmind.com>
Date: Mon, 19 Sep 2016 14:31:57 -0700
Subject: [PATCH] Fix panic with embedded struct. GitHub #28

---
 decoder.go     | 27 +++++++++++++++++++--------
 reader_test.go | 18 ++++++++++++++++++
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/decoder.go b/decoder.go
index df3030c..53ab390 100644
--- a/decoder.go
+++ b/decoder.go
@@ -139,19 +139,30 @@ func (d *decoder) unmarshalBool(size uint, offset uint, result reflect.Value) (u
 	return newOffset, newUnmarshalTypeError(value, result.Type())
 }
 
-// follow pointers and create values as necessary
+// indirect follows pointers and create values as necessary. This is
+// heavily based on encoding/json as my original version had a subtle
+// bug. This method should be considered to be licensed under
+// https://golang.org/LICENSE
 func (d *decoder) indirect(result reflect.Value) reflect.Value {
 	for {
-		if result.Kind() == reflect.Ptr {
-			if result.IsNil() {
-				result.Set(reflect.New(result.Type().Elem()))
+		// Load value from interface, but only if the result will be
+		// usefully addressable.
+		if result.Kind() == reflect.Interface && !result.IsNil() {
+			e := result.Elem()
+			if e.Kind() == reflect.Ptr && !e.IsNil() {
+				result = e
+				continue
 			}
-			result = result.Elem()
-		} else if result.Kind() == reflect.Interface && !result.IsNil() {
-			result = result.Elem()
-		} else {
+		}
+
+		if result.Kind() != reflect.Ptr {
 			break
 		}
+
+		if result.IsNil() {
+			result.Set(reflect.New(result.Type().Elem()))
+		}
+		result = result.Elem()
 	}
 	return result
 }
diff --git a/reader_test.go b/reader_test.go
index bd2fb64..5d3a0b0 100644
--- a/reader_test.go
+++ b/reader_test.go
@@ -173,6 +173,24 @@ func TestNonEmptyNilInterface(t *testing.T) {
 	assert.Equal(t, err.Error(), "maxminddb: cannot unmarshal map into type maxminddb.TestInterface")
 }
 
+type CityTraits struct {
+	AutonomousSystemNumber uint `json:"autonomous_system_number,omitempty" maxminddb:"autonomous_system_number"`
+}
+
+type City struct {
+	Traits CityTraits `maxminddb:"traits"`
+}
+
+func TestEmbeddedStructAsInterface(t *testing.T) {
+	var city City
+	var result interface{} = city.Traits
+
+	db, err := Open("test-data/test-data/GeoIP2-ISP-Test.mmdb")
+	require.Nil(t, err)
+
+	assert.Nil(t, db.Lookup(net.ParseIP("1.128.0.0"), &result))
+}
+
 type BoolInterface interface {
 	true() bool
 }