Merge pull request #419 from liquity/sp-arithmetic-4

Hold received SP yield when total deposited amount is low

contracts/src/ActivePool.sol

@@ -258,9 +258,11 @@ contract ActivePool is IActivePool {
  uint256 remainderToLPs = mintedAmount - spYield;
 
  _boldToken.mint(address(interestRouter), remainderToLPs);
- _boldToken.mint(address(stabilityPool), spYield);
 
- stabilityPool.triggerBoldRewards(spYield);
+ if (spYield > 0) {
+ _boldToken.mint(address(stabilityPool), spYield);
+ stabilityPool.triggerBoldRewards(spYield);
+ }
  }
 
  lastAggUpdateTime = block.timestamp;

contracts/src/Interfaces/IStabilityPool.sol

@@ -76,6 +76,7 @@ interface IStabilityPool is ILiquityBase, IBoldRewardsReceiver {
  function getTotalBoldDeposits() external view returns (uint256);
 
  function getYieldGainsOwed() external view returns (uint256);
+ function getYieldGainsPending() external view returns (uint256);
 
  /*
  * Calculates the Coll gain earned by the deposit since its last snapshots were taken.

contracts/src/StabilityPool.sol

@@ -146,6 +146,9 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
  // TODO: from the contract's perspective, this is a write-only variable. It is only ever read in tests, so it would
  // be better to keep it outside the core contract.
  uint256 internal yieldGainsOwed;
+ // Total remaining Bold yield gains (from Trove interest mints) held by SP, not yet paid out to depositors,
+ // and not accounted for because they were received when the total deposits were too small
+ uint256 internal yieldGainsPending;
 
  // --- Data structures ---
 
@@ -227,6 +230,10 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
  return yieldGainsOwed;
  }
 
+ function getYieldGainsPending() external view override returns (uint256) {
+ return yieldGainsPending;
+ }
+
  // --- External Depositor Functions ---
 
  /* provideToSP():
@@ -263,10 +270,16 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
 
  _updateDepositAndSnapshots(msg.sender, newDeposit, newStashedColl);
  boldToken.sendToPool(msg.sender, address(this), _topUp);
- _updateTotalBoldDeposits(_topUp + keptYieldGain, 0);
+ uint256 totalBoldDepositsCached = _updateTotalBoldDeposits(_topUp + keptYieldGain, 0);
  _decreaseYieldGainsOwed(currentYieldGain);
  _sendBoldtoDepositor(msg.sender, yieldGainToSend);
  _sendCollGainToDepositor(collToSend);
+
+ // If there were pending yields and with the new deposit we are reaching the threshold, let’s move the yield to owed
+ uint256 yieldGainsPendingCached = yieldGainsPending;
+ if (yieldGainsPendingCached > 0 && totalBoldDepositsCached >= DECIMAL_PRECISION) {
+ _updateYieldRewardsSum(yieldGainsPendingCached, totalBoldDepositsCached);
+ }
  }
 
  function _getYieldToKeepOrSend(uint256 _currentYieldGain, bool _doClaim) internal pure returns (uint256, uint256) {
@@ -319,9 +332,15 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
 
  _updateDepositAndSnapshots(msg.sender, newDeposit, newStashedColl);
  _decreaseYieldGainsOwed(currentYieldGain);
- _updateTotalBoldDeposits(keptYieldGain, boldToWithdraw);
+ uint256 totalBoldDepositsCached = _updateTotalBoldDeposits(keptYieldGain, boldToWithdraw);
  _sendBoldtoDepositor(msg.sender, boldToWithdraw + yieldGainToSend);
  _sendCollGainToDepositor(collToSend);
+
+ // If there were pending yields and with the new deposit we are reaching the threshold, let’s move the yield to owed
+ uint256 yieldGainsPendingCached = yieldGainsPending;
+ if (yieldGainsPendingCached > 0 && totalBoldDepositsCached >= DECIMAL_PRECISION) {
+ _updateYieldRewardsSum(yieldGainsPendingCached, totalBoldDepositsCached);
+ }
  }
 
