fix: revert changes on debt outstanding (yearn#381)

* fix: do not use totalDebt to calculate debtLimit

* feat: test failure

* docs: add comment

* refactor: move test

Co-authored-by: Poolpi Tako <poolpitako@protonmail.com>

contracts/Vault.vy

@@ -964,6 +964,8 @@ def _reportLoss(strategy: address, loss: uint256):
 
     # Also, make sure we reduce our trust with the strategy by the amount of loss
     if self.debtRatio != 0: # if vault with single strategy that is set to EmergencyOne
+        # NOTE: The context to this calculation is different than the calculation in `_reportLoss`,
+        # this calculation intentionally approximates via `totalDebt` to avoid manipulatable results
         ratio_change: uint256 = min(
             # NOTE: This calculation isn't 100% precise, the adjustment is ~10%-20% more severe due to EVM math
             loss * self.debtRatio / self.totalDebt,
@@ -1437,10 +1439,11 @@ def _debtOutstanding(strategy: address) -> uint256:
     # See note on `debtOutstanding()`.
     if self.debtRatio == 0:
         return self.strategies[strategy].totalDebt
+
     strategy_debtLimit: uint256 = (
         self.strategies[strategy].debtRatio
-        * self.totalDebt
-        / self.debtRatio
+        * self._totalAssets()
+        / MAX_BPS
     )
     strategy_totalDebt: uint256 = self.strategies[strategy].totalDebt
 

tests/functional/strategy/test_misc.py

@@ -186,3 +186,13 @@ def test_set_metadataURI(gov, strategy, strategist, rando):
     assert strategy.metadataURI() == "ipfs://test3"
     with brownie.reverts("!authorized"):
         strategy.setMetadataURI("ipfs://fake", {"from": rando})
+
+
+def test_reduce_debt_ratio(strategy, vault, gov, chain):
+    chain.sleep(1)
+    strategy.harvest({"from": gov})
+    assert vault.strategies(strategy).dict()["totalDebt"] > 0
+    old_debt_ratio = vault.strategies(strategy).dict()["debtRatio"]
+    vault.updateStrategyDebtRatio(strategy, old_debt_ratio // 2, {"from": gov})
+
+    assert vault.debtOutstanding(strategy) > 0