Add a beacon proxy contract

contracts/mocks/BadBeacon.sol

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "../proxy/IBeacon.sol";
+
+contract BadBeaconNoImpl {
+}
+
+contract BadBeaconNotContract is IBeacon {
+    function implementation() external view override returns (address) {
+        return address(0x1);
+    }
+}

contracts/mocks/DummyImplementation.sol

@@ -19,11 +19,11 @@ contract DummyImplementation {
     value = 100;
   }
 
-  function initializeNonPayable(uint256 _value) public {
+  function initializeNonPayableWithValue(uint256 _value) public {
     value = _value;
   }
 
-  function initializePayable(uint256 _value) public payable {
+  function initializePayableWithValue(uint256 _value) public payable {
     value = _value;
   }
 
@@ -42,7 +42,7 @@ contract DummyImplementation {
   }
 
   function reverts() public pure {
-    require(false);
+    require(false, "DummyImplementation reverted");
   }
 }
 

contracts/proxy/BeaconProxy.sol

@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "./Proxy.sol";
+import "../utils/Address.sol";
+import "./IBeacon.sol";
+
+/**
+ * @dev This contract implements a proxy that gets the implementation address for each call from a {UpgradeableBeacon}.
+ *
+ * The beacon address is stored in storage slot `uint256(keccak256('eip1967.proxy.beacon')) - 1`, so that it doesn't
+ * conflict with the storage layout of the implementation behind the proxy.
+ */
+contract BeaconProxy is Proxy {
+    /**
+     * @dev The storage slot of the UpgradeableBeacon contract which defines the implementation for this proxy.
+     * This is bytes32(uint256(keccak256('eip1967.proxy.beacon')) - 1)) and is validated in the constructor.
+     */
+    bytes32 private constant _BEACON_SLOT = 0xa3f0ad74e5423aebfd80d3ef4346578335a9a72aeaee59ff6cb3582b35133d50;
+
+    /**
+     * @dev Initializes the proxy with `beacon`.
+     *
+     * If `data` is nonempty, it's used as data in a delegate call to the implementation returned by the beacon. This
+     * will typically be an encoded function call, and allows initializating the storage of the proxy like a Solidity
+     * constructor.
+     *
+     * Requirements:
+     *
+     * - `beacon` must be a contract with the interface {IBeacon}.
+     */
+    constructor(address beacon, bytes memory data) public payable {
+        assert(_BEACON_SLOT == bytes32(uint256(keccak256("eip1967.proxy.beacon")) - 1));
+        _setBeacon(beacon, data);
+    }
+
+    /**
+     * @dev Returns the current beacon address.
+     */
+    function _beacon() internal view returns (address beacon) {
+        bytes32 slot = _BEACON_SLOT;
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            beacon := sload(slot)
+        }
+    }
+
+    /**
+     * @dev Returns the current implementation address of the associated beacon.
+     */
+    function _implementation() internal view override returns (address) {
+        return IBeacon(_beacon()).implementation();
+    }
+
+    /**
+     * @dev Changes the proxy to use a new beacon.
+     *
+     * If `data` is nonempty, it's used as data in a delegate call to the implementation returned by the beacon. 
+     *
+     * Requirements:
+     *
+     * - `beacon` must be a contract.
+     * - The implementation returned by `beacon` must be a contract.
+     */
+    function _setBeacon(address beacon, bytes memory data) internal {
+        require(
+            Address.isContract(beacon),
+            "BeaconProxy: beacon is not a contract"
+        );
+        require(
+            Address.isContract(IBeacon(beacon).implementation()),
+            "BeaconProxy: beacon implementation is not a contract"
+        );
+        bytes32 slot = _BEACON_SLOT;
+
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            sstore(slot, beacon)
+        }
+
+        if (data.length > 0) {
+            Address.functionDelegateCall(_implementation(), data, "BeaconProxy: function call failed");
+        }
+    }
+}

contracts/proxy/IBeacon.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+/**
+ * @dev This is the interface that {BeaconProxy} expects of its beacon.
+ */
+interface IBeacon {
+    /**
+     * @dev Must return an address that can be used as a delegate call target.
+     *
+     * {BeaconProxy} will check that this address is a contract.
+     */
+    function implementation() external view returns (address);
+}

contracts/proxy/README.adoc

@@ -9,6 +9,8 @@ The abstract {Proxy} contract implements the core delegation functionality. If t
 
 Upgradeability is implemented in the {UpgradeableProxy} contract, although it provides only an internal upgrade interface. For an upgrade interface exposed externally to an admin, we provide {TransparentUpgradeableProxy}. Both of these contracts use the storage slots specified in https://eips.ethereum.org/EIPS/eip-1967[EIP1967] to avoid clashes with the storage of the implementation contract behind the proxy.
 