  function _getNewStashedCollAndCollToSend(address _depositor, uint256 _currentCollGain, bool _doClaim)
@@ -358,20 +377,27 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
 
  function triggerBoldRewards(uint256 _boldYield) external {
  _requireCallerIsActivePool();
+ assert(_boldYield > 0); // TODO: remove before deploying
 
  uint256 totalBoldDepositsCached = totalBoldDeposits; // cached to save an SLOAD
- /*
- * When total deposits is 0, B is not updated. In this case, the BOLD issued can not be obtained by later
- * depositors - it is missed out on, and remains in the balance of the SP.
- *
- */
- if (totalBoldDepositsCached == 0 || _boldYield == 0) {
+
+ // When total deposits is very small, B is not updated. In this case, the BOLD issued is hold
+ // until the total deposits reach 1 BOLD (remains in the balance of the SP).
+ if (totalBoldDepositsCached < DECIMAL_PRECISION) {
+ yieldGainsPending += _boldYield;
  return;
  }
 
- yieldGainsOwed += _boldYield;
+ _updateYieldRewardsSum(yieldGainsPending + _boldYield, totalBoldDepositsCached);
+ }
 
- uint256 yieldPerUnitStaked = _computeYieldPerUnitStaked(_boldYield, totalBoldDepositsCached);
+ function _updateYieldRewardsSum(uint256 _accumulatedYield, uint256 _totalBoldDeposits) internal {
+ assert(_accumulatedYield > 0); // TODO: remove before deploying
+
+ yieldGainsOwed += _accumulatedYield;
+ yieldGainsPending = 0;
+
+ uint256 yieldPerUnitStaked = _computeYieldPerUnitStaked(_accumulatedYield, _totalBoldDeposits);
 
  uint256 marginalYieldGain = yieldPerUnitStaked * P;
  epochToScaleToB[currentEpoch][currentScale] = epochToScaleToB[currentEpoch][currentScale] + marginalYieldGain;
@@ -535,11 +561,13 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
  emit StabilityPoolCollBalanceUpdated(newCollBalance);
  }
 
- function _updateTotalBoldDeposits(uint256 _depositIncrease, uint256 _depositDecrease) internal {
- if (_depositIncrease == 0 && _depositDecrease == 0) return;
+ function _updateTotalBoldDeposits(uint256 _depositIncrease, uint256 _depositDecrease) internal returns (uint256) {
+ if (_depositIncrease == 0 && _depositDecrease == 0) return totalBoldDeposits;
  uint256 newTotalBoldDeposits = totalBoldDeposits + _depositIncrease - _depositDecrease;
  totalBoldDeposits = newTotalBoldDeposits;
  emit StabilityPoolBoldBalanceUpdated(newTotalBoldDeposits);
+
+ return newTotalBoldDeposits;
  }
 
  function _decreaseYieldGainsOwed(uint256 _amount) internal {
@@ -584,11 +612,11 @@ contract StabilityPool is LiquityBase, IStabilityPool, IStabilityPoolEvents {
 
  Snapshots memory snapshots = depositSnapshots[_depositor];
 
- uint256 pendingSPYield = activePool.calcPendingSPYield();
+ uint256 pendingSPYield = activePool.calcPendingSPYield() + yieldGainsPending;
  uint256 firstPortionPending;
  uint256 secondPortionPending;
 
- if (pendingSPYield > 0 && snapshots.epoch == currentEpoch) {
+ if (pendingSPYield > 0 && snapshots.epoch == currentEpoch && totalBoldDeposits >= DECIMAL_PRECISION) {
  uint256 yieldNumerator = pendingSPYield * DECIMAL_PRECISION + lastYieldError;
  uint256 yieldPerUnitStaked = yieldNumerator / totalBoldDeposits;
  uint256 marginalYieldGain = yieldPerUnitStaked * P;