Revert "encoding/json: reuse values when decoding map elements"

This reverts golang.org/cl/179337.

Reason for revert: broke a few too many reasonably valid Go programs.
The previous behavior was perhaps less consistent, but the docs were
never very clear about when the decoder merges with existing values,
versus replacing existing values altogether.

Fixes #39149.

Change-Id: I1c1d857709b8398969fe421aa962f6b62f91763a
Reviewed-on: https://go-review.googlesource.com/c/go/+/234559
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

2 files changed, 45 insertions, 112 deletions

diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 5acc6d8b26..5f34af44ea 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -677,6 +677,7 @@ func (d *decodeState) object(v reflect.Value) error {
 		return nil
 	}
 
+	var mapElem reflect.Value
 	origErrorContext := d.errorContext
 
 	for {
@@ -700,66 +701,17 @@ func (d *decodeState) object(v reflect.Value) error {
 		}
 
 		// Figure out field corresponding to key.
-		var kv, subv reflect.Value
+		var subv reflect.Value
 		destring := false // whether the value is wrapped in a string to be decoded first
 
 		if v.Kind() == reflect.Map {
-			// First, figure out the key value from the input.
-			kt := t.Key()
-			switch {
-			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
-				kv = reflect.New(kt)
-				if err := d.literalStore(item, kv, true); err != nil {
-					return err
-				}
-				kv = kv.Elem()
-			case kt.Kind() == reflect.String:
-				kv = reflect.ValueOf(key).Convert(kt)
-			default:
-				switch kt.Kind() {
-				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-					s := string(key)
-					n, err := strconv.ParseInt(s, 10, 64)
-					if err != nil || reflect.Zero(kt).OverflowInt(n) {
-						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
-						break
-					}
-					kv = reflect.ValueOf(n).Convert(kt)
-				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
-					s := string(key)
-					n, err := strconv.ParseUint(s, 10, 64)
-					if err != nil || reflect.Zero(kt).OverflowUint(n) {
-						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
-						break
-					}
-					kv = reflect.ValueOf(n).Convert(kt)
-				default:
-					panic("json: Unexpected key type") // should never occur
-				}
-			}
-
-			// Then, decide what element value we'll decode into.
-			et := t.Elem()
-			if kv.IsValid() {
-				if existing := v.MapIndex(kv); !existing.IsValid() {
-					// Nothing to reuse.
-				} else if et.Kind() == reflect.Ptr {
-					// Pointer; decode directly into it if non-nil.
-					if !existing.IsNil() {
-						subv = existing
-					}
-				} else {
-					// Non-pointer. Make a copy and decode into the
-					// addressable copy. Don't just use a new/zero
-					// value, as that would lose existing data.
-					subv = reflect.New(et).Elem()
-					subv.Set(existing)
-				}
-			}
-			if !subv.IsValid() {
-				// We couldn't reuse an existing value.
-				subv = reflect.New(et).Elem()
+			elemType := t.Elem()
+			if !mapElem.IsValid() {
+				mapElem = reflect.New(elemType).Elem()
+			} else {
+				mapElem.Set(reflect.Zero(elemType))
 			}
+			subv = mapElem
 		} else {
 			var f *field
 			if i, ok := fields.nameIndex[string(key)]; ok {
@@ -838,8 +790,43 @@ func (d *decodeState) object(v reflect.Value) error {
 
 		// Write value back to map;
 		// if using struct, subv points into struct already.
-		if v.Kind() == reflect.Map && kv.IsValid() {
-			v.SetMapIndex(kv, subv)
+		if v.Kind() == reflect.Map {
+			kt := t.Key()
+			var kv reflect.Value
+			switch {
+			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
+				kv = reflect.New(kt)
+				if err := d.literalStore(item, kv, true); err != nil {
+					return err
+				}
+				kv = kv.Elem()
+			case kt.Kind() == reflect.String:
+				kv = reflect.ValueOf(key).Convert(kt)
+			default:
+				switch kt.Kind() {
+				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+					s := string(key)
+					n, err := strconv.ParseInt(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowInt(n) {
+						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
+						break
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+					s := string(key)
+					n, err := strconv.ParseUint(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowUint(n) {
+						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
+						break
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				default:
+					panic("json: Unexpected key type") // should never occur
+				}
+			}
+			if kv.IsValid() {
+				v.SetMapIndex(kv, subv)
+			}
 		}
 
 		// Next token must be , or }.
diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index a62488d447..5ac1022207 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -2569,57 +2569,3 @@ func TestUnmarshalMaxDepth(t *testing.T) {
 		}
 	}
 }
-
-func TestUnmarshalMapPointerElem(t *testing.T) {
-	type S struct{ Unchanged, Changed int }
-	input := []byte(`{"S":{"Changed":5}}`)
-	want := S{1, 5}
-
-	// First, a map with struct pointer elements. The key-value pair exists,
-	// so reuse the existing value.
-	s := &S{1, 2}
-	ptrMap := map[string]*S{"S": s}
-	if err := Unmarshal(input, &ptrMap); err != nil {
-		t.Fatal(err)
-	}
-	if s != ptrMap["S"] {
-		t.Fatal("struct pointer element in map was completely replaced")
-	}
-	if got := *s; got != want {
-		t.Fatalf("want %#v, got %#v", want, got)
-	}
-
-	// Second, a map with struct elements. The key-value pair exists, but
-	// the value isn't addresable, so make a copy and use that.
-	s = &S{1, 2}
-	strMap := map[string]S{"S": *s}
-	if err := Unmarshal(input, &strMap); err != nil {
-		t.Fatal(err)
-	}
-	if *s == strMap["S"] {
-		t.Fatal("struct element in map wasn't copied")
-	}
-	if got := strMap["S"]; got != want {
-		t.Fatalf("want %#v, got %#v", want, got)
-	}
-
-	// Finally, check the cases where the key-value pair exists, but the
-	// value is zero.
-	want = S{0, 5}
-
-	ptrMap = map[string]*S{"S": nil}
-	if err := Unmarshal(input, &ptrMap); err != nil {
-		t.Fatal(err)
-	}
-	if got := *ptrMap["S"]; got != want {
-		t.Fatalf("want %#v, got %#v", want, got)
-	}
-
-	strMap = map[string]S{"S": {}}
-	if err := Unmarshal(input, &strMap); err != nil {
-		t.Fatal(err)
-	}
-	if got := strMap["S"]; got != want {
-		t.Fatalf("want %#v, got %#v", want, got)
-	}
-}