add returnValue message on tracing

x/evm/keeper/grpc_query.go

@@ -544,11 +544,15 @@ func (k *Keeper) traceTx(
 	// Depending on the tracer type, format and return the trace result data.
 	switch tracer := tracer.(type) {
 	case *logger.StructLogger:
-		// TODO: Return proper returnValue
+		returnVal := fmt.Sprintf("%x", res.Return())
+		revert := res.Revert()
+		if len(revert) > 0 {
+			returnVal = fmt.Sprintf("%x", revert)
+		}
 		result = types.ExecutionResult{
 			Gas:         res.GasUsed,
 			Failed:      res.Failed(),
-			ReturnValue: "",
+			ReturnValue: returnVal,
 			StructLogs:  types.FormatLogs(tracer.StructLogs()),
 		}
 	case tracers.Tracer:

x/evm/types/tx.go

@@ -1,6 +1,29 @@
 package types
 
+import (
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/vm"
+)
+
 // Failed returns if the contract execution failed in vm errors
 func (m *MsgEthereumTxResponse) Failed() bool {
 	return len(m.VmError) > 0
 }
+
+// Return is a helper function to help caller distinguish between revert reason
+// and function return. Return returns the data after execution if no error occurs.
+func (m *MsgEthereumTxResponse) Return() []byte {
+	if m.Failed() {
+		return nil
+	}
+	return common.CopyBytes(m.Ret)
+}
+
+// Revert returns the concrete revert reason if the execution is aborted by `REVERT`
+// opcode. Note the reason can be nil if no data supplied with revert opcode.
+func (m *MsgEthereumTxResponse) Revert() []byte {
+	if m.VmError != vm.ErrExecutionReverted.Error() {
+		return nil
+	}
+	return common.CopyBytes(m.Ret)
+}