Add Multicall module

CHANGELOG.md

@@ -5,6 +5,7 @@
  * `IERC20Metadata`: add a new extended interface that includes the optional `name()`, `symbol()` and `decimals()` functions. ([#2561](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2561))
  * `ERC777`: make reception acquirement optional in `_mint`. ([#2552](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2552))
  * `ERC20Permit`: add a `_useNonce` to enable further usage of ERC712 signatures. ([#2565](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2565))
+ * `Multicall`: add abstract contract with `Multicall(bytes[] calldata data)` function to bundle multiple calls together ([#2608](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2608))
 
 ## 4.0.0 (2021-03-23)
 

contracts/mocks/BatchCallTokenMock.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../utils/Multicall.sol";
+import "./ERC20Mock.sol";
+
+contract MulticallTokenMock is ERC20Mock, Multicall {
+    constructor (uint256 initialBalance) ERC20Mock("MulticallToken", "BCT", msg.sender, initialBalance) {}
+}

contracts/utils/Multicall.sol

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "./Address.sol";
+
+/**
+ * @dev Provides a function to batch together multiple calls in a single external call.
+ */
+abstract contract Multicall {
+    /**
+    * @dev Receives and executes a batch of function calls on this contract.
+    */
+    function multicall(bytes[] calldata data) external payable returns (bytes[] memory results) {
+        results = new bytes[](data.length);
+        for (uint i = 0; i < data.length; i++) {
+            results[i] = Address.functionDelegateCall(address(this), data[i]);
+        }
+        return results;
+    }
+}

contracts/utils/README.adoc

@@ -6,6 +6,7 @@ NOTE: This document is better viewed at https://docs.openzeppelin.com/contracts/
 Miscellaneous contracts and libraries containing utility functions you can use to improve security, work with new data types, or safely use low-level primitives.
 
 The {Address}, {Arrays} and {Strings} libraries provide more operations related to these native data types, while {SafeCast} adds ways to safely convert between the different signed and unsigned numeric types.
+{Multicall} provides a function to batch together multiple calls in a single external call.
 
 For new data types:
 
@@ -92,3 +93,5 @@ Note that, in all cases, accounts simply _declare_ their interfaces, but they ar
 {{Counters}}
 
 {{Strings}}
+
+{{Multicall}}

docs/modules/ROOT/pages/utilities.adoc

@@ -99,3 +99,41 @@ Want to check if an address is a contract? Use xref:api:utils.adoc#Address[`Addr
 
 Want to keep track of some numbers that increment by 1 every time you want another one? Check out xref:api:utils.adoc#Counters[`Counters`]. This is useful for lots of things, like creating incremental identifiers, as shown on the xref:erc721.adoc[ERC721 guide].
 
+=== Multicall
+
+The `Multicall` abstract contract comes with a `multicall` function that bundles together multiple calls in a single external call. With it, external accounts may perform atomic operations comprising several function calls. This is not only useful for EOAs to make multiple calls in a single transaction, it's also a way to revert a previous call if a later one fails.
+
+Consider this dummy contract:
+
+[source,solidity]
+----
+// contracts/Box.sol
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/utils/Multicall.sol";
+
+contract Box is Multicall {
+    function foo() public {
+        ...
+    }
+
+    function bar() public {
+        ...
+    }
+}
+----
+
+This is how to call the `multicall` function using Truffle, allowing `foo` and `bar` to be called in a single transaction:
+[source,javascript]
+----
+// scripts/foobar.js
+
+const Box = artifacts.require('Box');
+const instance = await Box.new();
+
+await instance.multicall([
+    instance.contract.methods.foo().encodeABI(),
+    instance.contract.methods.bar().encodeABI()
+]);
+----

test/utils/BatchCall.test.js

@@ -0,0 +1,33 @@
+const { BN, expectRevert } = require('@openzeppelin/test-helpers');
+const MulticallTokenMock = artifacts.require('MulticallTokenMock');
+
+contract('MulticallToken', function (accounts) {
+  const [deployer, alice, bob] = accounts;
+  const amount = 12000;
+
+  beforeEach(async function () {
+    this.multicallToken = await MulticallTokenMock.new(new BN(amount), { from: deployer });
+  });
+
+  it('batches function calls', async function () {
+    expect(await this.multicallToken.balanceOf(alice)).to.be.bignumber.equal(new BN('0'));
+    expect(await this.multicallToken.balanceOf(bob)).to.be.bignumber.equal(new BN('0'));
+
+    await this.multicallToken.multicall([
+      this.multicallToken.contract.methods.transfer(alice, amount / 2).encodeABI(),
+      this.multicallToken.contract.methods.transfer(bob, amount / 3).encodeABI(),
+    ], { from: deployer });
+
+    expect(await this.multicallToken.balanceOf(alice)).to.be.bignumber.equal(new BN(amount / 2));
+    expect(await this.multicallToken.balanceOf(bob)).to.be.bignumber.equal(new BN(amount / 3));
+  });
+
+  it('bubbles up revert reasons', async function () {
+    const call = this.multicallToken.multicall([
+      this.multicallToken.contract.methods.transfer(alice, amount).encodeABI(),
+      this.multicallToken.contract.methods.transfer(bob, amount).encodeABI(),
+    ], { from: deployer });
+
+    await expectRevert(call, 'ERC20: transfer amount exceeds balance');
+  });
+});