add gas price=0 unit test, comments (#528)

* add handler comment

* add gas price=0 test

Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

x/evm/handler.go

@@ -60,9 +60,12 @@ func handleMsgEthereumTx(ctx sdk.Context, k Keeper, msg types.MsgEthereumTx) (*s
 		Simulate:     ctx.IsCheckTx(),
 	}
 
-	// Prepare db for logs
-	// TODO: block hash
+	// since the txCount is used by the stateDB, and a simulated tx is run only on the node it's submitted to,
+	// then this will cause the txCount/stateDB of the node that ran the simulated tx to be different than the
+	// other nodes, causing a consensus error
 	if !st.Simulate {
+		// Prepare db for logs
+		// TODO: block hash
 		k.CommitStateDB.Prepare(ethHash, common.Hash{}, k.TxCount)
 		k.TxCount++
 	}
@@ -146,8 +149,8 @@ func handleMsgEthermint(ctx sdk.Context, k Keeper, msg types.MsgEthermint) (*sdk
 		st.Recipient = &to
 	}
 
-	// Prepare db for logs
 	if !st.Simulate {
+		// Prepare db for logs
 		k.CommitStateDB.Prepare(ethHash, common.Hash{}, k.TxCount)
 		k.TxCount++
 	}

x/evm/types/msg_test.go

@@ -47,6 +47,7 @@ func TestMsgEthermintValidation(t *testing.T) {
 		{amount: sdk.NewInt(0), gasPrice: sdk.NewInt(100000), expectPass: true},
 		{amount: sdk.NewInt(-1), gasPrice: sdk.NewInt(100000), expectPass: false},
 		{amount: sdk.NewInt(100), gasPrice: sdk.NewInt(-1), expectPass: false},
+		{amount: sdk.NewInt(100), gasPrice: sdk.NewInt(0), expectPass: false},
 	}
 
 	for i, tc := range testCases {
@@ -116,6 +117,7 @@ func TestMsgEthereumTxValidation(t *testing.T) {
 		{msg: "pass", amount: big.NewInt(100), gasPrice: big.NewInt(100000), expectPass: true},
 		{msg: "invalid amount", amount: big.NewInt(-1), gasPrice: big.NewInt(100000), expectPass: false},
 		{msg: "invalid gas price", amount: big.NewInt(100), gasPrice: big.NewInt(-1), expectPass: false},
+		{msg: "invalid gas price", amount: big.NewInt(100), gasPrice: big.NewInt(0), expectPass: false},
 	}
 
 	for i, tc := range testCases {