fix(levm): sstore refund logic (#1559)

**Motivation**

There was a discrepancies in gas usage and refunds. The issue was
related to an improper management of gas refunds in `sstore`.

**Description**

There was an error when calculating the gas refund. Adjust the
conditional logic to subtract refunds when reverting storage changes and
adding refunds appropriately. Also, reordered refund and cost logic.

crates/vm/levm/src/gas_cost.rs

@@ -367,17 +367,7 @@ pub fn sstore(
     storage_slot: &StorageSlot,
     new_value: U256,
     storage_slot_was_cold: bool,
-    current_call_frame: &CallFrame,
 ) -> Result<u64, VMError> {
-    // EIP-2200
-    let gas_left = current_call_frame
-        .gas_limit
-        .checked_sub(current_call_frame.gas_used)
-        .ok_or(OutOfGasError::ConsumedGasOverflow)?;
-    if gas_left <= SSTORE_STIPEND {
-        return Err(VMError::OutOfGas(OutOfGasError::MaxGasLimitExceeded));
-    }
-
     let static_gas = SSTORE_STATIC;
 
     let mut base_dynamic_gas = if new_value == storage_slot.current_value {

crates/vm/levm/src/opcode_handlers/stack_memory_storage_flow.rs

@@ -2,7 +2,7 @@ use crate::{
     call_frame::CallFrame,
     constants::{WORD_SIZE, WORD_SIZE_IN_BYTES_USIZE},
     errors::{OpcodeSuccess, OutOfGasError, VMError},
-    gas_cost,
+    gas_cost::{self, SSTORE_STIPEND},
     memory::{self, calculate_memory_size},
     vm::VM,
 };
@@ -164,6 +164,15 @@ impl VM {
         let storage_slot_key = current_call_frame.stack.pop()?;
         let new_storage_slot_value = current_call_frame.stack.pop()?;
 
+        // EIP-2200
+        let gas_left = current_call_frame
+            .gas_limit
+            .checked_sub(current_call_frame.gas_used)
+            .ok_or(OutOfGasError::ConsumedGasOverflow)?;
+        if gas_left <= SSTORE_STIPEND {
+            return Err(VMError::OutOfGas(OutOfGasError::MaxGasLimitExceeded));
+        }
+
         // Convert key from U256 to H256
         let mut bytes = [0u8; 32];
         storage_slot_key.to_big_endian(&mut bytes);
@@ -172,38 +181,27 @@ impl VM {
         let (storage_slot, storage_slot_was_cold) =
             self.access_storage_slot(current_call_frame.to, key)?;
 
-        self.increase_consumed_gas(
-            current_call_frame,
-            gas_cost::sstore(
-                &storage_slot,
-                new_storage_slot_value,
-                storage_slot_was_cold,
-                current_call_frame,
-            )?,
-        )?;
-
         // Gas Refunds
         // Sync gas refund with global env, ensuring consistency accross contexts.
         let mut gas_refunds = self.env.refunded_gas;
 
         if new_storage_slot_value != storage_slot.current_value {
-            if storage_slot.current_value == storage_slot.original_value {
-                if !storage_slot.original_value.is_zero() && new_storage_slot_value.is_zero() {
-                    gas_refunds = gas_refunds
-                        .checked_add(4800)
-                        .ok_or(VMError::GasRefundsOverflow)?;
-                }
-            } else if !storage_slot.original_value.is_zero() {
-                if storage_slot.current_value.is_zero() {
-                    gas_refunds = gas_refunds
-                        .checked_sub(4800)
-                        .ok_or(VMError::GasRefundsUnderflow)?;
-                } else if new_storage_slot_value.is_zero() {
-                    gas_refunds = gas_refunds
-                        .checked_add(4800)
-                        .ok_or(VMError::GasRefundsOverflow)?;
-                }
-            } else if new_storage_slot_value == storage_slot.original_value {
+            if !storage_slot.original_value.is_zero()
+                && !storage_slot.current_value.is_zero()
+                && new_storage_slot_value.is_zero()
+            {
+                gas_refunds = gas_refunds
+                    .checked_add(4800)
+                    .ok_or(VMError::GasRefundsOverflow)?;
+            }
+
+            if !storage_slot.original_value.is_zero() && storage_slot.current_value.is_zero() {
+                gas_refunds = gas_refunds
+                    .checked_sub(4800)
+                    .ok_or(VMError::GasRefundsUnderflow)?;
+            }
+
+            if new_storage_slot_value == storage_slot.original_value {
                 if storage_slot.original_value.is_zero() {
                     gas_refunds = gas_refunds
                         .checked_add(19900)
@@ -214,10 +212,15 @@ impl VM {
                         .ok_or(VMError::GasRefundsOverflow)?;
                 }
             }
-        };
+        }
 
         self.env.refunded_gas = gas_refunds;
 
+        self.increase_consumed_gas(
+            current_call_frame,
+            gas_cost::sstore(&storage_slot, new_storage_slot_value, storage_slot_was_cold)?,
+        )?;
+
         self.update_account_storage(current_call_frame.to, key, new_storage_slot_value)?;
         Ok(OpcodeSuccess::Continue)
     }