Allow a specified address to disable/enable the Exchange

packages/protocol/contracts/baklava/Freezable.sol

@@ -0,0 +1,43 @@
+pragma solidity ^0.5.3;
+
+
+contract Freezable {
+  bool public frozen;
+  address public freezer;
+
+  // onlyFreezer functions can only be called by the specified `freezer` address
+  modifier onlyFreezer() {
+    require(msg.sender == freezer);
+    _;
+  }
+
+  // onlyWhenNotFrozen functions can only be called when `frozen` is false, otherwise they will
+  // revert.
+  modifier onlyWhenNotFrozen() {
+    require(!frozen);
+    _;
+  }
+
+  /**
+   * @notice Sets the address that is allowed to freeze/unfreeze the contract.
+   * @param _freezer The address that is allowed to freeze/unfree the contract
+   * @dev This function is `internal` and leaves its permissioning up to the inheriting contract.
+   */
+  function _setFreezer(address _freezer) internal {
+    freezer = _freezer;
+  }
+
+  /**
+   * @notice Freezes the contract, disabling `onlyWhenNotFrozen` functions.
+   */
+  function freeze() external onlyFreezer {
+    frozen = true;
+  }
+
+  /**
+   * @notice Unreezes the contract, enabling `onlyWhenNotFrozen` functions.
+   */
+  function unfreeze() external onlyFreezer {
+    frozen = false;
+  }
+}

packages/protocol/contracts/baklava/test/FreezableTest.sol

@@ -0,0 +1,20 @@
+pragma solidity ^0.5.8;
+
+import "../Freezable.sol";
+
+
+contract FreezableTest is Freezable {
+  event FunctionCalled();
+
+  function setFreezer(address _freezer) external {
+    _setFreezer(_freezer);
+  }
+
+  function freezableFunction() external onlyWhenNotFrozen {
+    emit FunctionCalled();
+  }
+
+  function nonfreezableFunction() external {
+    emit FunctionCalled();
+  }
+}

packages/protocol/contracts/stability/Exchange.sol

@@ -10,14 +10,15 @@ import "./interfaces/IStableToken.sol";
 import "../common/FractionUtil.sol";
 import "../common/Initializable.sol";
 import "../common/FixidityLib.sol";
+import "../baklava/Freezable.sol";
 import "../common/UsingRegistry.sol";
 
 
 /**
  * @title Contract that allows to exchange StableToken for GoldToken and vice versa
  * using a Constant Product Market Maker Model
  */
-contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, ReentrancyGuard {
+contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, ReentrancyGuard, Freezable {
   using SafeMath for uint256;
   using FractionUtil for FractionUtil.Fraction;
   using FixidityLib for FixidityLib.Fraction;
@@ -89,6 +90,7 @@ contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, Reentranc
    */
   function initialize(
     address registryAddress,
+    address _freezer,
     address stableToken,
     uint256 _spread,
     uint256 _reserveFraction,
@@ -99,6 +101,7 @@ contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, Reentranc
     initializer
   {
     _transferOwnership(msg.sender);
+    setFreezer(_freezer);
     setRegistry(registryAddress);
     setStableToken(stableToken);
     setSpread(_spread);
@@ -116,13 +119,15 @@ contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, Reentranc
    * transaction to succeed
    * @param sellGold `true` if gold is the sell token
    * @return The amount of buyToken that was transfered
+   * @dev This function can be frozen using the Freezable interface.
    */
   function exchange(
     uint256 sellAmount,
     uint256 minBuyAmount,
     bool sellGold
   )
     external
+    onlyWhenNotFrozen
     updateBucketsIfNecessary
     nonReentrant
     returns (uint256)
@@ -254,6 +259,10 @@ contract Exchange is IExchange, Initializable, Ownable, UsingRegistry, Reentranc
     emit MinimumReportsSet(newMininumReports);
   }
 
+  function setFreezer(address freezer) public onlyOwner {
+    _setFreezer(freezer);
+  }
+
   /**
     * @notice Allows owner to set the Stable Token address
     * @param newStableToken The new address for Stable Token

packages/protocol/contracts/stability/interfaces/IExchange.sol

@@ -4,6 +4,7 @@ pragma solidity ^0.5.3;
 interface IExchange {
 
   function initialize(
+    address,
     address,
     address,
     uint256,

packages/protocol/migrations/09_exchange.ts

@@ -8,14 +8,17 @@ import {
 import { config } from '@celo/protocol/migrationsConfig'
 import { toFixed } from '@celo/utils/lib/fixidity'
 import { ExchangeInstance, ReserveInstance, StableTokenInstance } from 'types'
+const truffle = require('@celo/protocol/truffle-config.js')
 
