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Hedera Forking for System Contracts

This projects allows Smart Contract developers working on Hedera to use fork testing while using Hedera System Contracts. It does so by providing an emulation layer for the Hedera Token Service (more System Contracts to come) written in Solidity. Given it is written in Solidity, it can be executed in a forked network environment, such as Foundry or Hardhat.

You can use either our Foundry library or Hardhat plugin to enable HTS emulation in your project.

See Hedera Token Service Supported Methods for a list of methods currently implemented as part of this project.

Important

The HTS emulation contract SHOULD BE ONLY used to ease development workflow when working with Hedera Tokens. The HTS emulation contract DOES NOT replicate Hedera Token Services fully. That is, behavior might differ when switching from local development to a real Hedera network. Always test your contracts against a real Hedera network before launching your contracts.

For detailed information on how this project works, see Internals.

Foundry library

We provide a Foundry library that enables fork testing when using HTS Tokens.

To see how this library works in details, see Foundry library in Internals.

Installation

First, install our library in your Foundry project

forge install hashgraph/hedera-forking

Set up

To use this library in your tests, you need to enable ffi. You can do so by adding the following lines to your .toml file

[profile.default]
ffi = true

Alternatively, you can add the --ffi flag to your execution script.

This is necessary because our library relies on curl to make HTTP requests to the Hedera remote network. This enables the library to fetch token state in the remote network. Given curl is an external command, ffi needs to be enabled.

Note

The Foundry library assumes it is running on a Unix-like operating system. In addition to curl, it uses bash, tail, sed and tr and to make requests and parse responses.

To activate HTS emulation in your tests, you need to add the following setup code in your test files. Import our wrapper function to deploy HTS emulation and enable cheat codes for it.

import {htsSetup} from "hedera-forking/contracts/htsSetup.sol";

and then invoke it in your test setup

    function setUp() public {
        htsSetup();
    }

Running your Tests

Now you can use Hedera Token Services and remote tokens as if they were deployed locally when fork testing. For example

// SPDX-License-Identifier: Apache-2.0
pragma solidity ^0.8.0;

import {Test} from "forge-std/Test.sol";
import {htsSetup} from "hedera-forking/contracts/htsSetup.sol";
import {IERC20} from "hedera-forking/contracts/IERC20.sol";

contract USDCExampleTest is Test {
    // https://hashscan.io/mainnet/token/0.0.456858
    address USDC_mainnet = 0x000000000000000000000000000000000006f89a;

    address private user1;

    function setUp() external {
        htsSetup();

        user1 = makeAddr("user1");
        deal(USDC_mainnet, user1, 1000 * 10e8);
    }

    function test_get_balance_of_existing_account() view external {
        // https://hashscan.io/mainnet/account/0.0.1528
        address usdcHolder = 0x00000000000000000000000000000000000005f8;
        // Balance retrieved from mainnet at block 72433403
        assertEq(IERC20(USDC_mainnet).balanceOf(usdcHolder), 28_525_752677);
    }

    function test_dealt_balance_of_local_account() view external {
        assertEq(IERC20(USDC_mainnet).balanceOf(user1), 1000 * 10e8);
    }
}

To run your tests, use the usual command

forge test --fork-url https://mainnet.hashio.io/api

You can also include a specific block number. For example, the test above is known to work at block 72433403

forge test --fork-url https://mainnet.hashio.io/api --fork-block-number 72433403

You can use all the tools and cheatcodes Foundry provides, e.g., console.log

// SPDX-License-Identifier: Apache-2.0
pragma solidity ^0.8.0;

import {Test, console} from "forge-std/Test.sol";
import {htsSetup} from "hedera-forking/contracts/htsSetup.sol";
import {IERC20} from "hedera-forking/contracts/IERC20.sol";

contract USDCConsoleExampleTest is Test {
    function setUp() external {
        htsSetup();
    }

    function test_using_console_log() view external {
        // https://hashscan.io/mainnet/token/0.0.456858
        address USDC_mainnet = 0x000000000000000000000000000000000006f89a;

        string memory name = IERC20(USDC_mainnet).name();
        string memory symbol = IERC20(USDC_mainnet).symbol();
        uint8 decimals = IERC20(USDC_mainnet).decimals();
        assertEq(name, "USD Coin");
        assertEq(symbol, "USDC");
        assertEq(decimals, 6);

        console.log("name: %s, symbol: %s, decimals: %d", name, symbol, decimals);
    }
}

Hardhat plugin

We provide a Hardhat plugin that enables fork testing when using HTS Tokens.

