Prevented users from front-running the Draw timestamp on Ethereum (#146)

pooltogether · Sep 30, 2021 · 42cc633 · 42cc633
1 parent cdaa546
commit 42cc633
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Showing 2 changed files with 15 additions and 5 deletions.
diff --git a/contracts/DrawBeacon.sol b/contracts/DrawBeacon.sol
@@ -198,6 +198,12 @@ contract DrawBeacon is IDrawBeacon,
     return isRngRequested() && isRngCompleted();
   }
 
+  /// @notice Calculates the next beacon start time, assuming all beacon periods have occurred between the last and now.
+  /// @return The next beacon period start time
+  function calculateNextBeaconPeriodStartTimeFromCurrentTime() external view returns (uint64) {
+    return _calculateNextBeaconPeriodStartTime(beaconPeriodStartedAt, beaconPeriodSeconds, _currentTime());
+  }
+
   /// @inheritdoc IDrawBeacon
   function calculateNextBeaconPeriodStartTime(uint256 currentTime) external view override returns (uint64) {
     return _calculateNextBeaconPeriodStartTime(beaconPeriodStartedAt, beaconPeriodSeconds, uint64(currentTime));
@@ -230,7 +236,8 @@ contract DrawBeacon is IDrawBeacon,
     DrawLib.Draw memory _draw = DrawLib.Draw({
       winningRandomNumber: randomNumber,
       drawId: _nextDrawId,
-      timestamp: _time,
+      // must use the startAward() timestamp to prevent front-running
+      timestamp: rngRequest.requestedAt,
       beaconPeriodStartedAt: _beaconPeriodStartedAt,
       beaconPeriodSeconds: _beaconPeriodSeconds
     });
@@ -239,13 +246,12 @@ contract DrawBeacon is IDrawBeacon,
       * The DrawBeacon (deployed on L1) will have a Manager role authorized to push history onto DrawHistory.
      */
     drawHistory.pushDraw(_draw);
-    
+
     // to avoid clock drift, we should calculate the start time based on the previous period start time.
     _beaconPeriodStartedAt = _calculateNextBeaconPeriodStartTime(_beaconPeriodStartedAt, _beaconPeriodSeconds, _time);
     beaconPeriodStartedAt = _beaconPeriodStartedAt;
     nextDrawId = _nextDrawId + 1;
 
-
     // Reset the rngReqeust state so Beacon period can start again.
     delete rngRequest;
 

diff --git a/test/DrawBeacon.test.ts b/test/DrawBeacon.test.ts
@@ -4,6 +4,7 @@ import { Signer } from '@ethersproject/abstract-signer';
 import { SignerWithAddress } from '@nomiclabs/hardhat-ethers/signers';
 import { constants, Contract, ContractFactory, utils } from 'ethers';
 import { expect } from 'chai';
+import { increaseTime } from './helpers/increaseTime';
 
 const debug = require('debug')('pt:DrawBeacon.test.ts');
 
@@ -319,6 +320,9 @@ describe('DrawBeacon', () => {
           beaconPeriodSeconds
         ]).returns(1)
 
+        await drawBeacon.setCurrentTime((await drawBeacon.beaconPeriodEndAt()).add(1000))
+        const nextStartTime = await drawBeacon.calculateNextBeaconPeriodStartTimeFromCurrentTime()
+
         expect(await drawBeacon.completeDraw())
           .to.emit(drawBeacon, 'DrawCompleted')
           .withArgs(
@@ -328,10 +332,10 @@ describe('DrawBeacon', () => {
           .and.to.emit(drawBeacon, 'BeaconPeriodStarted')
           .withArgs(
             wallet.address,
-            beaconPeriodEndAt
+            nextStartTime
           )
 
-        expect(await drawBeacon.beaconPeriodStartedAt()).to.equal(beaconPeriodEndAt);
+        expect(await drawBeacon.getBeaconPeriodStartedAt()).to.equal(nextStartTime)
       });
     });
   })