-const initializeArgs = async (): Promise<any[]> => {
+const initializeArgs = async (networkName: string): Promise<any[]> => {
+  const network: any = truffle.networks[networkName]
   const stableToken: StableTokenInstance = await getDeployedProxiedContract<StableTokenInstance>(
     'StableToken',
     artifacts
   )
   return [
     config.registry.predeployedProxyAddress,
+    network.from,
     stableToken.address,
     toFixed(config.exchange.spread).toString(),
     toFixed(config.exchange.reserveFraction).toString(),

packages/protocol/test/baklava/freezable.ts

@@ -0,0 +1,91 @@
+import { assertSameAddress, assertLogMatches2, assertRevert } from '@celo/protocol/lib/test-utils'
+import { FreezableTestInstance } from 'types'
+
+contract('Freezable', (accounts: string[]) => {
+  const FreezableTest = artifacts.require('FreezableTest')
+  let freezableTest: FreezableTestInstance
+
+  beforeEach(async () => {
+    freezableTest = await FreezableTest.new()
+    freezableTest.setFreezer(accounts[0])
+  })
+
+  describe('_setFreezer', () => {
+    it('should allow owner to change the freezer', async () => {
+      // _setFreezer is internal in Freezable, FreezableTest wraps around it with setFreezer
+      await freezableTest.setFreezer(accounts[1])
+      const freezer = await freezableTest.freezer()
+      assertSameAddress(freezer, accounts[1])
+    })
+  })
+
+  describe('when the contract is not frozen', () => {
+    it('should allow freezable functions to be called', async () => {
+      const resp = await freezableTest.freezableFunction()
+      assert.equal(resp.logs.length, 1)
+      const log = resp.logs[0]
+      assertLogMatches2(log, {
+        event: 'FunctionCalled',
+        args: {},
+      })
+    })
+
+    it('should allow non-freezable functions to be called', async () => {
+      const resp = await freezableTest.nonfreezableFunction()
+      assert.equal(resp.logs.length, 1)
+      const log = resp.logs[0]
+      assertLogMatches2(log, {
+        event: 'FunctionCalled',
+        args: {},
+      })
+    })
+  })
+
+  describe('freeze', () => {
+    it('should allow freezer to freeze the contract', async () => {
+      await freezableTest.freeze()
+      const frozen = await freezableTest.frozen()
+      assert.isTrue(frozen)
+    })
+
+    it('should not allow a non-freezer to freeze the contract', async () => {
+      await assertRevert(freezableTest.freeze({ from: accounts[1] }))
+    })
+  })
+
+  describe('unfreeze', () => {
+    beforeEach(async () => {
+      freezableTest.freeze()
+    })
+
+    it('should allow freezer to unfreeze the contract', async () => {
+      await freezableTest.unfreeze()
+      const frozen = await freezableTest.frozen()
+      assert.isFalse(frozen)
+    })
+
+    it('should not allow a non-freezer to unfreeze the contract', async () => {
+      await assertRevert(freezableTest.freeze({ from: accounts[1] }))
+    })
+  })
+
+  describe('when the contract is frozen', () => {
+    beforeEach(async () => {
+      await freezableTest.freeze()
+    })
+
+    it('should revert a freezable function', async () => {
+      await assertRevert(freezableTest.freezableFunction())
+    })
+
+    it('should not affect a non-freezable function', async () => {
+      const resp = await freezableTest.nonfreezableFunction()
+      assert.equal(resp.logs.length, 1)
+      const log = resp.logs[0]
+      assertLogMatches2(log, {
+        event: 'FunctionCalled',
+        args: {},
+      })
+    })
+  })
+})

packages/protocol/test/stability/exchange.ts

@@ -122,6 +122,7 @@ contract('Exchange', (accounts: string[]) => {
     exchange = await Exchange.new()
     await exchange.initialize(
       registry.address,
+      accounts[0],
       stableToken.address,
       spread,
       reserveFraction,
@@ -141,6 +142,7 @@ contract('Exchange', (accounts: string[]) => {
       await assertRevert(
         exchange.initialize(
           registry.address,
+          accounts[0],
           stableToken.address,
           spread,
           reserveFraction,
@@ -794,5 +796,16 @@ contract('Exchange', (accounts: string[]) => {
         })
       })
     })
+
+    describe('when the contract is frozen', () => {
+      beforeEach(async () => {
+        await exchange.freeze()
+      })
+
+      it('should revert', async () => {
+        await goldToken.approve(exchange.address, 1000)
+        await assertRevert(exchange.exchange(1000, 1, true))
+      })
+    })
   })
 })