Add a governor module to protect against late quorum

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## Unreleased
+ * `GovernorExtendedVoting`: add new module to ensure a minimum voting duration is available after the quorum is reached. ([#2973](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2973))
+
 ## Unreleased
 
  * `Ownable`: add an internal `_transferOwnership(address)`. ([#2568](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2568))

contracts/governance/Governor.sol

@@ -110,23 +110,36 @@ abstract contract Governor is Context, ERC165, EIP712, IGovernor {
      * @dev See {IGovernor-state}.
      */
     function state(uint256 proposalId) public view virtual override returns (ProposalState) {
-        ProposalCore memory proposal = _proposals[proposalId];
+        ProposalCore storage proposal = _proposals[proposalId];
 
         if (proposal.executed) {
             return ProposalState.Executed;
-        } else if (proposal.canceled) {
+        }
+
+        if (proposal.canceled) {
             return ProposalState.Canceled;
-        } else if (proposal.voteStart.getDeadline() >= block.number) {
+        }
+
+        uint256 snapshot = proposalSnapshot(proposalId);
+
+        if (snapshot == 0) {
+            revert("Governor: unknown proposal id");
+        }
+
+        if (snapshot >= block.number) {
             return ProposalState.Pending;
-        } else if (proposal.voteEnd.getDeadline() >= block.number) {
+        }
+
+        uint256 deadline = proposalDeadline(proposalId);
+
+        if (deadline >= block.number) {
             return ProposalState.Active;
-        } else if (proposal.voteEnd.isExpired()) {
-            return
-                _quorumReached(proposalId) && _voteSucceeded(proposalId)
-                    ? ProposalState.Succeeded
-                    : ProposalState.Defeated;
+        }
+
+        if (_quorumReached(proposalId) && _voteSucceeded(proposalId)) {
+            return ProposalState.Succeeded;
         } else {
-            revert("Governor: unknown proposal id");
+            return ProposalState.Defeated;
         }
     }
 

contracts/governance/README.adoc

@@ -42,6 +42,8 @@ Other extensions can customize the behavior or interface in multiple ways.
 
 * {GovernorSettings}: Manages some of the settings (voting delay, voting period duration, and proposal threshold) in a way that can be updated through a governance proposal, without requiering an upgrade.
 
+* {GovernorPreventLateQuorum}: Ensures there is a minimum voting period after quorum is reached as a security protection against large voters.
+
 In addition to modules and extensions, the core contract requires a few virtual functions to be implemented to your particular specifications:
 
 * <<Governor-votingDelay-,`votingDelay()`>>: Delay (in number of blocks) since the proposal is submitted until voting power is fixed and voting starts. This can be used to enforce a delay after a proposal is published for users to buy tokens, or delegate their votes.
@@ -74,6 +76,8 @@ NOTE: Functions of the `Governor` contract do not include access control. If you
 
