eth_feeHistory - fix reward calculation from MsgEthereumTx (evmos#990)

* eth_feeHistory - fix reward calculation from MsgEthereumTx

Problem:
- rewards calculation based on percentiles took into consideration the tendermint tx count instead of `MsgEthereumTx` count.
- `sorter := make(sortGasAndReward, tendermintTxCount)` creates an array of `{0 <nil>}` values.
The `nil` value from the `reward` attribute was never rewritten for tendermint Tx without any `MsgEthereumTx`

Fixes:
- return early if there are 0 `MsgEthereumTx`
- use the `MsgEthereumTx` count to calculate rewards

* Add change log

CHANGELOG.md

@@ -43,6 +43,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Bug Fixes
 
+* (rpc) [tharsis#975](https://github.com/tharsis/ethermint/pull/975) Fix unexpected `nil` values for `reward`, returned by `EffectiveGasTipValue(blockBaseFee)` in the `eth_feeHistory` RPC method.
 * (rpc) [#970] (https://github.com/tharsis/ethermint/pull/970) Fix unexpected nil reward values on `eth_feeHistory` response
 
 ### API Breaking

rpc/ethereum/backend/feebackend.go

@@ -61,14 +61,15 @@ func (e *EVMBackend) processBlock(
 	rewardCount := len(rewardPercentiles)
 	targetOneFeeHistory.Reward = make([]*big.Int, rewardCount)
 	for i := 0; i < rewardCount; i++ {
-		targetOneFeeHistory.Reward[i] = big.NewInt(2000)
+		targetOneFeeHistory.Reward[i] = big.NewInt(0)
 	}
 
 	// check tendermintTxs
 	tendermintTxs := tendermintBlock.Block.Txs
 	tendermintTxResults := tendermintBlockResult.TxsResults
 	tendermintTxCount := len(tendermintTxs)
-	sorter := make(sortGasAndReward, tendermintTxCount)
+
+	var sorter sortGasAndReward
 
 	for i := 0; i < tendermintTxCount; i++ {
 		eachTendermintTx := tendermintTxs[i]
@@ -90,29 +91,28 @@ func (e *EVMBackend) processBlock(
 			if reward == nil {
 				reward = big.NewInt(0)
 			}
-			sorter[i] = txGasAndReward{gasUsed: txGasUsed, reward: reward}
-			break
+			sorter = append(sorter, txGasAndReward{gasUsed: txGasUsed, reward: reward})
 		}
 	}
+
+	// return an all zero row if there are no transactions to gather data from
+	ethTxCount := len(sorter)
+	if ethTxCount == 0 {
+		return nil
+	}
+
 	sort.Sort(sorter)
 
 	var txIndex int
-	sumGasUsed := uint64(0)
-	if len(sorter) > 0 {
-		sumGasUsed = sorter[0].gasUsed
-	}
+	sumGasUsed := sorter[0].gasUsed
+
 	for i, p := range rewardPercentiles {
 		thresholdGasUsed := uint64(blockGasUsed * p / 100)
-		for sumGasUsed < thresholdGasUsed && txIndex < tendermintTxCount-1 {
+		for sumGasUsed < thresholdGasUsed && txIndex < ethTxCount-1 {
 			txIndex++
 			sumGasUsed += sorter[txIndex].gasUsed
 		}
-
-		chosenReward := big.NewInt(0)
-		if 0 <= txIndex && txIndex < len(sorter) {
-			chosenReward = sorter[txIndex].reward
-		}
-		targetOneFeeHistory.Reward[i] = chosenReward
+		targetOneFeeHistory.Reward[i] = sorter[txIndex].reward
 	}
 
 	return nil