Add refund to execution result in Go binding (#690)

This PR modifies the `Execute()` function from the Go bindings to return a `Result` struct instead of a list of result values to support the retrieval of the amount of refunded gas. This information is available in [evmc.h](https://github.com/ethereum/evmc/blob/v10.1.0/include/evmc/evmc.h)'s [evmc_result](https://github.com/ethereum/evmc/blob/v10.1.0/include/evmc/evmc.h#L404-L410), but was not exposed in the Go binding so far.

Author: HerbertJordan

bindings/go/evmc/evmc.go

@@ -195,10 +195,16 @@ func (vm *VM) SetOption(name string, value string) (err error) {
 	return err
 }
 
+type Result struct {
+	Output    []byte
+	GasLeft   int64
+	GasRefund int64
+}
+
 func (vm *VM) Execute(ctx HostContext, rev Revision,
 	kind CallKind, static bool, depth int, gas int64,
 	recipient Address, sender Address, input []byte, value Hash,
-	code []byte) (output []byte, gasLeft int64, err error) {
+	code []byte) (res Result, err error) {
 
 	flags := C.uint32_t(0)
 	if static {
@@ -216,8 +222,9 @@ func (vm *VM) Execute(ctx HostContext, rev Revision,
 		bytesPtr(code), C.size_t(len(code)))
 	removeHostContext(ctxId)
 
-	output = C.GoBytes(unsafe.Pointer(result.output_data), C.int(result.output_size))
-	gasLeft = int64(result.gas_left)
+	res.Output = C.GoBytes(unsafe.Pointer(result.output_data), C.int(result.output_size))
+	res.GasLeft = int64(result.gas_left)
+	res.GasRefund = int64(result.gas_refund)
 	if result.status_code != C.EVMC_SUCCESS {
 		err = Error(result.status_code)
 	}
@@ -226,7 +233,7 @@ func (vm *VM) Execute(ctx HostContext, rev Revision,
 		C.evmc_release_result(&result)
 	}
 
-	return output, gasLeft, err
+	return res, err
 }
 
 var (

bindings/go/evmc/evmc_test.go

@@ -47,13 +47,16 @@ func TestExecuteEmptyCode(t *testing.T) {
 
 	addr := Address{}
 	h := Hash{}
-	output, gasLeft, err := vm.Execute(nil, Byzantium, Call, false, 1, 999, addr, addr, nil, h, nil)
+	result, err := vm.Execute(nil, Byzantium, Call, false, 1, 999, addr, addr, nil, h, nil)
 
-	if bytes.Compare(output, []byte("")) != 0 {
-		t.Errorf("execution unexpected output: %x", output)
+	if !bytes.Equal(result.Output, []byte("")) {
+		t.Errorf("execution unexpected output: %x", result.Output)
 	}
-	if gasLeft != 999 {
-		t.Errorf("execution gas left is incorrect: %d", gasLeft)
+	if result.GasLeft != 999 {
+		t.Errorf("execution gas left is incorrect: %d", result.GasLeft)
+	}
+	if result.GasRefund != 0 {
+		t.Errorf("execution gas refund is incorrect: %d", result.GasRefund)
 	}
 	if err != nil {
 		t.Errorf("execution returned unexpected error: %v", err)

bindings/go/evmc/host_test.go

@@ -82,15 +82,17 @@ func TestGetBlockNumberFromTxContext(t *testing.T) {
 	host := &testHostContext{}
 	addr := Address{}
 	h := Hash{}
-	output, gasLeft, err := vm.Execute(host, Byzantium, Call, false, 1, 100, addr, addr, nil, h, code)
+	result, err := vm.Execute(host, Byzantium, Call, false, 1, 100, addr, addr, nil, h, code)
+	output := result.Output
+	gasLeft := result.GasLeft
 
 	if len(output) != 32 {
 		t.Errorf("unexpected output size: %d", len(output))
 	}
 
 	// Should return value 42 (0x2a) as defined in GetTxContext().
 	expectedOutput := []byte("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x2a")
-	if bytes.Compare(output, expectedOutput) != 0 {
+	if !bytes.Equal(output, expectedOutput) {
 		t.Errorf("execution unexpected output: %x", output)
 	}
 	if gasLeft != 94 {
@@ -111,12 +113,14 @@ func TestCall(t *testing.T) {
 	host := &testHostContext{}
 	addr := Address{}
 	h := Hash{}
-	output, gasLeft, err := vm.Execute(host, Byzantium, Call, false, 1, 100, addr, addr, nil, h, code)
+	result, err := vm.Execute(host, Byzantium, Call, false, 1, 100, addr, addr, nil, h, code)
+	output := result.Output
+	gasLeft := result.GasLeft
 
 	if len(output) != 34 {
 		t.Errorf("execution unexpected output length: %d", len(output))
 	}
-	if bytes.Compare(output, []byte("output from testHostContext.Call()")) != 0 {
+	if !bytes.Equal(output, []byte("output from testHostContext.Call()")) {
 		t.Errorf("execution unexpected output: %s", output)
 	}
 	if gasLeft != 89 {