## Account Abstraction Lesson 23: zkSync Transaction Simulations We have gone through a lot, but now we are ready to do some coding. In this lesson we will be working inside the `validateTransaction` function. A heavy focus here will be on steps 2 and 3 of the validation phase - validate and update the nonce. We will: - Add Headers - Update foundryup for zkSync - Learn about System Contract Calls - Call NonceHolder - increment nonce by 1 Let's get it! --- ### Headers and Updates Let's start by making two updates to our code. 1. adding some headers to our code 2. updating foundryup for zksync in our terminal --- ```js foudryup - zksync; ``` ```js forge build-zksync ``` --- > ❗ **NOTE** This may take a moment and the output will have a lot of yellow warnings. These can be safely ignored. **External Functions** Paste this above your `validateTransactions` function. --- ```js /*////////////////////////////////////////////////////////////// EXTERNAL FUNCTIONS //////////////////////////////////////////////////////////////*/ ``` --- **Internal Functions** Paste this below your `prepareForPaymaster` function. --- ```js /*////////////////////////////////////////////////////////////// INTERNAL FUNCTIONS //////////////////////////////////////////////////////////////*/ ``` --- ### Validate Transaction Let's get started with our `validateTransaction` function. Step 2 of the validation phase requires this function to update the nonce and validate the transaction. Additionally, we will use this function to check if we have enough money in our account. ```js /*////////////////////////////////////////////////////////////// EXTERNAL FUNCTIONS //////////////////////////////////////////////////////////////*/ /** * @notice must increase the nonce * @notice must validate the transaction (check the owner signed the transaction) * @notice also check to see if we have enough money in our account */ function validateTransaction(bytes32 _txHash, bytes32 _suggestedSignedHash, Transaction memory _transaction) external payable returns (bytes4 magic) { // Call NonceHolder // increment nonce by 1 } ``` --- In order for us to make this work, we'll have to make a systems contract call. In [NonceHolder.sol](https://github.com/Cyfrin/foundry-era-contracts/blob/3f99de4a37b126c5cb0466067f37be0c932167b2/src/system-contracts/contracts/NonceHolder.sol) we will need to call the `incrementMinNonceIfEquals` function. This will increment the nonce by 1. --- **<span style="color:red">NonceHolder.sol</span>** ```js /// @notice A convenience method to increment the minimal nonce if it is equal /// to the `_expectedNonce`. /// @param _expectedNonce The expected minimal nonce for the account. function incrementMinNonceIfEquals(uint256 _expectedNonce) external onlySystemCall { uint256 addressAsKey = uint256(uint160(msg.sender)); uint256 oldRawNonce = rawNonces[addressAsKey]; (, uint256 oldMinNonce) = _splitRawNonce(oldRawNonce); require(oldMinNonce == _expectedNonce, "Incorrect nonce"); unchecked { rawNonces[addressAsKey] = oldRawNonce + 1; } } ``` --- ### Calling System Contracts To do this, we are first going to have to add a flag under our `remappings` in `foundry.toml`. ```toml is-system = true ``` --- ### <span style="color:red">VERY IMPORTANT NOTE!</span> > ❗ **NOTE** As of the current recording (July 2024) we wil instead have to add `--system-mode=true` in the command line when we compile. More on this later in the course. As alway, please refer to the GitHub repo for up-to-date code. --- For now, all you need to know is that when you pass the isSystem flag to the contract, it will in-line replace all the simulations with the zkSync isSystem call counterpart. It transforms it into a system contract call. **This is very complex and not something you need to have nailed down. Don't get bogged down here.