makerdao / tx-manager

Solidity contract to make multiple calls in one Ethereum transaction. Used by Maker keepers.

Overview

This repository contains a transaction manager contract, allowing its owner to make multiple contract calls in one Ethereum transaction. This saves time, but what's most important it allows things like multi-step arbitrage to be executed in one transaction.

This transaction manager also helps to deal with ERC20 token transfers. If we haven't done anything to solve this issue, the transaction manager contract would have to own all funds (ERC20 tokens) any of the calls made by it may need. In our approach, requested ERC20 tokens all transferred from the caller to the transaction manager contract before the first call is made and returned to the caller after the last call.

The transaction manager needs to be approved to access ERC20 tokens. Because of that, we only allow the transaction manager to be used only by its owner.

Contract deployment

The TxManager contract takes no parameters.

Use Dapp (https://github.com/dapphub/dapp) to build and deploy the contract:

dapp build
ETH_GAS=2000000 dapp create TxManager

Contract usage

The contract has only one public function:

function execute(address[] tokens, bytes script) { … }

The tokens parameter is an array of ERC20 token addresses. For each of them, the maximum available allowance will be transferred from the caller to the contract before the first call. Remaining balances of each of these tokens will be returned to the caller after the last call.

The script parameter is a byte array representing the sequence of contract calls to be made. It consists of multiple call records concatenated together, whereas each record is built as follows:

+-----------------+---------------------+-----------------------...------+
|     address     |   calldata length   |         calldata               |
|                 |                     |                                |
|   (20 bytes)    |      (32 bytes)     |                                |
+-----------------+---------------------+-----------------------...------+

For example, if you want to make one call to 0x11111222223333344444333332222211111122222, the script may look like this 111112222233333444443333322222111111222220000000000000000000000000000000000000000000000000000000000000024a39f1c6c0000000000000000000000000000000000000000000000000000000000000064, the last part of it being calldata encoded with:

$ seth calldata 'cork(uint256)' 100
0xa39f1c6c0000000000000000000000000000000000000000000000000000000000000064

The script parameter should be a concatenation of records of all calls to be made.