EIP-3529: Reduction in gas refunds (#406)

* Implement changed gas refund rules according to EIP-3529

* Add a unit test for changed gas refunds

* Add missing comment

* Add fork block to spec.hard_forks

crates/ethcore/src/executive.rs

@@ -1463,12 +1463,18 @@ impl<'a, B: 'a + StateBackend> Executive<'a, B> {
             U256::from(schedule.suicide_refund_gas) * U256::from(substate.suicides.len());
         let refunds_bound = sstore_refunds + suicide_refunds;
 
-        // real ammount to refund
+        // real amount to refund
         let gas_left_prerefund = match result {
             Ok(FinalizationResult { gas_left, .. }) => gas_left,
             _ => 0.into(),
         };
-        let refunded = cmp::min(refunds_bound, (t.tx().gas - gas_left_prerefund) >> 1);
+        let refunded = if refunds_bound.is_zero() {
+            refunds_bound
+        } else {
+            let gas_used = t.tx().gas - gas_left_prerefund;
+            let max_refund = gas_used / schedule.max_refund_quotient;
+            cmp::min(max_refund, refunds_bound)
+        };
         let gas_left = gas_left_prerefund + refunded;
 
         let gas_used = t.tx().gas.saturating_sub(gas_left);

crates/ethcore/src/spec/spec.rs

@@ -139,6 +139,8 @@ pub struct CommonParams {
     pub eip2929_transition: BlockNumber,
     /// Number of first block where EIP-2930 rules begin.
     pub eip2930_transition: BlockNumber,
+    /// Number of first block where EIP-3529 rules begin.
+    pub eip3529_transition: BlockNumber,
     /// Number of first block where dust cleanup rules (EIP-168 and EIP169) begin.
     pub dust_protection_transition: BlockNumber,
     /// Nonce cap increase per block. Nonce cap is only checked if dust protection is enabled.
@@ -245,6 +247,11 @@ impl CommonParams {
             schedule.sload_gas = ::vm::schedule::EIP2929_WARM_STORAGE_READ_COST;
             schedule.sstore_reset_gas = ::vm::schedule::EIP2929_SSTORE_RESET_GAS;
         }
+        if block_number >= self.eip3529_transition {
+            schedule.suicide_refund_gas = 0;
+            schedule.sstore_refund_gas = ::vm::schedule::EIP3529_SSTORE_CLEARS_SCHEDULE;
+            schedule.max_refund_quotient = ::vm::schedule::EIP3529_MAX_REFUND_QUOTIENT;
+        }
 
         if block_number >= self.dust_protection_transition {
             schedule.kill_dust = match self.remove_dust_contracts {
@@ -378,6 +385,9 @@ impl From<ethjson::spec::Params> for CommonParams {
             eip2930_transition: p
                 .eip2930_transition
                 .map_or_else(BlockNumber::max_value, Into::into),
+            eip3529_transition: p
+                .eip3529_transition
+                .map_or_else(BlockNumber::max_value, Into::into),
             dust_protection_transition: p
                 .dust_protection_transition
                 .map_or_else(BlockNumber::max_value, Into::into),
@@ -667,6 +677,7 @@ impl Spec {
             params.eip2315_transition,
             params.eip2929_transition,
             params.eip2930_transition,
+            params.eip3529_transition,
             params.dust_protection_transition,
             params.wasm_activation_transition,
             params.wasm_disable_transition,

crates/ethjson/src/spec/params.rs

@@ -112,6 +112,8 @@ pub struct Params {
     /// See `CommonParams` docs.
     pub eip2930_transition: Option<Uint>,
     /// See `CommonParams` docs.
+    pub eip3529_transition: Option<Uint>,
+    /// See `CommonParams` docs.
     pub dust_protection_transition: Option<Uint>,
     /// See `CommonParams` docs.
     pub nonce_cap_increment: Option<Uint>,

crates/vm/evm/src/tests.rs

@@ -1661,6 +1661,61 @@ fn test_access_list_cheap_expensive_cheap(factory: super::Factory) {
     assert_eq!(gas_left, U256::from(0));
 }
 
+evm_test! {test_refund_post_london: test_refund_post_london_int}
+fn test_refund_post_london(factory: super::Factory) {
+    // Compare EIP-3529 for the test cases
+
+    let code = hex!("60006000556000600055").to_vec();
+    london_refund_test(&factory, code, &[], 0);
+
+    let code = hex!("60006000556001600055").to_vec();
+    london_refund_test(&factory, code, &[], 0);
+
+    let code = hex!("60016000556000600055").to_vec();
+    london_refund_test(&factory, code, &[], 19900);
+
+    let code = hex!("60006000556000600055").to_vec();
+    london_refund_test(&factory, code, &[1], 4800);
+
+    let code = hex!("60006000556001600055").to_vec();
+    london_refund_test(&factory, code, &[1], 2800);
+
+    let code = hex!("60006000556002600055").to_vec();
+    london_refund_test(&factory, code, &[1], 0);
+
+    let code = hex!("60026000556000600055").to_vec();
+    london_refund_test(&factory, code, &[1], 4800);
+
+    let code = hex!("60026000556001600055").to_vec();
+    london_refund_test(&factory, code, &[1], 2800);
+
+    let code = hex!("60016000556000600055").to_vec();
+    london_refund_test(&factory, code, &[], 19900);
+
+    let code = hex!("600060005560016000556000600055").to_vec();
+    london_refund_test(&factory, code, &[1], 7600);
+}
+
+fn london_refund_test(
+    factory: &super::Factory,
+    code: Vec<u8>,
+    fill: &[u64],
+    expected_refund: i128,
+) {
+    let mut params = ActionParams::default();
+    params.gas = U256::from(22318);
+    params.code = Some(Arc::new(code));
+    let mut ext = FakeExt::new_london(
+        Address::from_str("0000000000000000000000000000000000000000").unwrap(),
+        Address::from_str("000000000000000000000000636F6E7472616374").unwrap(),
+        &[],
+    );
+    ext.prefill(fill);
+    let vm = factory.create(params, ext.schedule(), ext.depth());
+    vm.exec(&mut ext).ok().unwrap().unwrap();
+    assert_eq!(ext.sstore_clears, expected_refund);
+}
+
 fn push_two_pop_one_constantinople_test(
     factory: &super::Factory,
     opcode: u8,

