Set the block gas limit to the value returned by a contract call

ethcore/block-gas-limit/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+description = "A crate to interact with the block gas limit contract"
+name = "block-gas-limit"
+version = "0.1.0"
+authors = ["Parity Technologies <[email protected]>"]
+edition = "2018"
+license = "GPL-3.0"
+
+[dependencies]
+client-traits = { path = "../client-traits" }
+common-types = { path = "../types" }
+ethabi = "9.0.1"
+ethabi-derive = "9.0.1"
+ethabi-contract = "9.0.0"
+ethereum-types = "0.8.0"
+log = "0.4"
+
+[dev-dependencies]
+ethcore = { path = "..", features = ["test-helpers"] }
+spec = { path = "../spec" }

ethcore/block-gas-limit/res/block_gas_limit.json

@@ -0,0 +1,16 @@
+[
+	{
+		"constant": true,
+		"inputs": [],
+		"name": "blockGasLimit",
+		"outputs": [
+			{
+				"name": "",
+				"type": "uint256"
+			}
+		],
+		"payable": false,
+		"stateMutability": "view",
+		"type": "function"
+	}
+]

ethcore/block-gas-limit/src/lib.rs

@@ -0,0 +1,39 @@
+// Copyright 2015-2019 Parity Technologies (UK) Ltd.
+// This file is part of Parity Ethereum.
+
+// Parity Ethereum is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Ethereum is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Ethereum.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A client interface for interacting with the block gas limit contract.
+
+use client_traits::BlockChainClient;
+use common_types::{header::Header, ids::BlockId};
+use ethabi::FunctionOutputDecoder;
+use ethabi_contract::use_contract;
+use ethereum_types::{Address, U256};
+use log::{debug, error};
+
+use_contract!(contract, "res/block_gas_limit.json");
+
+pub fn block_gas_limit(full_client: &dyn BlockChainClient, header: &Header, address: Address) -> Option<U256> {
+	let (data, decoder) = contract::functions::block_gas_limit::call();
+	let value = full_client.call_contract(BlockId::Hash(*header.parent_hash()), address, data).map_err(|err| {
+		error!(target: "block_gas_limit", "Contract call failed. Not changing the block gas limit. {:?}", err);
+	}).ok()?;
+	if value.is_empty() {
+		debug!(target: "block_gas_limit", "Contract call returned nothing. Not changing the block gas limit.");
+		None
+	} else {
+		decoder.decode(&value).ok()
+	}
+}

ethcore/engine/src/engine.rs

@@ -347,6 +347,12 @@ pub trait Engine: Sync + Send {
 		Ok(*header.author())
 	}
 
+	/// Overrides the block gas limit. Whenever this returns `Some` for a header, the next block's gas limit must be
+	/// exactly that value.
+	fn gas_limit_override(&self, _header: &Header) -> Option<U256> {
+		None
+	}
+
 	/// Get the general parameters of the chain.
 	fn params(&self) -> &CommonParams;
 
@@ -397,6 +403,11 @@ pub trait Engine: Sync + Send {
 	fn decode_transaction(&self, transaction: &[u8]) -> Result<UnverifiedTransaction, transaction::Error> {
 		self.machine().decode_transaction(transaction)
 	}
+
+	/// The configured minimum gas limit.
+	fn min_gas_limit(&self) -> U256 {
+		self.params().min_gas_limit
+	}
 }
 
 /// Verifier for all blocks within an epoch with self-contained state.

ethcore/engines/authority-round/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 license = "GPL-3.0"
 
 [dependencies]
+block-gas-limit = { path = "../../block-gas-limit" }
 block-reward = { path = "../../block-reward" }
 client-traits = { path = "../../client-traits" }
 common-types = { path = "../../types" }

ethcore/engines/authority-round/src/lib.rs

@@ -42,6 +42,7 @@ use std::u64;
 
 use client_traits::{EngineClient, ForceUpdateSealing, TransactionRequest};
 use engine::{Engine, ConstructedVerifier};
+use block_gas_limit::block_gas_limit;
 use block_reward::{self, BlockRewardContract, RewardKind};
 use ethjson;
 use machine::{
@@ -51,6 +52,7 @@ use machine::{
 use macros::map;
 use keccak_hash::keccak;
 use log::{info, debug, error, trace, warn};
+use lru_cache::LruCache;
 use engine::signer::EngineSigner;
 use parity_crypto::publickey::Signature;
 use io::{IoContext, IoHandler, TimerToken, IoService};
@@ -78,7 +80,6 @@ use common_types::{
 	transaction::SignedTransaction,
 };
 use unexpected::{Mismatch, OutOfBounds};
-
 use validator_set::{ValidatorSet, SimpleList, new_validator_set};
 
 mod finality;
@@ -124,10 +125,16 @@ pub struct AuthorityRoundParams {
 	pub strict_empty_steps_transition: u64,
 	/// If set, enables random number contract integration. It maps the transition block to the contract address.
 	pub randomness_contract_address: BTreeMap<u64, Address>,
+	/// The addresses of contracts that determine the block gas limit with their associated block
+	/// numbers.
+	pub block_gas_limit_contract_transitions: BTreeMap<u64, Address>,
 }
 
 const U16_MAX: usize = ::std::u16::MAX as usize;
 
