Keep track of historical quorum values (#3561)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com> (cherry picked from commit 8ea1fc8)

Author: Amxx

contracts/governance/extensions/GovernorVotesQuorumFraction.sol

@@ -4,6 +4,8 @@
 pragma solidity ^0.8.0;
 
 import "./GovernorVotes.sol";
+import "../../utils/Checkpoints.sol";
+import "../../utils/math/SafeCast.sol";
 
 /**
  * @dev Extension of {Governor} for voting weight extraction from an {ERC20Votes} token and a quorum expressed as a
@@ -12,7 +14,10 @@ import "./GovernorVotes.sol";
  * _Available since v4.3._
  */
 abstract contract GovernorVotesQuorumFraction is GovernorVotes {
- uint256 private _quorumNumerator;
+ using Checkpoints for Checkpoints.History;
+
+ uint256 private _quorumNumerator; // DEPRECATED
+ Checkpoints.History private _quorumNumeratorHistory;
 
  event QuorumNumeratorUpdated(uint256 oldQuorumNumerator, uint256 newQuorumNumerator);
 
@@ -31,7 +36,27 @@ abstract contract GovernorVotesQuorumFraction is GovernorVotes {
  * @dev Returns the current quorum numerator. See {quorumDenominator}.
  */
  function quorumNumerator() public view virtual returns (uint256) {
- return _quorumNumerator;
+ return _quorumNumeratorHistory._checkpoints.length == 0 ? _quorumNumerator : _quorumNumeratorHistory.latest();
+ }
+
+ /**
+ * @dev Returns the quorum numerator at a specific block number. See {quorumDenominator}.
+ */
+ function quorumNumerator(uint256 blockNumber) public view virtual returns (uint256) {
+ // If history is empty, fallback to old storage
+ uint256 length = _quorumNumeratorHistory._checkpoints.length;
+ if (length == 0) {
+ return _quorumNumerator;
+ }
+
+ // Optimistic search, check the latest checkpoint
+ Checkpoints.Checkpoint memory latest = _quorumNumeratorHistory._checkpoints[length - 1];
+ if (latest._blockNumber <= blockNumber) {
+ return latest._value;
+ }
+
+ // Otherwize, do the binary search
+ return _quorumNumeratorHistory.getAtBlock(blockNumber);
  }
 
  /**
@@ -45,7 +70,7 @@ abstract contract GovernorVotesQuorumFraction is GovernorVotes {
  * @dev Returns the quorum for a block number, in terms of number of votes: `supply * numerator / denominator`.
  */
  function quorum(uint256 blockNumber) public view virtual override returns (uint256) {
- return (token.getPastTotalSupply(blockNumber) * quorumNumerator()) / quorumDenominator();
+ return (token.getPastTotalSupply(blockNumber) * quorumNumerator(blockNumber)) / quorumDenominator();
  }
 
  /**
@@ -77,8 +102,17 @@ abstract contract GovernorVotesQuorumFraction is GovernorVotes {
  "GovernorVotesQuorumFraction: quorumNumerator over quorumDenominator"
  );
 
- uint256 oldQuorumNumerator = _quorumNumerator;
- _quorumNumerator = newQuorumNumerator;
+ uint256 oldQuorumNumerator = quorumNumerator();
+
+ // Make sure we keep track of the original numerator in contracts upgraded from a version without checkpoints.
+ if (oldQuorumNumerator != 0 && _quorumNumeratorHistory._checkpoints.length == 0) {
+ _quorumNumeratorHistory._checkpoints.push(
+ Checkpoints.Checkpoint({_blockNumber: 0, _value: SafeCast.toUint224(oldQuorumNumerator)})
+ );
+ }
+
+ // Set new quorum for future proposals
+ _quorumNumeratorHistory.push(newQuorumNumerator);
 
  emit QuorumNumeratorUpdated(oldQuorumNumerator, newQuorumNumerator);
  }

test/governance/extensions/GovernorWeightQuorumFraction.test.js renamed to test/governance/extensions/GovernorVotesQuorumFraction.test.js

@@ -104,6 +104,13 @@ contract('GovernorVotesQuorumFraction', function (accounts) {
 
  expect(await this.mock.quorumNumerator()).to.be.bignumber.equal(newRatio);
  expect(await this.mock.quorumDenominator()).to.be.bignumber.equal('100');
+
+ // it takes one block for the new quorum to take effect
+ expect(await time.latestBlock().then(blockNumber => this.mock.quorum(blockNumber.subn(1))))
+ .to.be.bignumber.equal(tokenSupply.mul(ratio).divn(100));
+
+ await time.advanceBlock();
+
  expect(await time.latestBlock().then(blockNumber => this.mock.quorum(blockNumber.subn(1))))
  .to.be.bignumber.equal(tokenSupply.mul(newRatio).divn(100));
  });