 {{GovernorSettings}}
 
+{{GovernorPreventLateQuorum}}
+
 {{GovernorCompatibilityBravo}}
 
 === Deprecated

contracts/governance/extensions/GovernorPreventLateQuorum.sol

@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../Governor.sol";
+import "../../utils/math/Math.sol";
+
+/**
+ * @dev A module that ensures there is a minimum voting period after quorum is reached. This prevents a large voter from
+ * swaying a vote and triggering quorum at the last minute, by ensuring there is always time for other voters to react
+ * and try to oppose the decision.
+ *
+ * If a vote causes quorum to be reached, the proposal's voting period may be extended so that it does not end before at
+ * least a given number of blocks have passed (the "vote extension" parameter). This parameter can be set by the
+ * governance executor (e.g. through a governance proposal).
+ *
+ * _Available since v4.5._
+ */
+abstract contract GovernorPreventLateQuorum is Governor {
+    using SafeCast for uint256;
+    using Timers for Timers.BlockNumber;
+
+    uint64 private _voteExtension;
+    mapping(uint256 => Timers.BlockNumber) private _extendedDeadlines;
+
+    /// @dev Emitted when a proposal deadline is pushed back due to reaching quorum late in its voting period.
+    event ProposalExtended(uint256 indexed proposalId, uint64 extendedDeadline);
+
+    /// @dev Emitted when the {lateQuorumVoteExtension} parameter is changed.
+    event LateQuorumVoteExtensionSet(uint64 oldVoteExtension, uint64 newVoteExtension);
+
+    /**
+     * @dev Initializes the vote extension parameter: the number of blocks that are required to pass since a proposal
+     * reaches quorum until its voting period ends. If necessary the voting period will be extended beyond the one set
+     * at proposal creation.
+     */
+    constructor(uint64 initialVoteExtension) {
+        _setLateQuorumVoteExtension(initialVoteExtension);
+    }
+
+    /**
+     * @dev Returns the proposal deadline, which may have been extended beyond that set at proposal creation, if the
+     * proposal reached quorum late in the voting period. See {Governor-proposalDeadline}.
+     */
+    function proposalDeadline(uint256 proposalId) public view virtual override returns (uint256) {
+        return Math.max(super.proposalDeadline(proposalId), _extendedDeadlines[proposalId].getDeadline());
+    }
+
+    /**
+     * @dev Casts a vote and detects if it caused quorum to be reached, potentially extending the voting period. See
+     * {Governor-_castVote}.
+     *
+     * May emit a {ProposalExtended} event.
+     */
+    function _castVote(
+        uint256 proposalId,
+        address account,
+        uint8 support,
+        string memory reason
+    ) internal virtual override returns (uint256) {
+        uint256 result = super._castVote(proposalId, account, support, reason);
+
+        Timers.BlockNumber storage extendedDeadline = _extendedDeadlines[proposalId];
+
+        if (extendedDeadline.isUnset() && _quorumReached(proposalId)) {
+            uint64 extendedDeadlineValue = block.number.toUint64() + lateQuorumVoteExtension();
+
+            if (extendedDeadlineValue > proposalDeadline(proposalId)) {
+                emit ProposalExtended(proposalId, extendedDeadlineValue);
+            }
+
+            extendedDeadline.setDeadline(extendedDeadlineValue);
+        }
+
+        return result;
+    }
+
+    /**
+     * @dev Returns the current value of the vote extension parameter: the number of blocks that are required to pass
+     * from the time a proposal reaches quorum until its voting period ends.
+     */
+    function lateQuorumVoteExtension() public view virtual returns (uint64) {
+        return _voteExtension;
+    }
+
+    /**
+     * @dev Changes the {lateQuorumVoteExtension}. This operation can only be performed by the governance executor,
+     * generally through a governance proposal.
+     *
+     * Emits a {LateQuorumVoteExtensionSet} event.
+     */
+    function setLateQuorumVoteExtension(uint64 newVoteExtension) public virtual onlyGovernance {
+        _setLateQuorumVoteExtension(newVoteExtension);
+    }
+
+    /**
+     * @dev Changes the {lateQuorumVoteExtension}. This is an internal function that can be exposed in a public function
+     * like {setLateQuorumVoteExtension} if another access control mechanism is needed.
+     *
+     * Emits a {LateQuorumVoteExtensionSet} event.
+     */
+    function _setLateQuorumVoteExtension(uint64 newVoteExtension) internal virtual {
+        emit LateQuorumVoteExtensionSet(_voteExtension, newVoteExtension);
+        _voteExtension = newVoteExtension;
+    }
+}

contracts/mocks/GovernorPreventLateQuorumMock.sol

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../governance/extensions/GovernorPreventLateQuorum.sol";
+import "../governance/extensions/GovernorSettings.sol";
+import "../governance/extensions/GovernorCountingSimple.sol";
+import "../governance/extensions/GovernorVotes.sol";
+
+contract GovernorPreventLateQuorumMock is
+    GovernorSettings,
+    GovernorVotes,
+    GovernorCountingSimple,
+    GovernorPreventLateQuorum
+{
+    uint256 private _quorum;
+
+    constructor(
+        string memory name_,
+        ERC20Votes token_,
+        uint256 votingDelay_,
+        uint256 votingPeriod_,
+        uint256 quorum_,
+        uint64 voteExtension_
+    )
+        Governor(name_)
+        GovernorSettings(votingDelay_, votingPeriod_, 0)
+        GovernorVotes(token_)
+        GovernorPreventLateQuorum(voteExtension_)
+    {
+        _quorum = quorum_;
+    }
+
+    function quorum(uint256) public view virtual override returns (uint256) {
+        return _quorum;
+    }
+
+    function proposalDeadline(uint256 proposalId)
+        public
+        view
+        virtual
+        override(Governor, GovernorPreventLateQuorum)
+        returns (uint256)
+    {
+        return super.proposalDeadline(proposalId);
+    }
+
+    function proposalThreshold() public view virtual override(Governor, GovernorSettings) returns (uint256) {
+        return super.proposalThreshold();
+    }
+
+    function _castVote(
+        uint256 proposalId,
+        address account,
+        uint8 support,
+        string memory reason
+    ) internal virtual override(Governor, GovernorPreventLateQuorum) returns (uint256) {
+        return super._castVote(proposalId, account, support, reason);
+    }
+}

test/governance/GovernorWorkflow.behavior.js

@@ -65,7 +65,7 @@ function runGovernorWorkflow () {
     // vote
     if (tryGet(this.settings, 'voters')) {
       this.receipts.castVote = [];
-      for (const voter of this.settings.voters) {
+      for (const voter of this.settings.voters.filter(({ support }) => !!support)) {
         if (!voter.signature) {
           this.receipts.castVote.push(
             await getReceiptOrRevert(