+/// The number of recent block hashes for which the gas limit override is memoized.
+const GAS_LIMIT_OVERRIDE_CACHE_CAPACITY: usize = 10;
+
 impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 	fn from(p: ethjson::spec::AuthorityRoundParams) -> Self {
 		let map_step_duration = |u: ethjson::uint::Uint| {
@@ -180,6 +187,12 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 				(block.as_u64(), addr.into())
 			}).collect()
 		});
+		let block_gas_limit_contract_transitions: BTreeMap<_, _> =
+			p.block_gas_limit_contract_transitions
+			.unwrap_or_default()
+			.into_iter()
+			.map(|(block_num, address)| (block_num.into(), address.into()))
+			.collect();
 		AuthorityRoundParams {
 			step_durations,
 			validators: new_validator_set(p.validators),
@@ -196,6 +209,7 @@ impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
 			two_thirds_majority_transition: p.two_thirds_majority_transition.map_or_else(BlockNumber::max_value, Into::into),
 			strict_empty_steps_transition: p.strict_empty_steps_transition.map_or(0, Into::into),
 			randomness_contract_address,
+			block_gas_limit_contract_transitions,
 		}
 	}
 }
@@ -565,6 +579,10 @@ pub struct AuthorityRound {
 	received_step_hashes: RwLock<BTreeMap<(u64, Address), H256>>,
 	/// If set, enables random number contract integration. It maps the transition block to the contract address.
 	randomness_contract_address: BTreeMap<u64, Address>,
+	/// The addresses of contracts that determine the block gas limit.
+	block_gas_limit_contract_transitions: BTreeMap<u64, Address>,
+	/// Memoized gas limit overrides, by block hash.
+	gas_limit_override_cache: Mutex<LruCache<H256, Option<U256>>>,
 }
 
 // header-chain validator.
@@ -867,6 +885,8 @@ impl AuthorityRound {
 				machine,
 				received_step_hashes: RwLock::new(Default::default()),
 				randomness_contract_address: our_params.randomness_contract_address,
+				block_gas_limit_contract_transitions: our_params.block_gas_limit_contract_transitions,
+				gas_limit_override_cache: Mutex::new(LruCache::new(GAS_LIMIT_OVERRIDE_CACHE_CAPACITY)),
 			});
 
 		// Do not initialize timeouts for tests.
@@ -1218,6 +1238,14 @@ impl Engine for AuthorityRound {
 
 		let score = calculate_score(parent_step, current_step, current_empty_steps_len);
 		header.set_difficulty(score);
+		if let Some(gas_limit) = self.gas_limit_override(header) {
+			trace!(target: "engine", "Setting gas limit to {} for block {}.", gas_limit, header.number());
+			let parent_gas_limit = *parent.gas_limit();
+			header.set_gas_limit(gas_limit);
+			if parent_gas_limit != gas_limit {
+				info!(target: "engine", "Block gas limit was changed from {} to {}.", parent_gas_limit, gas_limit);
+			}
+		}
 	}
 
 	fn sealing_state(&self) -> SealingState {
@@ -1834,6 +1862,30 @@ impl Engine for AuthorityRound {
 	fn params(&self) -> &CommonParams {
 		self.machine.params()
 	}
+
+	fn gas_limit_override(&self, header: &Header) -> Option<U256> {
+		let (_, &address) = self.block_gas_limit_contract_transitions.range(..=header.number()).last()?;
+		let client = match self.client.read().as_ref().and_then(|weak| weak.upgrade()) {
+			Some(client) => client,
+			None => {
+				error!(target: "engine", "Unable to prepare block: missing client ref.");
+				return None;
+			}
+		};
+		let full_client = match client.as_full_client() {
+			Some(full_client) => full_client,
+			None => {
+				error!(target: "engine", "Failed to upgrade to BlockchainClient.");
+				return None;
+			}
+		};
+		if let Some(limit) = self.gas_limit_override_cache.lock().get_mut(&header.hash()) {
+			return *limit;
+		}
+		let limit = block_gas_limit(full_client, header, address);
+		self.gas_limit_override_cache.lock().insert(header.hash(), limit);
+		limit
+	}
 }
 
 /// A helper accumulator function mapping a step duration and a step duration transition timestamp
@@ -1910,6 +1962,7 @@ mod tests {
 			strict_empty_steps_transition: 0,
 			two_thirds_majority_transition: 0,
 			randomness_contract_address: BTreeMap::new(),
+			block_gas_limit_contract_transitions: BTreeMap::new(),
 		};
 
 		// mutate aura params

ethcore/verification/src/verification.rs

@@ -61,6 +61,14 @@ pub fn verify_block_basic(block: &Unverified, engine: &dyn Engine, check_seal: b
 		}
 	}
 