In addition, this plugin provides the following features

  • Sets up Hardfork history for Hedera networks. This solves the issue No known hardfork for execution on historical block.
  • Avoids Missing access list error. Hardhat throws this error when a blockNumber is provided in the forking configuration. The plugin solves this issue.

This plugin intercepts the calls made by Hardhat to fetch remote state, _i.e., eth_getCode and eth_getStorageAt, to provide emulation for HTS. It assigns the Hedera Token Service code to the 0x167 address. In your tests, you will be able to query Hedera Token data as if they were stored as regular Smart Contracts.

To see how this plugin works in details, see Hardhat plugin in Internals.

Installation

To use this plugin, install it via your package manager.

If you are using npm

npm install --save-dev @hashgraph/system-contracts-forking

or using yarn

yarn add --dev @hashgraph/system-contracts-forking

Set up

Next, add the following line to the top of your Hardhat config file, e.g., hardhat.config.js

require('@hashgraph/system-contracts-forking/plugin');

or if you are using TypeScript, include the following line into your hardhat.config.ts

import '@hashgraph/system-contracts-forking/plugin';

This will automatically create a worker thread to intercept calls to fetch remote state. You can then proceed with writing your tests, and the plugin will allow you to query Hedera token data seamlessly.

Configuration

By default, the plugin uses the Hedera Mirror Node based on the guessed chain ID from the currently forked network.

Two additional values needs to be set in order to activate the plugin.

  • chainId. The call to eth_chainId of configuration parameter hardhat.forking.url needs to match the chainId configuration argument. Only chain IDs 295 (Mainnet), 296 (Testnet), and 297 (Previewnet) are supported.
  • workerPort. Any free port to start the worker to intercept Hardhat calls to fetch remote state.

For example

    networks: {
        hardhat: {
            forking: {
                url: 'https://mainnet.hashio.io/api',
                // This allows Hardhat to enable JSON-RPC's response cache.
                // Forking from a block is not fully integrated yet into HTS emulation.
                blockNumber: 70531900,
                chainId: 295,
                workerPort: 1235,
            },
        },
    },

Note

These configuration settings cannot be set automatically by the Plugin. This is because the way Hardhat plugins are loaded by Hardhat. The main issue is "since Hardhat needs to be loadable via require call, configuration must be synchronous". See here and here for more details.

Creating a Worker so we can hook into eth_getCode and eth_getStorageAt to provide HTS emulation and querying the chainId of a remote network are asynchronous operations. That is why we need to shift the setting of chainId and workerPort to the user.

Running Your Tests

For example, to query USDC information on mainnet you can use the following test

const { expect } = require('chai');
const { ethers: { getContractAt } } = require('hardhat');

describe('USDC example -- informational', function () {
    it('should get name, symbol and decimals', async function () {
        // https://hashscan.io/mainnet/token/0.0.456858
        const usdc = await getContractAt('IERC20', '0x000000000000000000000000000000000006f89a');
        expect(await usdc['name']()).to.be.equal('USD Coin');
        expect(await usdc['symbol']()).to.be.equal('USDC');
        expect(await usdc['decimals']()).to.be.equal(6n);
    });
});

Then run your Hardhat tests as usual

npx hardhat

You can also query an existing account's balance. For example using the blockNumber as shown above in Configuration, you can query the USDC balance for an existing account.

const { expect } = require('chai');
const { ethers: { getContractAt } } = require('hardhat');

describe('USDC example -- balanceOf', function () {
    it('should get `balanceOf` account holder', async function () {
        // https://hashscan.io/mainnet/token/0.0.456858
        const usdc = await getContractAt('IERC20', '0x000000000000000000000000000000000006f89a');

        // https://hashscan.io/mainnet/account/0.0.6279
        const holderAddress = '0x0000000000000000000000000000000000001887';
        expect(await usdc['balanceOf'](holderAddress)).to.be.equal(31_166_366226);
    });
});

You can also perform modifications in the forked network. Let's see an example where we execute the transfer method from an existing account to a local Hardhat signer. It is usually paired with Fixtures, so that each test can start in a known state. Moreover, you can use the tools you are already familiar with, for example, hardhat_impersonateAccount.

