Set a fixed fee transferred to the caller for publication

Avoids the risk of the gas used by the contract exceeding the gas presumed to
be used (causing an insolvency).

processor/ethereum/router/contracts/Router.sol

@@ -164,18 +164,16 @@ contract Router {
   // Execute a list of transactions if they were signed by the current key with the current nonce
   function execute(
     address coin,
-    uint256 fee_per_gas,
+    uint256 fee,
     OutInstruction[] calldata transactions,
     Signature calldata signature
   ) external {
-    uint256 gasLeftAtStart = gasleft();
-
     // Verify the signature
     // We hash the message here as we need the message's hash for the Executed event
     // Since we're already going to hash it, hashing it prior to verifying the signature reduces the
     // amount of words hashed by its challenge function (reducing our gas costs)
     bytes32 message =
-      keccak256(abi.encode("execute", block.chainid, _nonce, coin, fee_per_gas, transactions));
+      keccak256(abi.encode("execute", block.chainid, _nonce, coin, fee, transactions));
     if (!Schnorr.verify(_seraiKey, message, signature.c, signature.s)) {
       revert InvalidSignature();
     }
@@ -212,25 +210,8 @@ contract Router {
       }
     }
 
-    // Calculate the gas which will be used to transfer the fee out
-    // This is meant to be always over, never under, with any excess being a tip to the publisher
-    uint256 gasToTransferOut;
-    if (coin == address(0)) {
-      // 5,000 gas is explicitly allowed, with another 10,000 for whatever overhead remains
-      // unaccounted for
-      gasToTransferOut = 15_000;
-    } else {
-      // 100_000 gas is explicitly allowed, with another 15,000 for whatever overhead remains
-      // unaccounted for. More gas is given than for ETH due to needing to ABI encode the function
-      // call
-      gasToTransferOut = 115_000;
-    }
-
-    // Calculate the gas used
-    uint256 gasLeftAtEnd = gasleft();
-    uint256 gasUsed = gasLeftAtStart - gasLeftAtEnd;
     // Transfer to the caller the fee
-    _transferOut(msg.sender, coin, (gasUsed + gasToTransferOut) * fee_per_gas);
+    _transferOut(msg.sender, coin, fee);
   }
 
   function nonce() external view returns (uint256) {

processor/ethereum/router/src/lib.rs

@@ -325,27 +325,18 @@ impl Router {
     chain_id: U256,
     nonce: u64,
     coin: Coin,
-    fee_per_gas: U256,
+    fee: U256,
     outs: OutInstructions,
   ) -> Vec<u8> {
-    ("execute", chain_id, U256::try_from(nonce).unwrap(), coin.address(), fee_per_gas, outs.0)
-      .abi_encode()
+    ("execute", chain_id, U256::try_from(nonce).unwrap(), coin.address(), fee, outs.0).abi_encode()
   }
 
   /// Construct a transaction to execute a batch of `OutInstruction`s.
-  pub fn execute(
-    &self,
-    coin: Coin,
-    fee_per_gas: U256,
-    outs: OutInstructions,
-    sig: &Signature,
-  ) -> TxLegacy {
+  pub fn execute(&self, coin: Coin, fee: U256, outs: OutInstructions, sig: &Signature) -> TxLegacy {
     let outs_len = outs.0.len();
     TxLegacy {
       to: TxKind::Call(self.1),
-      input: abi::executeCall::new((coin.address(), fee_per_gas, outs.0, sig.into()))
-        .abi_encode()
-        .into(),
+      input: abi::executeCall::new((coin.address(), fee, outs.0, sig.into())).abi_encode().into(),
       // TODO
       gas_limit: 100_000 + ((200_000 + 10_000) * u128::try_from(outs_len).unwrap()),
       ..Default::default()

processor/ethereum/src/primitives/transaction.rs

@@ -18,7 +18,7 @@ use crate::{output::OutputId, machine::ClonableTransctionMachine};
 #[derive(Clone, PartialEq, Debug)]
 pub(crate) enum Action {
   SetKey { chain_id: U256, nonce: u64, key: PublicKey },
-  Batch { chain_id: U256, nonce: u64, coin: Coin, fee_per_gas: U256, outs: Vec<(Address, U256)> },
+  Batch { chain_id: U256, nonce: u64, coin: Coin, fee: U256, outs: Vec<(Address, U256)> },
 }
 
