eth_feeHistory - fix reward calculation from MsgEthereumTx

[loredanacirstea]

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, as in:
  https://github.com/ethereum/go-ethereum/blob/57cec892536270fc6dafae01ded2c528ffa370e9/eth/gasprice/feehistory.go#L106-L113
• use the MsgEthereumTx count to calculate rewards

Fixes #974

Closes: #XXX

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[loredanacirstea]

2000 was chosen as default in https://github.com/tharsis/ethermint/pull/734/files#diff-4d996264fd51e2936260254650a034814a870ddfa9f7df7398d53feba7c39ef6R64.
Standard default value is 0 and it was usually rewritten anyway by the code below this line.

[loredanacirstea]

Why the break? I see this same question was asked by @fedekunze :

I don't understand the purpose of this break here, are we only supporting 1 MsgEthereumTx message?
#734 (comment)

But I did not find the answer to it.

From what I understand, we should calculate the reward only from MsgEthereumTx and not from tendermintTx. But I have some questions:

• can we find multiple MsgEthereumTx in a tendermint Tx? (tx.GetMsgs())
• if yes, on what rules are the MsgEthereumTx batched in a Tx?

[fedekunze]

yes, we can find MsgEthereumTx in a Tendermint Tx. There are no filters, we can only iterate over the msgs

[fedekunze]

ACK, can you add a CHANGELOG entry? 👍