const { expect } = require('chai');
const { ethers: { getSigner, getSigners, getContractAt }, network: { provider } } = require('hardhat');
const { loadFixture } = require('@nomicfoundation/hardhat-toolbox/network-helpers');

describe('USDC example -- transfer', function () {
    async function id() {
        return [(await getSigners())[0]];
    }

    it("should `tranfer` tokens from account holder to one of Hardhat' signers", async function () {
        const [receiver] = await loadFixture(id);

        // https://hashscan.io/mainnet/account/0.0.6279
        const holderAddress = '0x0000000000000000000000000000000000001887';
        await provider.request({
            method: 'hardhat_impersonateAccount',
            params: [holderAddress],
        });
        const holder = await getSigner(holderAddress);

        // https://hashscan.io/mainnet/token/0.0.456858
        const usdc = await getContractAt('IERC20', '0x000000000000000000000000000000000006f89a');

        expect(await usdc['balanceOf'](receiver.address)).to.be.equal(0n);

        await usdc.connect(holder)['transfer'](receiver, 10_000_000n);

        expect(await usdc['balanceOf'](receiver.address)).to.be.equal(10_000_000n);
    });
});

Hedera Token Service Supported Methods

Given your HTS token address, you can invoke these functions whether the token is either fungible or non-fungible.

Fungible Tokens

The following methods and events are applicable to Fungible Tokens.

ERC20 Interface

Function Comment
allowance(address owner, address spender) view Returns the remaining number of tokens that spender will be allowed to spend on behalf of owner through {transferFrom}. This is zero by default. This value changes when {approve} or {transferFrom} are called.
approve(address spender, uint256 amount) Sets a value amount of tokens as the allowance of spender over the caller's tokens. Returns a boolean value indicating whether the operation succeeded.
balanceOf(address account) view Returns the value of tokens owned by account.
decimals() view Returns the decimals places of the token.
name() view Returns the name of the token.
symbol() view Returns the symbol of the token.
totalSupply() view Returns the value of tokens in existence.
transfer(address recipient, uint256 amount) Moves a value amount of tokens from the caller's account to to. Returns a boolean value indicating whether the operation succeeded.
transferFrom(address sender, address recipient, uint256 amount) Moves a value amount of tokens from from to to using the allowance mechanism. value is then deducted from the caller's allowance. Returns a boolean value indicating whether the operation succeeded.
Event Comment
Approval(address indexed owner, address indexed spender, uint256 amount) Emitted when the allowance of a spender for an owner is set by a call to {approve}. value is the new allowance.
Transfer(address indexed from, address indexed to, uint256 amount) Emitted when value tokens are moved from one account (from) to another (to). Note that value may be zero.

Association Methods Interface

Function Comment
associate() Associates the calling account with the token This function allows an account to opt-in to receive the token
dissociate() Dissociates the calling account from the token This function allows an account to opt-out from receiving the token
isAssociated() view Checks if the calling account is associated with the token This function returns the association status of the calling account

Non-Fungible Tokens

The following methods and events are applicable to Non-Fungible Tokens.

Note

ERC721 support coming soon!

Association Methods Interface

Function Comment
associate() Associates the calling account with the token This function allows an account to opt-in to receive the token
dissociate() Dissociates the calling account from the token This function allows an account to opt-out from receiving the token
isAssociated() view Checks if the calling account is associated with the token This function returns the association status of the calling account

Hedera Token Service (located at address 0x167)

The following methods can be invoked on the Hedera Token Service contract located at address 0x167.

Function Comment
burnToken(address token, int64 amount, int64[] serialNumbers) Burns an amount of the token from the defined treasury account
getTokenInfo(address token) Query token info
mintToken(address token, int64 amount, bytes[] metadata) Mints an amount of the token to the defined treasury account

JS Library

Service and application builders can use the core state emulation library. These functions can be embedded in a JSON-RPC implementation to enable System Contracts fork testing without using the Hardhat plugin.

const { getHtsCode, getHtsStorageAt, getHIP719Code } = require('@hashgraph/system-contracts-forking');
  • getHtsCode. Gets the runtime bytecode for the Solidity implementation of the HTS System Contract.
  • getHtsStorageAt. Allows the user to query the state of an HTS Token as if it were a standard smart contract.
  • getHIP719Code. Returns the token proxy contract bytecode for the given address. Based on the proxy contract defined by HIP-719.

