fix: divide by 0 error in _reportLoss (#306)

[fp-crypto]

This PR fixes a divide by 0 error that manifests when a vault has a total loss and harvest is called. The divide by 0 is only triggered when there is a total loss. Included is a new test that demonstrates the issue when run against the existing master branch but is fixed in this PR.

fixes #306

[fubuloubu]

Please fix your commit messages to align with conventional commits and run black . on your code updates to get this to pass the CI

[fubuloubu]

The issue here is that the actual bug is that the loss is accounted for before this ratio adjustment calculation is computed. That should be re-ordered to occur prior to the subtraction of the loss.

https://github.com/yearn/yearn-vaults/pull/307/files#diff-b583ba5c4d7b67e56b9084eff9fe883e12a2cdd94a2ea25029322c6f3568d807R974-R976

[fubuloubu]

This would be my suggestion for how to change this PR

  def _reportLoss(strategy: address, loss: uint256):
      # Loss can only be up the amount of debt issued to strategy
      totalDebt: uint256 = self.strategies[strategy].totalDebt
      assert totalDebt >= loss
+ 
+     # Also, make sure we reduce our trust with the strategy by the amount of loss
+     precisionFactor: uint256 = self.precisionFactor
+     ratio_change: uint256 = min(
+         precisionFactor * MAX_BPS * loss / self._totalAssets() / precisionFactor,
+         self.strategies[strategy].debtRatio,
+    )
+ 
+    # Finally, adjust our strategy's parameters by the loss
      self.strategies[strategy].totalLoss += loss
      self.strategies[strategy].totalDebt = totalDebt - loss
      self.totalDebt -= loss
-     # Also, make sure we reduce our trust with the strategy by the same amount
-     debtRatio: uint256 = self.strategies[strategy].debtRatio
-     precisionFactor: uint256 = self.precisionFactor
-     ratio_change: uint256 = min(precisionFactor * loss * MAX_BPS / self._totalAssets() / precisionFactor, debtRatio)
      self.strategies[strategy].debtRatio -= ratio_change
      self.debtRatio -= ratio_change

[ghost]

@fp-crypto need help?

[ghost]

Will take care of it.