vm: implement PUSHNULL opcode

pkg/vm/opcode/opcode.go

@@ -89,6 +89,7 @@ const (
 	PUSHDATA2   Opcode = 0x4D
 	PUSHDATA4   Opcode = 0x4E
 	PUSHM1      Opcode = 0x4F
+	PUSHNULL    Opcode = 0x50
 	PUSH1       Opcode = 0x51
 	PUSHT       Opcode = PUSH1
 	PUSH2       Opcode = 0x52

pkg/vm/vm.go

@@ -560,6 +560,9 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 	case opcode.PUSHDATA1, opcode.PUSHDATA2, opcode.PUSHDATA4:
 		v.estack.PushVal(parameter)
 
+	case opcode.PUSHNULL:
+		v.estack.PushVal(NullItem{})
+
 	// Stack operations.
 	case opcode.TOALTSTACK:
 		v.astack.Push(v.estack.Pop())

pkg/vm/vm_test.go

@@ -197,6 +197,15 @@ func TestStackLimitPUSH1Bad(t *testing.T) {
 	checkVMFailed(t, v)
 }
 
+func TestPUSHNULL(t *testing.T) {
+	prog := makeProgram(opcode.PUSHNULL, opcode.PUSHNULL, opcode.EQUAL)
+	v := load(prog)
+	require.NoError(t, v.Step())
+	require.Equal(t, 1, v.estack.Len())
+	runVM(t, v)
+	require.True(t, v.estack.Pop().Bool())
+}
+
 // appendBigStruct returns a program which:
 // 1. pushes size Structs on stack
 // 2. packs them into a new struct