See index.d.ts for more details.

At its core, the Hardhat plugin is supported by these functions.

Build

This repo consists of a Foundry project and an npm package. A Foundry project, to compile and test the HtsSystemContract.sol contract. And an npm package that implements both eth_getCode and eth_getStorageAt JSON-RPC methods when HTS emulation is involved together with the Hardhat plugin that uses these methods.

To compile the HtsSystemContract, its dependencies and the test contracts run

forge build

There is no compilation step to build the npm package. However, you can type-check it by running

npm run type-check

The code examples and the tables for supported methods included in this README are automatically generated. Run the following command to generate them

npm run make:readme

This allow us to ensure that all examples and tables are never outdated (if we change them we just need to rerun the above command) and that the examples are executed properly (we run them on CI).

Code fences that contains a file name after the language definition, e.g.,

  ```solidity examples/foundry-hts/USDC.t.sol
  ```

or comments such as

  <!-- !./scripts/abi-table.js out/IERC20.sol/IERC20.json out/IERC20.sol/IERC20Events.json -->
  <!-- -->

will be expanded with either the content of the file or, when the file descriptor starts with a !, with the standard output of the application.

Tests

Tip

The scripts/create-token.js file was used to deploy Tokens using HTS to testnet for testing purposes.

getHtsCode, getHtsStorageAt and getHIP719Code Unit Tests

These tests only involve testing getHtsCode, getHtsStorageAt and getHIP719Code functions. In other words, they do not use the HtsSystemContract bytecode, only its storageLayout definition. To run them, execute

npm run test

Tip

You may want to run the npm tests without the Hardhat plugin tests. To do so, you can use the --fgrep or --grep flags together with --invert, for example

npm run test -- --grep ::plugin --invert

All top-level mocha's describes should be prefixed with :: so it can be easy to filter by tests.

HtsSystemContract Solidity tests + local storage emulation (without forking)

We use Foundry cheatcodes, i.e., vm.store and vm.load, to provide local storage emulation that allow us to run HtsSystemContract Solidity tests without starting out a separate JSON-RPC process.

forge test

Tip

You can use the --match-contract flag to filter the tests to be executed if needed. In addition, usually when debugging the contract under test we are interested in getting the Solidity traces. We can use the forge flag -vvvv to display them. For example

forge test --match-contract TokenTest -vvvv

When running these tests without forking from a remote network, the package @hashgraph/hedera-forking is not under test.

HtsSystemContract Solidity tests + Mirror Node FFI (with mocked curl)

This is used to test HtsSystemContract(Json)+MirrorNodeFFI contracts. It is the implementation used by Foundry users.

We provide a curl mock in the test/scripts folder to avoid depending on a remote Mirror Node and thus making the tests more robust. By modifiying the PATH environment variable, the mocked curl is used instead.

PATH=./test/scripts:$PATH forge test --fork-url https://testnet.hashio.io/api --fork-block-number 8535327

In case needed, there is a trace log of requests made by mocked curl in scripts/curl.log

cat test/scripts/curl.log

HtsSystemContract Solidity tests + JSON-RPC mock server for storage emulation (with forking)

These Solidity tests are used to test both the HtsSystemContract and the npm package. It is the implementation used by Hardhat users.

Instead of starting a local-node or using a remote network, they use the json-rpc-mock.js script as a backend without the need for any additional services, thus making the tests more robust. This is the network Foundry's Anvil is forking from.

In a separate terminal run the JSON-RPC Mock Server

./test/scripts/json-rpc-mock.js

Then run

forge test --fork-url http://localhost:7546 --no-storage-caching

Important

The --no-storage-caching flag disables the JSON-RPC calls cache, which is important to make sure the json-rpc-mock.js is reached in each JSON-RPC call. See https://book.getfoundry.sh/reference/forge/forge-test#description for more information.

Support

If you have a question on how to use the product, please see our support guide.

Contributing

Contributions are welcome. Please see the contributing guide to see how you can get involved.

Code of Conduct

This project is governed by the Contributor Covenant Code of Conduct. By participating, you are expected to uphold this code of conduct. Please report unacceptable behavior to oss@hedera.com.

License

Apache License 2.0

Security

Please do not file a public ticket mentioning the vulnerability. Refer to the security policy defined in the SECURITY.md.

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