VM:Implement THROW opcode (#219)

[VM]

- Changed vmstate from HALT to FAULT in Sub opcode
- Implemented THROW opcode + tests
- Renamed TestSimpleRun test to TestThrowIfNot

pkg/vm/vm_ops.go

@@ -12,6 +12,7 @@ var opFunc = map[stack.Instruction]stackInfo{
 	stack.RET:         RET,
 	stack.EQUAL:       EQUAL,
 	stack.THROWIFNOT:  THROWIFNOT,
+	stack.THROW:       THROW,
 }
 
 func init() {

pkg/vm/vm_ops_exceptions.go

@@ -31,3 +31,9 @@ func THROWIFNOT(op stack.Instruction, ctx *stack.Context, istack *stack.Invocati
 	}
 	return NONE, nil
 }
+
+// THROW returns a FAULT VM state. This indicate that there is an error in the
+// current context loaded program.
+func THROW(op stack.Instruction, ctx *stack.Context, istack *stack.Invocation, rstack *stack.RandomAccess) (Vmstate, error) {
+	return FAULT, errors.New("the execution of the script program end with an error")
+}

pkg/vm/vm_ops_maths.go

@@ -34,7 +34,7 @@ func Sub(op stack.Instruction, ctx *stack.Context, istack *stack.Invocation, rst
 	}
 	res, err := operandB.Sub(operandA)
 	if err != nil {
-		return HALT, err
+		return FAULT, err
 	}
 
 	ctx.Estack.Push(res)

pkg/vm/vm_test.go

@@ -61,7 +61,7 @@ func TestPushAdd(t *testing.T) {
 
 }
 
-func TestSimpleRun(t *testing.T) {
+func TestThrowIfNot(t *testing.T) {
 
 	// Program pushes 20 and 34 to the stack
 	// Adds them together
@@ -89,6 +89,33 @@ func TestSimpleRun(t *testing.T) {
 	// ResultStack should be nil
 	assert.Equal(t, -1, vm.ResultStack.Len())
 
+	// InvocationStack should be empty
+	assert.Equal(t, 0, vm.InvocationStack.Len())
+
+}
+
+func TestThrow(t *testing.T) {
+
+	// Program pushes 20 to the stack
+	// exits with an error
+
+	// Push(20)
+	// THROW
+
+	builder := stack.NewBuilder()
+	builder.EmitInt(20).EmitOpcode(stack.THROW)
+
+	// Pass program to VM
+	vm := NewVM(builder.Bytes())
+
+	// Runs vm with program
+	_, err := vm.Run()
+	assert.NotNil(t, err)
+
+	ctx, err := vm.InvocationStack.CurrentContext()
+	assert.Equal(t, nil, err)
+	assert.Equal(t, 1, ctx.Estack.Len())
+	assert.Equal(t, -1, ctx.Astack.Len())
 }
 
 // returns true if the value at the top of the evaluation stack is a integer