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -36,11 +36,11 @@ impl Action {
       Action::SetKey { chain_id, nonce, key } => {
         Router::update_serai_key_message(*chain_id, *nonce, key)
       }
-      Action::Batch { chain_id, nonce, coin, fee_per_gas, outs } => Router::execute_message(
+      Action::Batch { chain_id, nonce, coin, fee, outs } => Router::execute_message(
         *chain_id,
         *nonce,
         *coin,
-        *fee_per_gas,
+        *fee,
         OutInstructions::from(outs.as_ref()),
       ),
     }
@@ -51,10 +51,9 @@ impl Action {
       Self::SetKey { chain_id: _, nonce, key } => {
         Executed::SetKey { nonce: *nonce, key: key.eth_repr() }
       }
-      Self::Batch { nonce, .. } => Executed::Batch {
-        nonce: *nonce,
-        message_hash: keccak256(self.message()),
-      },
+      Self::Batch { nonce, .. } => {
+        Executed::Batch { nonce: *nonce, message_hash: keccak256(self.message()) }
+      }
     })
   }
 }
@@ -106,9 +105,9 @@ impl SignableTransaction for Action {
       1 => {
         let coin = Coin::read(reader)?;
 
-        let mut fee_per_gas = [0; 32];
-        reader.read_exact(&mut fee_per_gas)?;
-        let fee_per_gas = U256::from_le_bytes(fee_per_gas);
+        let mut fee = [0; 32];
+        reader.read_exact(&mut fee)?;
+        let fee = U256::from_le_bytes(fee);
 
         let mut outs_len = [0; 4];
         reader.read_exact(&mut outs_len)?;
@@ -124,7 +123,7 @@ impl SignableTransaction for Action {
 
           outs.push((address, amount));
         }
-        Action::Batch { chain_id, nonce, coin, fee_per_gas, outs }
+        Action::Batch { chain_id, nonce, coin, fee, outs }
       }
       _ => unreachable!(),
     })
@@ -137,12 +136,12 @@ impl SignableTransaction for Action {
         writer.write_all(&nonce.to_le_bytes())?;
         writer.write_all(&key.eth_repr())
       }
-      Self::Batch { chain_id, nonce, coin, fee_per_gas, outs } => {
+      Self::Batch { chain_id, nonce, coin, fee, outs } => {
         writer.write_all(&[1])?;
         writer.write_all(&chain_id.as_le_bytes())?;
         writer.write_all(&nonce.to_le_bytes())?;
         coin.write(writer)?;
-        writer.write_all(&fee_per_gas.as_le_bytes())?;
+        writer.write_all(&fee.as_le_bytes())?;
         writer.write_all(&u32::try_from(outs.len()).unwrap().to_le_bytes())?;
         for (address, amount) in outs {
           borsh::BorshSerialize::serialize(address, writer)?;

processor/ethereum/src/publisher.rs

@@ -89,8 +89,8 @@ impl<D: Db> signers::TransactionPublisher<Transaction> for TransactionPublisher<
       // Convert from an Action (an internal representation of a signable event) to a TxLegacy
       let tx = match tx.0 {
         Action::SetKey { chain_id: _, nonce: _, key } => router.update_serai_key(&key, &tx.1),
-        Action::Batch { chain_id: _, nonce: _, coin, fee_per_gas, outs } => {
-          router.execute(coin, fee_per_gas, OutInstructions::from(outs.as_ref()), &tx.1)
+        Action::Batch { chain_id: _, nonce: _, coin, fee, outs } => {
+          router.execute(coin, fee, OutInstructions::from(outs.as_ref()), &tx.1)
         }
       };
 

processor/ethereum/src/scheduler.rs

@@ -111,16 +111,19 @@ impl<D: Db> smart_contract_scheduler::SmartContract<Rpc<D>> for SmartContract {
 
       // Push each batch onto the result
       for outs in batches {
-        let base_gas = BASE_GAS.div_ceil(u32::try_from(outs.len()).unwrap());
+        let mut total_gas = 0;
+
+        let base_gas_per_payment = BASE_GAS.div_ceil(u32::try_from(outs.len()).unwrap());
         // Deduce the fee from each out
         for out in &mut outs {
-          let payment_gas = base_gas +
+          let payment_gas = base_gas_per_payment +
             match out.0 {
               Address::Address(_) => ADDRESS_PAYMENT_GAS,
               Address::Contract(deployment) => CONTRACT_PAYMENT_GAS + deployment.gas_limit(),
             };
+          total_gas += payment_gas;
 
-          let payment_gas_cost = fee_per_gas * U256::try_from(payment_gas).unwrap();
+          let payment_gas_cost = U256::try_from(payment_gas).unwrap() * fee_per_gas;
           out.1 -= payment_gas_cost;
         }
 
@@ -128,7 +131,7 @@ impl<D: Db> smart_contract_scheduler::SmartContract<Rpc<D>> for SmartContract {
           chain_id: self.chain_id,
           nonce,
           coin: coin_to_ethereum_coin(coin),
-          fee_per_gas,
+          fee: U256::try_from(total_gas).unwrap() * fee_per_gas,
           outs,
         });
         nonce += 1;