fix: buy debt

contracts/VaultV3.vy

@@ -1390,9 +1390,8 @@ def buy_debt(strategy: address, amount: uint256):
     @dev This should only ever be used in an emergency in place
     of force revoking a strategy in order to not report a loss.
     It allows the DEBT_PURCHASER role to buy the strategies debt
-    for an equal amount of `asset`. It's important to note that 
-    this does rely on the strategies `convertToShares` function to
-    determine the amount of shares to buy.
+    for an equal amount of `asset`. 
+    
     @param strategy The strategy to buy the debt for
     @param amount The amount of debt to buy from the vault.
     """
@@ -1401,35 +1400,37 @@ def buy_debt(strategy: address, amount: uint256):
 
     # Cache the current debt.
     current_debt: uint256 = self.strategies[strategy].current_debt
-
+    _amount: uint256 = amount
+
     assert current_debt > 0, "nothing to buy"
-    assert amount > 0, "nothing to buy with"
+    assert _amount > 0, "nothing to buy with"
+
+    if _amount > current_debt:
+        _amount = current_debt
 
-    # Get the current shares value for the amount.
-    shares: uint256 = IStrategy(strategy).convertToShares(amount)
+    # We get the proportion of the debt that is being bought and
+    # transfer the equivalant shares. We assume this is being used
+    # due to strategy issues so won't rely on its conversion rates.
+    shares: uint256 = IStrategy(strategy).balanceOf(self) * _amount / current_debt
 
     assert shares > 0, "can't buy 0"
-    assert shares <= IStrategy(strategy).balanceOf(self), "not enough shares"
-
-    self._erc20_safe_transfer_from(ASSET.address, msg.sender, self, amount)
 
-    # Adjust if needed to not underflow on math
-    bought: uint256 = min(current_debt, amount)
+    self._erc20_safe_transfer_from(ASSET.address, msg.sender, self, _amount)
 
     # Lower strategy debt
-    self.strategies[strategy].current_debt -= bought
+    self.strategies[strategy].current_debt -= _amount
     # lower total debt
-    self.total_debt -= bought
+    self.total_debt -= _amount
     # Increase total idle
-    self.total_idle += bought
+    self.total_idle += _amount
 
     # log debt change
-    log DebtUpdated(strategy, current_debt, current_debt - bought)
+    log DebtUpdated(strategy, current_debt, current_debt - _amount)
 
     # Transfer the strategies shares out.
     self._erc20_safe_transfer(strategy, msg.sender, shares)
 
-    log DebtPurchased(strategy, bought)
+    log DebtPurchased(strategy, _amount)
 
 ## STRATEGY MANAGEMENT ##
 @external

tests/unit/vault/test_buy_debt.py

@@ -75,7 +75,7 @@ def test_buy_debt__no_amount__reverts(
         vault.buy_debt(strategy, 0, sender=gov)
 
 
-def test_buy_debt__to_many_shares__reverts(
+def test_buy_debt__more_than_available__withdraws_current_debt(
     gov,
     asset,
     vault,
@@ -94,8 +94,30 @@ def test_buy_debt__to_many_shares__reverts(
     asset.mint(gov.address, amount, sender=gov)
     asset.approve(vault.address, amount, sender=gov)
 
-    with ape.reverts("not enough shares"):
-        vault.buy_debt(strategy, amount * 2, sender=gov)
+    before_balance = asset.balanceOf(gov)
+    before_shares = strategy.balanceOf(gov)
+
+    tx = vault.buy_debt(strategy, amount * 2, sender=gov)
+
+    logs = list(tx.decode_logs(vault.DebtPurchased))[0]
+
+    assert logs.strategy == strategy.address
+    assert logs.amount == amount
+
+    logs = list(tx.decode_logs(vault.DebtUpdated))
+
+    assert len(logs) == 1
+    assert logs[0].strategy == strategy.address
+    assert logs[0].current_debt == amount
+    assert logs[0].new_debt == 0
+
+    assert vault.totalIdle() == amount
+    assert vault.totalDebt() == 0
+    assert vault.pricePerShare() == 10 ** asset.decimals()
+    assert vault.strategies(strategy)["current_debt"] == 0
+    # assert shares got moved
+    assert asset.balanceOf(gov) == before_balance - amount
+    assert strategy.balanceOf(gov) == before_shares + amount
 
 
 def test_buy_debt__full_debt(
@@ -120,7 +142,19 @@ def test_buy_debt__full_debt(
     before_balance = asset.balanceOf(gov)
     before_shares = strategy.balanceOf(gov)
 
-    vault.buy_debt(strategy, amount, sender=gov)
+    tx = vault.buy_debt(strategy, amount, sender=gov)
+
+    logs = list(tx.decode_logs(vault.DebtPurchased))[0]
+
+    assert logs.strategy == strategy.address
+    assert logs.amount == amount
+
+    logs = list(tx.decode_logs(vault.DebtUpdated))
+
+    assert len(logs) == 1
+    assert logs[0].strategy == strategy.address
+    assert logs[0].current_debt == amount
+    assert logs[0].new_debt == 0
 
     assert vault.totalIdle() == amount
     assert vault.totalDebt() == 0
@@ -155,7 +189,19 @@ def test_buy_debt__half_debt(
     before_balance = asset.balanceOf(gov)
     before_shares = strategy.balanceOf(gov)
 
-    vault.buy_debt(strategy, to_buy, sender=gov)
+    tx = vault.buy_debt(strategy, to_buy, sender=gov)
+
+    logs = list(tx.decode_logs(vault.DebtPurchased))[0]
+
+    assert logs.strategy == strategy.address
+    assert logs.amount == to_buy
+
+    logs = list(tx.decode_logs(vault.DebtUpdated))
+
+    assert len(logs) == 1
+    assert logs[0].strategy == strategy.address
+    assert logs[0].current_debt == amount
+    assert logs[0].new_debt == amount - to_buy
 
     assert vault.totalIdle() == to_buy
     assert vault.totalDebt() == amount - to_buy