From c76e7eb471241cc17b2d6e61b182755bdad22434 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thomas=20P=C3=A9cseli?= <thomas.pecseli@trili.tech>
Date: Fri, 25 Aug 2023 16:10:36 +0100
Subject: [PATCH] EVM on WASM: Refund unused gas

---
 src/kernel_evm/CHANGES.md                   |  1 +
 src/kernel_evm/evm_execution/src/handler.rs | 25 +++++++++++++++++++++
 src/kernel_evm/evm_execution/src/lib.rs     | 11 ++++++++-
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/src/kernel_evm/CHANGES.md b/src/kernel_evm/CHANGES.md
index 50dcb3d1e3a0..f677afcf9b6c 100644
--- a/src/kernel_evm/CHANGES.md
+++ b/src/kernel_evm/CHANGES.md
@@ -14,6 +14,7 @@
 - Adds a new type of message in simulation mode, to verify that a transaction is valid by checking if the nonce is neither too low nor too high. (!9679)
 - Simulate if a transaction has a correct chain id. (!9752)
 - Fees are now paid by the sender. (!9480)
+- Unused gas is refunded after succesful transactions. (!9915)
 
 ### EVM Node
 
diff --git a/src/kernel_evm/evm_execution/src/handler.rs b/src/kernel_evm/evm_execution/src/handler.rs
index f5d7993c9213..2264a5cc5bf1 100644
--- a/src/kernel_evm/evm_execution/src/handler.rs
+++ b/src/kernel_evm/evm_execution/src/handler.rs
@@ -273,6 +273,31 @@ impl<'a, Host: Runtime> EvmHandler<'a, Host> {
         }
     }
 
+    /// Repay unused gas
+    pub fn repay_gas(
+        &mut self,
+        caller: H160,
+        unused_gas: Option<u64>,
+    ) -> Result<(), EthereumError> {
+        match unused_gas {
+            Some(unused_gas) => {
+                let amount = U256::from(unused_gas) * self.block.gas_price;
+
+                debug_msg!(
+                    self.host,
+                    "{:?} refunded {:?} for transaction",
+                    caller,
+                    amount
+                );
+
+                self.get_or_create_account(caller)?
+                    .balance_add(self.host, amount)
+                    .map_err(EthereumError::from)
+            }
+            None => Ok(()),
+        }
+    }
+
     /// Execute a SputnikVM runtime with this handler
     fn execute(
         &mut self,
diff --git a/src/kernel_evm/evm_execution/src/lib.rs b/src/kernel_evm/evm_execution/src/lib.rs
index 5cc54c9bc002..62463cb8bf1c 100644
--- a/src/kernel_evm/evm_execution/src/lib.rs
+++ b/src/kernel_evm/evm_execution/src/lib.rs
@@ -162,6 +162,11 @@ where
 
         handler.increment_nonce(caller)?;
 
+        if result.is_success {
+            let unused_gas = gas_limit.map(|gl| gl - result.gas_used);
+            handler.repay_gas(caller, unused_gas)?;
+        }
+
         Ok(Some(result))
     } else {
         // caller was unable to pay for the gas limit
@@ -1624,9 +1629,13 @@ mod test {
                 .unwrap(),
             None
         );
+
+        let funds_total =
+            1_000_000 + all_the_gas - expected_result.unwrap().unwrap().gas_used;
+
         assert_eq!(
             get_balance(&mut mock_runtime, &mut evm_account_storage, &caller),
-            1_000_000.into()
+            funds_total.into()
         );
     }
 
-- 
GitLab