vm: fix PACKMAP operation

Close #3613

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/vm/stackitem/item.go

@@ -824,7 +824,8 @@ func NewMap() *Map {
 	}
 }
 
-// NewMapWithValue returns a new Map object filled with the specified value.
+// NewMapWithValue returns a new Map object filled with the specified value
+// without value validation.
 func NewMapWithValue(value []MapElement) *Map {
 	if value != nil {
 		return &Map{

pkg/vm/stackitem/serialization_test.go

@@ -219,7 +219,7 @@ func TestSerializeLimited(t *testing.T) {
 		bigMap := make([]MapElement, customLimit/2)
 		for i := range bigMap {
 			bigMap[i] = MapElement{
-				Key:   NewByteArray([]byte("key")),
+				Key:   NewByteArray([]byte("key" + strconv.Itoa(i))),
 				Value: NewBool(true),
 			}
 		}

pkg/vm/vm.go

@@ -1214,15 +1214,16 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 			panic("invalid length")
 		}
 
-		items := make([]stackitem.MapElement, n)
-		for i := range n {
+		m := stackitem.NewMap()
+		for range n {
 			key := v.estack.Pop()
-			validateMapKey(key)
 			val := v.estack.Pop().value
-			items[i].Key = key.value
-			items[i].Value = val
+			if key.Item() == nil {
+				panic("no key found")
+			}
+			m.Add(key.value, val)
 		}
-		v.estack.PushItem(stackitem.NewMapWithValue(items))
+		v.estack.PushItem(m)
 
 	case opcode.PACKSTRUCT, opcode.PACK:
 		n := toInt(v.estack.Pop().BigInt())

pkg/vm/vm_test.go

@@ -2083,6 +2083,45 @@ func TestUNPACKBadNotArray(t *testing.T) {
 	runWithArgs(t, prog, nil, 1)
 }
 
+func TestPACKMAPDuplicateKeys(t *testing.T) {
+	prog := makeProgram(opcode.PACKMAP)
+	vm := load(prog)
+
+	keys := []string{"duplicateKey", "uniqueKey", "duplicateKey", "anotherUniqueKey"}
+	values := []string{"value1", "value2", "value3", "value4"}
+
+	for i := range keys {
+		vm.estack.PushVal(values[i])
+		vm.estack.PushVal(keys[i])
+	}
+
+	vm.estack.PushVal(len(keys))
+	runVM(t, vm)
+
+	require.Equal(t, 1, vm.estack.Len())
+	packedItem := vm.estack.Pop().Item()
+	require.Equal(t, stackitem.MapT, packedItem.Type())
+
+	packedMap := packedItem.(*stackitem.Map)
+	require.Equal(t, 3, packedMap.Len())
+
+	expected := []stackitem.MapElement{
+		{
+			Key:   stackitem.NewByteArray([]byte("anotherUniqueKey")),
+			Value: stackitem.NewByteArray([]byte("value4")),
+		},
+		{
+			Key:   stackitem.NewByteArray([]byte("duplicateKey")),
+			Value: stackitem.NewByteArray([]byte("value1")),
+		},
+		{
+			Key:   stackitem.NewByteArray([]byte("uniqueKey")),
+			Value: stackitem.NewByteArray([]byte("value2")),
+		},
+	}
+	require.Equal(t, expected, packedMap.Value())
+}
+
 func TestUNPACKGood(t *testing.T) {
 	prog := makeProgram(opcode.UNPACK)
 	elements := []int{55, 34, 42}