+An alternative upgradeability mechanism is provided in <<UpgradeableBeacon>>. This pattern, popularized by Dharma, allows multiple proxies to be upgraded to a different implementation in a single transaction. In this pattern, the proxy contract doesn't hold the implementation address in storage like {UpgradeableProxy}, but the address of a {UpgradeableBeacon} contract, which is where the implementation address is actually stored and retrieved from. The `upgrade` operations that change the implementation contract address are then sent to the beacon instead of to the proxy contract, and all proxies that follow that beacon are automatically upgraded.
+
 CAUTION: Using upgradeable proxies correctly and securely is a difficult task that requires deep knowledge of the proxy pattern, Solidity, and the EVM. Unless you want a lot of low level control, we recommend using the xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins] for Truffle and Buidler.
 
 == Core
@@ -19,6 +21,14 @@ CAUTION: Using upgradeable proxies correctly and securely is a difficult task th
 
 {{TransparentUpgradeableProxy}}
 
+== UpgradeableBeacon
+
+{{BeaconProxy}}
+
+{{IBeacon}}
+
+{{UpgradeableBeacon}}
+
 == Utilities
 
 {{Initializable}}

contracts/proxy/UpgradeableBeacon.sol

@@ -0,0 +1,64 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "./IBeacon.sol";
+import "../access/Ownable.sol";
+import "../utils/Address.sol";
+
+/**
+ * @dev This contract is used in conjunction with one or more instances of {BeaconProxy} to determine their
+ * implementation contract, which is where they will delegate all function calls.
+ *
+ * An owner is able to change the implementation the beacon points to, thus upgrading the proxies that use this beacon.
+ */
+contract UpgradeableBeacon is IBeacon, Ownable {
+    address private _implementation;
+
+    /**
+     * @dev Emitted when the implementation returned by the beacon is changed.
+     */
+    event Upgraded(address indexed implementation);
+
+    /**
+     * @dev Sets the address of the initial implementation, and the deployer account as the owner who can upgrade the
+     * beacon.
+     */
+    constructor(address implementation_) public {
+        _setImplementation(implementation_);
+    }
+
+    /**
+     * @dev Returns the current implementation address.
+     */
+    function implementation() public view override returns (address) {
+        return _implementation;
+    }
+
+    /**
+     * @dev Upgrades the beacon to a new implementation.
+     *
+     * Emits an {Upgraded} event.
+     *
+     * Requirements:
+     *
+     * - msg.sender must be the owner of the contract.
+     * - `newImplementation` must be a contract.
+     */
+    function upgradeTo(address newImplementation) public onlyOwner {
+        _setImplementation(newImplementation);
+        emit Upgraded(newImplementation);
+    }
+
+    /**
+     * @dev Sets the implementation contract address for this beacon
+     *
+     * Requirements:
+     *
+     * - `newImplementation` must be a contract.
+     */
+    function _setImplementation(address newImplementation) private {
+        require(Address.isContract(newImplementation), "UpgradeableBeacon: implementation is not a contract");
+        _implementation = newImplementation;
+    }
+}

