encoding/json: reuse values when decoding map elements

When we decode into a struct, each input key-value may be decoded into
one of the struct's fields. Particularly, existing data isn't dropped,
so that some sub-fields can be decoded into without zeroing all other
data.

However, decoding into a map behaved in the opposite way. Whenever a
key-value was decoded, it completely replaced the previous map element.
If the map contained any non-zero data in that key, it's dropped.

Instead, try to reuse the existing element value if possible. If the map
element type is a pointer, and the value is non-nil, we can decode
directly into it. If it's not a pointer, make a copy and decode into
that copy, as map element values aren't addressable.

This means we have to parse and convert the map element key before the
value, to be able to obtain the existing element value. This is fine,
though. Moreover, reporting errors on the key before the value follows
the input order more closely.

Finally, add a test to explore the four combinations, involving pointer
and non-pointer, and non-zero and zero values. A table-driven test
wasn't used, as each case required different checks, such as checking
that the non-nil pointer case doesn't end up with a different pointer.

Fixes #31924.

Change-Id: I5ca40c9963a98aaf92f26f0b35843c021028dfca
Reviewed-on: https://go-review.googlesource.com/c/go/+/179337
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/decode.go

@@ -677,7 +677,6 @@ func (d *decodeState) object(v reflect.Value) error {
 		return nil
 	}
 
-	var mapElem reflect.Value
 	origErrorContext := d.errorContext
 
 	for {
@@ -701,17 +700,66 @@ func (d *decodeState) object(v reflect.Value) error {
 		}
 
 		// Figure out field corresponding to key.
-		var subv reflect.Value
+		var kv, subv reflect.Value
 		destring := false // whether the value is wrapped in a string to be decoded first
 
 		if v.Kind() == reflect.Map {
-			elemType := t.Elem()
-			if !mapElem.IsValid() {
-				mapElem = reflect.New(elemType).Elem()
-			} else {
-				mapElem.Set(reflect.Zero(elemType))
+			// 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()
 			}
-			subv = mapElem
 		} else {
 			var f *field
 			if i, ok := fields.nameIndex[string(key)]; ok {
@@ -790,43 +838,8 @@ 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 {
-			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)
-			}
+		if v.Kind() == reflect.Map && kv.IsValid() {
+			v.SetMapIndex(kv, subv)
 		}
 
 		// Next token must be , or }.