expose bank module functionalities to evm smart contracts

[yihuang]

We have already exposed some native functionalities to EVM smart contracts through events with special signatures.
This proposal adds a new one to the family: __BankModuleTransfer(bytes from, bytes to, uint256 amount), it gives full control of a denom named after the contract address in bank module to the evm contract, the denom is named as crc20{contract_address}.

Implementation

The event handler in native module:

func BankModuleTransferHandler(contract address, from, to []byte, amount *big.Int) {
  denom := "crc20" + strings.ToLower(contract.String())
  coins := sdk.NewCoins(sdk.NewCoin(denom, amount))
  if len(from) == 0 {
    // mint
    bankKeeper.MintCoins(to, coins)
  } else if len(to) == 0 {
    // burn
    bankKeeper.BurnCoins(from, coins)
  } else {
    // normal transfer
    bankKeeper.Transfer(from, to, coins)
  }
}

How it's used in solidity:

event __BankModuleTransfer(address from, address to, uint256 amount);
contract CRC20 {
  function trasfer_to_bank_module(uint256 amount) {
    burn(msg.sender, amount);
    emit __BankModuleTransfer(0, addr, amount);
  }
  function transfer_from_bank_module(uint256 amount) {
    mint(msg.sender, amount);
    emit __BankModuleTransfer(addr, 0, amount);
  }
}

Security

DDos Attack

The cost of the bank module is not reflected in the gas consumption calculated by the EVM, could be vulnerable to abusive usage.