test/proxy/BeaconProxy.test.js

@@ -0,0 +1,163 @@
+const { BN, expectRevert } = require('@openzeppelin/test-helpers');
+const ethereumjsUtil = require('ethereumjs-util');
+const { keccak256 } = ethereumjsUtil;
+
+const { expect } = require('chai');
+
+const UpgradeableBeacon = artifacts.require('UpgradeableBeacon');
+const BeaconProxy = artifacts.require('BeaconProxy');
+const DummyImplementation = artifacts.require('DummyImplementation');
+const DummyImplementationV2 = artifacts.require('DummyImplementationV2');
+const BadBeaconNoImpl = artifacts.require('BadBeaconNoImpl');
+const BadBeaconNotContract = artifacts.require('BadBeaconNotContract');
+
+function toChecksumAddress (address) {
+  return ethereumjsUtil.toChecksumAddress('0x' + address.replace(/^0x/, '').padStart(40, '0'));
+}
+
+const BEACON_LABEL = 'eip1967.proxy.beacon';
+const BEACON_SLOT = '0x' + new BN(keccak256(Buffer.from(BEACON_LABEL))).subn(1).toString(16);
+
+contract('BeaconProxy', function (accounts) {
+  const [anotherAccount] = accounts;
+
+  describe('bad beacon is not accepted', async function () {
+    it('non-contract beacon', async function () {
+      await expectRevert(
+        BeaconProxy.new(anotherAccount, '0x'),
+        'BeaconProxy: beacon is not a contract',
+      );
+    });
+
+    it('non-compliant beacon', async function () {
+      const beacon = await BadBeaconNoImpl.new();
+      await expectRevert.unspecified(
+        BeaconProxy.new(beacon.address, '0x'),
+      );
+    });
+
+    it('non-contract implementation', async function () {
+      const beacon = await BadBeaconNotContract.new();
+      await expectRevert(
+        BeaconProxy.new(beacon.address, '0x'),
+        'BeaconProxy: beacon implementation is not a contract',
+      );
+    });
+  });
+
+  before('deploy implementation', async function () {
+    this.implementationV0 = await DummyImplementation.new();
+    this.implementationV1 = await DummyImplementationV2.new();
+  });
+
+  describe('initialization', function () {
+    before(function () {
+      this.assertInitialized = async ({ value, balance }) => {
+        const beaconAddress = toChecksumAddress(await web3.eth.getStorageAt(this.proxy.address, BEACON_SLOT));
+        expect(beaconAddress).to.equal(this.beacon.address);
+
+        const dummy = new DummyImplementation(this.proxy.address);
+        expect(await dummy.value()).to.bignumber.eq(value);
+
+        expect(await web3.eth.getBalance(this.proxy.address)).to.bignumber.eq(balance);
+      };
+    });
+
+    beforeEach('deploy beacon', async function () {
+      this.beacon = await UpgradeableBeacon.new(this.implementationV0.address);
+    });
+
+    it('no initialization', async function () {
+      const data = Buffer.from('');
+      const balance = '10';
+      this.proxy = await BeaconProxy.new(this.beacon.address, data, { value: balance });
+      await this.assertInitialized({ value: '0', balance });
+    });
+
+    it('non-payable initialization', async function () {
+      const value = '55';
+      const data = this.implementationV0.contract.methods
+        .initializeNonPayableWithValue(value)
+        .encodeABI();
+      this.proxy = await BeaconProxy.new(this.beacon.address, data);
+      await this.assertInitialized({ value, balance: '0' });
+    });
+
+    it('payable initialization', async function () {
+      const value = '55';
+      const data = this.implementationV0.contract.methods
+        .initializePayableWithValue(value)
+        .encodeABI();
+      const balance = '100';
+      this.proxy = await BeaconProxy.new(this.beacon.address, data, { value: balance });
+      await this.assertInitialized({ value, balance });
+    });
+
+    it('reverting initialization', async function () {
+      const data = this.implementationV0.contract.methods.reverts().encodeABI();
+      await expectRevert(
+        BeaconProxy.new(this.beacon.address, data),
+        'DummyImplementation reverted',
+      );
+    });
+  });
+
+  it('upgrade a proxy by upgrading its beacon', async function () {
+    const beacon = await UpgradeableBeacon.new(this.implementationV0.address);
+
+    const value = '10';
+    const data = this.implementationV0.contract.methods
+      .initializeNonPayableWithValue(value)
+      .encodeABI();
+    const proxy = await BeaconProxy.new(beacon.address, data);
+
+    const dummy = new DummyImplementation(proxy.address);
+
+    // test initial values
+    expect(await dummy.value()).to.bignumber.eq(value);
+
+    // test initial version
+    expect(await dummy.version()).to.eq('V1');
+
+    // upgrade beacon
+    await beacon.upgradeTo(this.implementationV1.address);
+
+    // test upgraded version
+    expect(await dummy.version()).to.eq('V2');
+  });
+
+  it('upgrade 2 proxies by upgrading shared beacon', async function () {
+    const value1 = '10';
+    const value2 = '42';
+
+    const beacon = await UpgradeableBeacon.new(this.implementationV0.address);
+
+    const proxy1InitializeData = this.implementationV0.contract.methods
+      .initializeNonPayableWithValue(value1)
+      .encodeABI();
+    const proxy1 = await BeaconProxy.new(beacon.address, proxy1InitializeData);
+
+    const proxy2InitializeData = this.implementationV0.contract.methods
+      .initializeNonPayableWithValue(value2)
+      .encodeABI();
+    const proxy2 = await BeaconProxy.new(beacon.address, proxy2InitializeData);
+
+    const dummy1 = new DummyImplementation(proxy1.address);
+    const dummy2 = new DummyImplementation(proxy2.address);
+
+    // test initial values
+    expect(await dummy1.value()).to.bignumber.eq(value1);
+    expect(await dummy2.value()).to.bignumber.eq(value2);
+
+    // test initial version
+    expect(await dummy1.version()).to.eq('V1');
+    expect(await dummy2.version()).to.eq('V1');
+
+    // upgrade beacon
+    await beacon.upgradeTo(this.implementationV1.address);
+
+    // test upgraded version
+    expect(await dummy1.version()).to.eq('V2');
+    expect(await dummy2.version()).to.eq('V2');
+  });
+});