refactor(Flashlend): add ability to set fee basis; reject 0 fee lends

Author: fubuloubu

contracts/accessories/Flashlend.storageLayout.json

@@ -0,0 +1,7 @@
+{
+  "feeBasis": {
+    "type": "HashMap[address, uint256]",
+    "n_slots": 0,
+    "slot": 154682873977518743260419555346329208424049161262037279244905357072823178018
+  }
+}

contracts/accessories/Flashlend.vy

@@ -23,19 +23,29 @@ interface IERC3156:
 
 implements: IERC3156
 
+# @custom:storage-location erc7201:purse.accessories.Flashlend
+# keccak256(abi.encode(uint256(keccak256("purse.accessories.Flashlend")) - 1)) & ~bytes32(uint256(0xff))
+feeBasis: HashMap[IERC20, uint256]  # 0x578c22acf65ce07623403df1f4aaaea33129ac33b22142a968e7c121335322
+
+
+# NOTE: Can watch for generic events of this type to find Purses that have "opted-in" to flashloans
+event FlashFeeUpdated:
+    token: indexed(IERC20)
+    milli_bps: uint256
 
-# TODO: Configure which tokens are allowed to be transferred?
 
 @view
 @external
 def maxFlashLoan(token: IERC20) -> uint256:
+    if self.feeBasis[token] == 0:
+        return 0
+
     return staticcall token.balanceOf(self)
 
 
 @view
 def _fee(token: IERC20, amount: uint256) -> uint256:
-    # TODO: Make this configurable?
-    return amount // 100  # 1% fee
+    return self.feeBasis[token] * amount // 1_000_000
 
 
 @view
@@ -45,6 +55,17 @@ def flashFee(token: IERC20, amount: uint256) -> uint256:
 
 
 @external
+def setFlashFee(token: IERC20, feeBasis: uint256):
+    assert feeBasis <= 1_000_000, "Flashlend:incorrect-fee-basis"
+    # NOTE: Can only work in a EIP-7702 context from Purse
+    assert tx.origin == self, "Flashlend:!authorized"
+
+    self.feeBasis[token] = feeBasis
+    log FlashFeeUpdated(token=token, milli_bps=feeBasis)
+
+
+@external
+# NOTE: Non-reentrant
 def flashLoan(
     receiver: IERC3156FlashBorrower,
     token: IERC20,
@@ -56,14 +77,20 @@ def flashLoan(
 
     # Tell receiver about the flashloan
     fee: uint256 = self._fee(token, amount)
+    assert fee > 0, "Flashlend:!token-allowed"
     assert (
         # NOTE: `msg.sender` is original caller of delegatecall
         extcall receiver.onFlashLoan(msg.sender, token, amount, fee, data)
         # NOTE: Magic value per ERC-3156
         == keccak256("ERC3156FlashBorrower.onFlashLoan")
-    )
+    ), "Flashlend:!receiver-returndata-invalid"
 
     # Get our tokens back
-    assert extcall token.transferFrom(receiver.address, self, amount + fee, default_return_value=True)
+    assert extcall token.transferFrom(
+        receiver.address,
+        self,
+        amount + fee,
+        default_return_value=True,
+    )
 
     return True