vm: add TryBytes() to StackItem interface

Conversion should be done in a StackItem, not in an Element.

pkg/vm/context.go

@@ -170,6 +170,11 @@ func (c *Context) Dup() StackItem {
 	return c
 }
 
+// TryBytes implements StackItem interface.
+func (c *Context) TryBytes() ([]byte, error) {
+	return nil, errors.New("can't convert Context to ByteArray")
+}
+
 // ToContractParameter implements StackItem interface.
 func (c *Context) ToContractParameter(map[StackItem]bool) smartcontract.Parameter {
 	return smartcontract.Parameter{

pkg/vm/stack.go

@@ -125,21 +125,11 @@ func (e *Element) Bool() bool {
 // Bytes attempts to get the underlying value of the element as a byte array.
 // Will panic if the assertion failed which will be caught by the VM.
 func (e *Element) Bytes() []byte {
-	switch t := e.value.(type) {
-	case *ByteArrayItem:
-		return t.value
-	case *BigIntegerItem:
-		return t.Bytes() // neoVM returns in LE
-	case *BoolItem:
-		if t.value {
-			return []byte{1}
-		}
-		// return []byte{0}
-		// FIXME revert when NEO 3.0 https://github.com/nspcc-dev/neo-go/issues/477
-		return []byte{}
-	default:
-		panic("can't convert to []byte: " + t.String())
+	bs, err := e.value.TryBytes()
+	if err != nil {
+		panic(err)
 	}
+	return bs
 }
 
 // Array attempts to get the underlying value of the element as an array of

pkg/vm/stack_item.go

@@ -4,6 +4,7 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 	"reflect"
@@ -18,6 +19,8 @@ type StackItem interface {
 	Value() interface{}
 	// Dup duplicates current StackItem.
 	Dup() StackItem
+	// TryBytes converts StackItem to a byte slice.
+	TryBytes() ([]byte, error)
 	// ToContractParameter converts StackItem to smartcontract.Parameter
 	ToContractParameter(map[StackItem]bool) smartcontract.Parameter
 }
@@ -118,6 +121,11 @@ func (i *StructItem) Dup() StackItem {
 	return i
 }
 
+// TryBytes implements StackItem interface.
+func (i *StructItem) TryBytes() ([]byte, error) {
+	return nil, errors.New("can't convert Struct to ByteArray")
+}
+
 // ToContractParameter implements StackItem interface.
 func (i *StructItem) ToContractParameter(seen map[StackItem]bool) smartcontract.Parameter {
 	var value []smartcontract.Parameter
@@ -167,6 +175,11 @@ func (i *BigIntegerItem) Bytes() []byte {
 	return emit.IntToBytes(i.value)
 }
 
+// TryBytes implements StackItem interface.
+func (i *BigIntegerItem) TryBytes() ([]byte, error) {
+	return i.Bytes(), nil
+}
+
 // Value implements StackItem interface.
 func (i *BigIntegerItem) Value() interface{} {
 	return i.value
@@ -226,6 +239,21 @@ func (i *BoolItem) Dup() StackItem {
 	return &BoolItem{i.value}
 }
 
+// Bytes converts BoolItem to bytes.
+func (i *BoolItem) Bytes() []byte {
+	if i.value {
+		return []byte{1}
+	}
+	// return []byte{0}
+	// FIXME revert when NEO 3.0 https://github.com/nspcc-dev/neo-go/issues/477
+	return []byte{}
+}
+
+// TryBytes implements StackItem interface.
+func (i *BoolItem) TryBytes() ([]byte, error) {
+	return i.Bytes(), nil
+}
+
 // ToContractParameter implements StackItem interface.
 func (i *BoolItem) ToContractParameter(map[StackItem]bool) smartcontract.Parameter {
 	return smartcontract.Parameter{
@@ -260,6 +288,11 @@ func (i *ByteArrayItem) String() string {
 	return "ByteArray"
 }
 
+// TryBytes implements StackItem interface.
+func (i *ByteArrayItem) TryBytes() ([]byte, error) {
+	return i.value, nil
+}
+
 // Dup implements StackItem interface.
 func (i *ByteArrayItem) Dup() StackItem {
 	a := make([]byte, len(i.value))
@@ -301,6 +334,11 @@ func (i *ArrayItem) String() string {
 	return "Array"
 }
 
+// TryBytes implements StackItem interface.
+func (i *ArrayItem) TryBytes() ([]byte, error) {
+	return nil, errors.New("can't convert Array to ByteArray")
+}
+
 // Dup implements StackItem interface.
 func (i *ArrayItem) Dup() StackItem {
 	// reference type
@@ -341,6 +379,11 @@ func (i *MapItem) Value() interface{} {
 	return i.value
 }
 
+// TryBytes implements StackItem interface.
+func (i *MapItem) TryBytes() ([]byte, error) {
+	return nil, errors.New("can't convert Map to ByteArray")
+}
+
 func (i *MapItem) String() string {
 	return "Map"
 }
@@ -438,6 +481,11 @@ func (i *InteropItem) Dup() StackItem {
 	return i
 }
 
+// TryBytes implements StackItem interface.
+func (i *InteropItem) TryBytes() ([]byte, error) {
+	return nil, errors.New("can't convert Interop to ByteArray")
+}
+
 // ToContractParameter implements StackItem interface.
 func (i *InteropItem) ToContractParameter(map[StackItem]bool) smartcontract.Parameter {
 	return smartcontract.Parameter{