+	if let Some(gas_limit) = engine.gas_limit_override(&block.header) {
+		if *block.header.gas_limit() != gas_limit {
+			return Err(From::from(BlockError::InvalidGasLimit(
+				OutOfBounds { min: Some(gas_limit), max: Some(gas_limit), found: *block.header.gas_limit() }
+			)));
+		}
+	}
+
 	for t in &block.transactions {
 		engine.verify_transaction_basic(t, &block.header)?;
 	}
@@ -284,22 +292,24 @@ pub(crate) fn verify_header_params(header: &Header, engine: &dyn Engine, check_s
 			found: *header.gas_used()
 		})));
 	}
-	let min_gas_limit = engine.params().min_gas_limit;
-	if header.gas_limit() < &min_gas_limit {
-		return Err(From::from(BlockError::InvalidGasLimit(OutOfBounds {
-			min: Some(min_gas_limit),
-			max: None,
-			found: *header.gas_limit()
-		})));
-	}
-	if let Some(limit) = engine.maximum_gas_limit() {
-		if header.gas_limit() > &limit {
+	if engine.gas_limit_override(header).is_none() {
+		let min_gas_limit = engine.min_gas_limit();
+		if header.gas_limit() < &min_gas_limit {
 			return Err(From::from(BlockError::InvalidGasLimit(OutOfBounds {
-				min: None,
-				max: Some(limit),
+				min: Some(min_gas_limit),
+				max: None,
 				found: *header.gas_limit()
 			})));
 		}
+		if let Some(limit) = engine.maximum_gas_limit() {
+			if header.gas_limit() > &limit {
+				return Err(From::from(BlockError::InvalidGasLimit(OutOfBounds {
+					min: None,
+					max: Some(limit),
+					found: *header.gas_limit()
+				})));
+			}
+		}
 	}
 	let maximum_extra_data_size = engine.maximum_extra_data_size();
 	if header.number() != 0 && header.extra_data().len() > maximum_extra_data_size {
@@ -358,8 +368,6 @@ fn verify_parent(header: &Header, parent: &Header, engine: &dyn Engine) -> Resul
 	assert!(header.parent_hash().is_zero() || &parent.hash() == header.parent_hash(),
 			"Parent hash should already have been verified; qed");
 
-	let gas_limit_divisor = engine.params().gas_limit_bound_divisor;
-
 	if !engine.is_timestamp_valid(header.timestamp(), parent.timestamp()) {
 		let now = SystemTime::now();
 		let min = CheckedSystemTime::checked_add(now, Duration::from_secs(parent.timestamp().saturating_add(1)))
@@ -382,16 +390,18 @@ fn verify_parent(header: &Header, parent: &Header, engine: &dyn Engine) -> Resul
 			found: header.number()
 		}).into());
 	}
-
-	let parent_gas_limit = *parent.gas_limit();
-	let min_gas = parent_gas_limit - parent_gas_limit / gas_limit_divisor;
-	let max_gas = parent_gas_limit + parent_gas_limit / gas_limit_divisor;
-	if header.gas_limit() <= &min_gas || header.gas_limit() >= &max_gas {
-		return Err(From::from(BlockError::InvalidGasLimit(OutOfBounds {
-			min: Some(min_gas),
-			max: Some(max_gas),
-			found: *header.gas_limit()
-		})));
+	if engine.gas_limit_override(header).is_none() {
+		let gas_limit_divisor = engine.params().gas_limit_bound_divisor;
+		let parent_gas_limit = *parent.gas_limit();
+		let min_gas = parent_gas_limit - parent_gas_limit / gas_limit_divisor;
+		let max_gas = parent_gas_limit + parent_gas_limit / gas_limit_divisor;
+		if header.gas_limit() <= &min_gas || header.gas_limit() >= &max_gas {
+			return Err(From::from(BlockError::InvalidGasLimit(OutOfBounds {
+				min: Some(min_gas),
+				max: Some(max_gas),
+				found: *header.gas_limit()
+			})));
+		}
 	}
 
 	Ok(())
@@ -522,7 +532,7 @@ mod tests {
 			// that's an invalid transaction list rlp
 			let invalid_transactions = vec![vec![0u8]];
 			header.set_transactions_root(ordered_trie_root(&invalid_transactions));
-			header.set_gas_limit(engine.params().min_gas_limit);
+			header.set_gas_limit(engine.min_gas_limit());
 			rlp.append(&header);
 			rlp.append_list::<Vec<u8>, _>(&invalid_transactions);
 			rlp.append_raw(&rlp::EMPTY_LIST_RLP, 1);
@@ -541,7 +551,7 @@ mod tests {
 		let spec = spec::new_test();
 		let engine = &*spec.engine;
 
-		let min_gas_limit = engine.params().min_gas_limit;
+		let min_gas_limit = engine.min_gas_limit();
 		good.set_gas_limit(min_gas_limit);
 		good.set_timestamp(40);
 		good.set_number(10);