** To make things a bit easier for us we are going to import the [SystemsContractCaller library.](https://github.com/Cyfrin/foundry-era-contracts/blob/3f99de4a37b126c5cb0466067f37be0c932167b2/src/system-contracts/contracts/libraries/SystemContractsCaller.sol) --- ```js import { SystemContractsCaller } from "lib/foundry-era-contracts/src/system-contracts/contracts/libraries/SystemContractsCaller.sol"; ``` --- Around line 142 you will see the `systemCallWithPropagatedRevert` function. This will help us a lot with our systems contract calls. **<span style="color:red">SystemContractsCaller.sol</span>** ```js /// @notice Makes a call with the `isSystem` flag. /// @param gasLimit The gas limit for the call. /// @param to The address to call. /// @param value The value to pass with the transaction. /// @param data The calldata. /// @return returnData The returndata of the transaction. In case the transaction reverts, the error /// bubbles up to the parent frame. /// @dev Note, that the `isSystem` flag can only be set when calling system contracts. function systemCallWithPropagatedRevert( uint32 gasLimit, address to, uint128 value, bytes memory data ) internal returns (bytes memory returnData) { bool success; (success, returnData) = systemCallWithReturndata(gasLimit, to, value, data); if (!success) { assembly { let size := mload(returnData) revert(add(returnData, 0x20), size) } } } ``` --- ### Call and Increment the Nonce Now that we've got this, let's add it to our `validateTransaction` function back in our `ZkMinimalAccount` contract. If you look back at the systemCallWithPropagatedRevert, you'll see that we will need to pass: - `uint32 gasLimit` what ever gas is left for us - `address to` the Nonce Holder System Contract (import from `Constants.sol`) - `uint128 value` 0 ether transferred - `bytes memory data` we'll use `abi.encodeCall()`, and we'll need to: - import `INonceHolder` - pass `INonceHolder.incrementMinNonceIfEquals` to take nonce - then add one to that nonce `(_transaction.nonce)` > ❗ **NOTE** Addresses in Constants.sol may change over time. Please consult the `Makefile` in our repo for updated versions. **<span style="color:red">ZkMinimalAccount.sol</span>** We'll need a couple of imports first. ```js import { NONCE_HOLDER_SYSTEM_CONTRACT } from "lib/foundry-era-contracts/src/system-contracts/contracts/Constants.sol"; import { INonceHolder } from "lib/foundry-era-contracts/src/system-contracts/contracts/interfaces/INonceHolder.sol"; ``` **Inside `validateTransaction` function** ```js { // Call NonceHolder // increment nonce by 1 // call(x,y,z ) -> system contract call SystemContractsCaller.systemCallWithPropagatedRevert( uint32(gasleft()), address(NONCE_HOLDER_SYSTEM_CONTRACT), 0, abi.encodeCall(INonceHolder.incrementMinNonceIfEquals, _transaction.nonce) ); } ``` --- ### Summary In summary, this code makes a system call to the NonceHolder contract to conditionally increment the transaction nonce, ensuring the nonce handling mechanism in zkSync's account abstraction is correctly followed. - `uint32(gasleft())` passes the remaining gas for the call to ensure the system call has enough gas to execute. - `address(NONCE_HOLDER_SYSTEM_CONTRACT)` specifies the target system contract responsible for managing nonces. - 0 specifies that no Ether is transferred with the call. - `abi.encodeCall(INonceHolder.incrementMinNonceIfEquals, (_transaction.nonce))` encodes the call to increment the nonce if it matches the provided nonce in `_transaction.nonce`. **Phew!** That was a lot to take in. If you need to, go back and have a look at some of these contracts and their functions that we have imported. As always, we'll do a bit of review and you can move on when you are ready. --- ### Questions for Review --- <summary>1. What does SystemContractsCaller.systemCallWithPropagatedRevert() do in our code?</summary> --- <details> **<summary><span style="color:red">Click for Answers</span></summary>** It makes a system call to the NonceHolder contract to conditionally increment the transaction nonce, ensuring the nonce handling mechanism in zkSync's account abstraction is correctly followed. </details> <summary>2. Which function in NonceHolder.sol is called to increment the nonce?</summary> --- <details> **<summary><span style="color:red">Click for Answers</span></summary>** incrementMinNonceIfEquals </details> <summary>3. What library did we import to help with system contract calls?</summary> --- <details> **<summary><span style="color:red">Click for Answers</span></summary>** SystemContractsCaller </details>