crates/vm/vm/src/schedule.rs

@@ -28,6 +28,12 @@ pub const EIP2929_SSTORE_RESET_GAS: usize = 5000 - EIP2929_COLD_SLOAD_COST;
 pub const EIP2930_ACCESS_LIST_STORAGE_KEY_COST: usize = 1900;
 /// Gas per received address
 pub const EIP2930_ACCESS_LIST_ADDRESS_COST: usize = 2400;
+/// Gas used per transaction divided by this number is the maximum refundable amount
+pub const MAX_REFUND_QUOTIENT: usize = 2;
+pub const EIP3529_MAX_REFUND_QUOTIENT: usize = 5;
+/// Reduced SSTORE refund as by EIP-3529
+pub const EIP3529_SSTORE_CLEARS_SCHEDULE: usize =
+    EIP2929_SSTORE_RESET_GAS + EIP2930_ACCESS_LIST_STORAGE_KEY_COST;
 
 /// Definition of the cost schedule and other parameterisations for the EVM.
 #[derive(Debug)]
@@ -155,6 +161,8 @@ pub struct Schedule {
     pub eip2929: bool,
     /// Enable EIP-2930 rules for optional access list transactions. it depends on EIP-2929
     pub eip2930: bool,
+    /// Gas used in transaction divided by this number is the maximum refundable amount.
+    pub max_refund_quotient: usize,
 }
 
 /// Wasm cost table
@@ -302,6 +310,7 @@ impl Schedule {
             wasm: None,
             eip2929: false,
             eip2930: false,
+            max_refund_quotient: MAX_REFUND_QUOTIENT,
         }
     }
 
@@ -359,6 +368,17 @@ impl Schedule {
         schedule
     }
 
+    pub fn new_london() -> Schedule {
+        let mut schedule = Self::new_berlin();
+
+        // EIP-3529 changes
+        schedule.suicide_refund_gas = 0;
+        schedule.sstore_refund_gas = EIP3529_SSTORE_CLEARS_SCHEDULE;
+        schedule.max_refund_quotient = EIP3529_MAX_REFUND_QUOTIENT;
+
+        schedule
+    }
+
     fn new(efcd: bool, hdc: bool, tcg: usize) -> Schedule {
         Schedule {
             exceptional_failed_code_deposit: efcd,
@@ -422,6 +442,7 @@ impl Schedule {
             wasm: None,
             eip2929: false,
             eip2930: false,
+            max_refund_quotient: MAX_REFUND_QUOTIENT,
         }
     }
 

crates/vm/vm/src/tests.rs

@@ -63,6 +63,7 @@ pub struct FakeCall {
 /// Can't do recursive calls.
 #[derive(Default)]
 pub struct FakeExt {
+    pub initial_store: HashMap<H256, H256>,
     pub store: HashMap<H256, H256>,
     pub suicides: HashSet<Address>,
     pub calls: HashSet<FakeCall>,
@@ -130,6 +131,12 @@ impl FakeExt {
         ext
     }
 
+    pub fn new_london(from: Address, to: Address, builtins: &[Address]) -> Self {
+        let mut ext = FakeExt::new_berlin(from, to, builtins);
+        ext.schedule = Schedule::new_london();
+        ext
+    }
+
     /// Alter fake externalities to allow wasm
     pub fn with_wasm(mut self) -> Self {
         self.schedule.wasm = Some(Default::default());
@@ -141,11 +148,30 @@ impl FakeExt {
         self.chain_id = chain_id;
         self
     }
+
+    pub fn set_initial_storage(&mut self, key: H256, value: H256) {
+        self.initial_store.insert(key, value);
+    }
+
+    /// Fill the storage before the transaction with `data`, i. e. set
+    /// both original and current values, beginning from address 0.
+    pub fn prefill(&mut self, data: &[u64]) {
+        for (k, v) in data.iter().enumerate() {
+            let key = H256::from_low_u64_be(k as u64);
+            let value = H256::from_low_u64_be(*v);
+            self.set_initial_storage(key, value);
+            self.set_storage(key, value)
+                .expect("FakeExt::set_storage() never returns an Err.");
+        }
+    }
 }
 
 impl Ext for FakeExt {
-    fn initial_storage_at(&self, _key: &H256) -> Result<H256> {
-        Ok(H256::default())
+    fn initial_storage_at(&self, key: &H256) -> Result<H256> {
+        match self.initial_store.get(key) {
+            Some(value) => Ok(*value),
+            None => Ok(H256::default()),
+        }
     }
 
     fn storage_at(&self, key: &H256) -> Result<H256> {