From bfe0d3c2f4c47d70d56f279e6c8867b05a3117cb Mon Sep 17 00:00:00 2001
From: Matt Keeler <mjkeeler7@gmail.com>
Date: Wed, 22 Jul 2020 11:52:35 -0400
Subject: [PATCH] Ensure that intermediate maps during struct to struct
 decoding are settable

---
 mapstructure.go           | 31 ++++++++++++++++---
 mapstructure_bugs_test.go | 64 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+), 5 deletions(-)

diff --git a/mapstructure.go b/mapstructure.go
index b384d9d9..f41bcc58 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -906,11 +906,22 @@ func (d *Decoder) decodeMapFromStruct(name string, dataVal reflect.Value, val re
 			mType := reflect.MapOf(vKeyType, vElemType)
 			vMap := reflect.MakeMap(mType)
 
-			err := d.decode(keyName, x.Interface(), vMap)
+			// Creating a pointer to a map so that other methods can completely
+			// overwrite the map if need be (looking at you decodeMapFromMap). The
+			// indirection allows the underlying map to be settable (CanSet() == true)
+			// where as reflect.MakeMap returns an unsettable map.
+			addrVal := reflect.New(vMap.Type())
+			reflect.Indirect(addrVal).Set(vMap)
+
+			err := d.decode(keyName, x.Interface(), reflect.Indirect(addrVal))
 			if err != nil {
 				return err
 			}
 
+			// the underlying map may have been completely overwritten so pull
+			// it indirectly out of the enclosing value.
+			vMap = reflect.Indirect(addrVal)
+
 			if squash {
 				for _, k := range vMap.MapKeys() {
 					valMap.SetMapIndex(k, vMap.MapIndex(k))
@@ -1154,13 +1165,23 @@ func (d *Decoder) decodeStruct(name string, data interface{}, val reflect.Value)
 		// Not the most efficient way to do this but we can optimize later if
 		// we want to. To convert from struct to struct we go to map first
 		// as an intermediary.
-		m := make(map[string]interface{})
-		mval := reflect.Indirect(reflect.ValueOf(&m))
-		if err := d.decodeMapFromStruct(name, dataVal, mval, mval); err != nil {
+
+		// Make a new map to hold our result
+		mapType := reflect.TypeOf((map[string]interface{})(nil))
+		mval := reflect.MakeMap(mapType)
+
+		// Creating a pointer to a map so that other methods can completely
+		// overwrite the map if need be (looking at you decodeMapFromMap). The
+		// indirection allows the underlying map to be settable (CanSet() == true)
+		// where as reflect.MakeMap returns an unsettable map.
+		addrVal := reflect.New(mval.Type())
+
+		reflect.Indirect(addrVal).Set(mval)
+		if err := d.decodeMapFromStruct(name, dataVal, reflect.Indirect(addrVal), mval); err != nil {
 			return err
 		}
 
-		result := d.decodeStructFromMap(name, mval, val)
+		result := d.decodeStructFromMap(name, reflect.Indirect(addrVal), val)
 		return result
 
 	default:
diff --git a/mapstructure_bugs_test.go b/mapstructure_bugs_test.go
index f030c2d6..8c87ebd4 100644
--- a/mapstructure_bugs_test.go
+++ b/mapstructure_bugs_test.go
@@ -3,6 +3,7 @@ package mapstructure
 import (
 	"reflect"
 	"testing"
+	"time"
 )
 
 // GH-1, GH-10, GH-96
@@ -475,3 +476,66 @@ func TestDecodeBadDataTypeInSlice(t *testing.T) {
 		t.Error("An error was expected, got nil")
 	}
 }
+
+// #202 Ensure that intermediate maps in the struct -> struct decode process are settable
+// and not just the elements within them.
+func TestDecodeIntermeidateMapsSettable(t *testing.T) {
+	type Timestamp struct {
+		Seconds int64
+		Nanos   int32
+	}
+
+	type TsWrapper struct {
+		Timestamp *Timestamp
+	}
+
+	type TimeWrapper struct {
+		Timestamp time.Time
+	}
+
+	input := TimeWrapper{
+		Timestamp: time.Unix(123456789, 987654),
+	}
+
+	expected := TsWrapper{
+		Timestamp: &Timestamp{
+			Seconds: 123456789,
+			Nanos:   987654,
+		},
+	}
+
+	timePtrType := reflect.TypeOf((*time.Time)(nil))
+	mapStrInfType := reflect.TypeOf((map[string]interface{})(nil))
+
+	var actual TsWrapper
+	decoder, err := NewDecoder(&DecoderConfig{
+		Result: &actual,
+		DecodeHook: func(from, to reflect.Type, data interface{}) (interface{}, error) {
+			if from == timePtrType && to == mapStrInfType {
+				ts := data.(*time.Time)
+				nanos := ts.UnixNano()
+
+				seconds := nanos / 1000000000
+				nanos = nanos % 1000000000
+
+				return &map[string]interface{}{
+					"Seconds": seconds,
+					"Nanos":   int32(nanos),
+				}, nil
+			}
+			return data, nil
+		},
+	})
+
+	if err != nil {
+		t.Fatalf("failed to create decoder: %v", err)
+	}
+
+	if err := decoder.Decode(&input); err != nil {
+		t.Fatalf("failed to decode input: %v", err)
+	}
+
+	if !reflect.DeepEqual(expected, actual) {
+		t.Fatalf("expected: %#[1]v (%[1]T), got: %#[2]v (%[2]T)", expected, actual)
+	}
+}