[EASY] Make gas offset configurable (#15)

Same as #7 but without the changes
to the simulation logic (which lead to problems).
This PR basically now just allows to configure the gas that should be
added to any swap proposed by dex solvers to have an easy knob to tweak
to adjust the cost coverage of the dex solvers.

src/domain/dex/mod.rs

@@ -108,6 +108,7 @@ impl Swap {
         gas_price: auction::GasPrice,
         sell_token: Option<auction::Price>,
         simulator: &infra::dex::Simulator,
+        gas_offset: eth::Gas,
     ) -> Option<solution::Solution> {
         let gas = if order.class == order::Class::Limit {
             match simulator.gas(order.owner(), &self).await {
@@ -141,7 +142,7 @@ impl Swap {
             interactions,
             gas,
         }
-        .into_solution(gas_price, sell_token)
+        .into_solution(gas_price, sell_token, gas_offset)
     }
 
     pub fn satisfies(&self, order: &domain::order::Order) -> bool {

src/domain/solution.rs

@@ -116,15 +116,13 @@ pub struct Single {
 }
 
 impl Single {
-    /// An approximation for the overhead of executing a trade in a settlement.
-    const SETTLEMENT_OVERHEAD: u64 = 106_391;
-
     /// Creates a full solution for a single order solution given gas and sell
     /// token prices.
     pub fn into_solution(
         self,
         gas_price: auction::GasPrice,
         sell_token: Option<auction::Price>,
+        gas_offset: eth::Gas,
     ) -> Option<Solution> {
         let Self {
             order,
@@ -147,7 +145,7 @@ impl Single {
             Fee::Surplus(
                 sell_token?.ether_value(eth::Ether(
                     swap.0
-                        .checked_add(Self::SETTLEMENT_OVERHEAD.into())?
+                        .checked_add(gas_offset.0)?
                         .checked_mul(gas_price.0 .0)?,
                 ))?,
             )
@@ -198,7 +196,7 @@ impl Single {
             ]),
             trades: vec![Trade::Fulfillment(Fulfillment::new(order, executed, fee)?)],
             interactions,
-            gas: Some(eth::Gas(Self::SETTLEMENT_OVERHEAD.into()) + self.gas),
+            gas: Some(gas_offset + self.gas),
         })
     }
 }

src/domain/solver/dex/mod.rs

@@ -6,6 +6,7 @@ use {
         domain::{
             auction,
             dex::{self, slippage},
+            eth,
             order::{self, Order},
             solution,
             solver::dex::fills::Fills,
@@ -38,6 +39,10 @@ pub struct Dex {
 
     /// Handles 429 Too Many Requests error with a retry mechanism
     rate_limiter: rate_limit::RateLimiter,
+
+    /// Amount of gas that gets added to each swap to tweak the cost coverage of
+    /// the solver.
+    gas_offset: eth::Gas,
 }
 
 /// The amount of time we aim the solver to finish before the final deadline is
@@ -61,6 +66,7 @@ impl Dex {
             concurrent_requests: config.concurrent_requests,
             fills: Fills::new(config.smallest_partial_fill),
             rate_limiter,
+            gas_offset: config.gas_offset,
         }
     }
 
@@ -177,7 +183,13 @@ impl Dex {
         let swap = self.try_solve(order, &dex_order, tokens, gas_price).await?;
         let sell = tokens.reference_price(&order.sell.token);
         let Some(solution) = swap
-            .into_solution(order.clone(), gas_price, sell, &self.simulator)
+            .into_solution(
+                order.clone(),
+                gas_price,
+                sell,
+                &self.simulator,
+                self.gas_offset,
+            )
             .await
         else {
             tracing::debug!("no solution for swap");

src/infra/config/dex/file.rs

@@ -58,6 +58,12 @@ struct Config {
 
     /// Settings specific to the wrapped dex API.
     dex: toml::Value,
+
+    /// Amount of gas that gets added to each swap to adjust the cost coverage
+    /// of the solver.
+    #[serde(default = "default_gas_offset")]
+    #[serde_as(as = "serialize::U256")]
+    gas_offset: eth::U256,
 }
 
 fn default_relative_slippage() -> BigDecimal {
@@ -84,6 +90,12 @@ fn default_max_back_off() -> Duration {
     Duration::from_secs(8)
 }
 
+fn default_gas_offset() -> eth::U256 {
+    // Rough estimation of the gas overhead of settling a single
+    // trade via the settlement contract.
+    106_391.into()
+}
+
 /// Loads the base solver configuration from a TOML file.
 ///
 /// # Panics
@@ -137,6 +149,7 @@ pub async fn load<T: DeserializeOwned>(path: &Path) -> (super::Config, T) {
             config.max_back_off,
         )
         .unwrap(),
+        gas_offset: eth::Gas(config.gas_offset),
     };
     (config, dex)
 }

src/infra/config/dex/mod.rs

@@ -23,4 +23,5 @@ pub struct Config {
     pub concurrent_requests: NonZeroUsize,
     pub smallest_partial_fill: eth::Ether,
     pub rate_limiting_strategy: rate_limit::Strategy,
+    pub gas_offset: eth::Gas,
 }