Simple Meta-Transaction Example

This repo shows how to implement an EIP-2771 compatible contract using the OpenZeppelin contract library. The example also relies on EIP-712 for typed data signing. The example uses the Hardhat framework for compilation and execution.

There is only one contract to inspect; Greeter.sol. The Greeter contract is nearly identical to the default Greeter contract created by the Hardhat sample project, except that the greet() function now emits a Greetings event that logs _msgSender.

Setup

git clone https://github.com/TtheBC01/Meta-Transaction-Example.git
cd Meta-Transaction-Example
npm install

Execution

Compile Greeter.sol like this:

npx hardhat compile

Run the unit tests that illustrate how to utilize EIP-2771 like this:

npx hardhat test

If you want to see the emitted Ethereum Events showing the value of _msgSender, use event tracing:

npx hardhat test --trace

Expected behavior

In the first unit test, a normal call to greet() is preformed by an account acting as an end user. You will see that the end user's funds are used in order to pay for the transaction fee. If you turn on event tracing, you will see the following event emitted:

CALL Greeter.greet()
   EVENT Greeter.Greatings(who=0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266, what="Hello, World!")

In the second unit test, a new account, relayer, will submit a transaction from an instance of the MinimalForwarder implementation which will implicitly make a call to the greet() function on behalf of the endUser account. After the transaction is mined, you will see the the balance of endUser remains the same and event tracing will show that the address of endUser was logged in the Ethereum event.

CALL MinimalForwarder.execute(req=[0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266, 0xDc64a140Aa3E981100a9becA4E685f962f0cF6C9, 0, 1000000, 0, 0xcfae3217], signature=0xfb9baab92637d5705af021d26a09fb9f074105a64e7ce75178a05ede934a86e066f31d4083f6a029e06fc136eed56e8812332a598473bab9a83c5a5638722b551c)
   STATICCALL UnknownContractAndFunction(to=0x0000000000000000000000000000000000000001, input=0x98046c9bd2d93e6f7746a1b40edb54e82cb00199c40fbe6e17647cf1d794751c000000000000000000000000000000000000000000000000000000000000001cfb9baab92637d5705af021d26a09fb9f074105a64e7ce75178a05ede934a86e066f31d4083f6a029e06fc136eed56e8812332a598473bab9a83c5a5638722b55, ret=0x000000000000000000000000f39fd6e51aad88f6f4ce6ab8827279cfffb92266)
   CALL Greeter.greet()
      EVENT Greeter.Greatings(who=0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266, what="Hello, World!")