Support variable session length for Rococo chains at genesis (#2167)

This pr adds support to change the session length of a Rococo chain at
genesis. This is rather useful because Rococo has a session length of
1 hour, while on rococo-local you will now get 1 minute. This improves
the dev experience, because a parachain is only going live at the
start of a new session.

polkadot/Cargo.lock

@@ -5672,6 +5672,7 @@ dependencies = [
  "sp-offchain",
  "sp-runtime",
  "sp-session",
+ "sp-state-machine",
  "sp-storage",
  "sp-transaction-pool",
  "sp-trie",

polkadot/node/service/Cargo.toml

@@ -39,6 +39,7 @@ sp-session = { git = "https://github.com/paritytech/substrate", branch = "master
 sp-storage = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sp-transaction-pool = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sp-trie = { git = "https://github.com/paritytech/substrate", branch = "master" }
+sp-state-machine = { git = "https://github.com/paritytech/substrate", branch = "master" }
 
 # Substrate Pallets
 pallet-babe = { git = "https://github.com/paritytech/substrate", branch = "master" }

polkadot/node/service/src/chain_spec.rs

@@ -66,7 +66,32 @@ pub type KusamaChainSpec = service::GenericChainSpec<kusama::GenesisConfig, Exte
 pub type WestendChainSpec = service::GenericChainSpec<westend::GenesisConfig, Extensions>;
 
 /// The `ChainSpec` parametrized for the rococo runtime.
-pub type RococoChainSpec = service::GenericChainSpec<rococo::GenesisConfig, Extensions>;
+pub type RococoChainSpec = service::GenericChainSpec<RococoGenesisExt, Extensions>;
+
+/// Extension for the Rococo genesis config to support a custom changes to the genesis state.
+#[derive(serde::Serialize, serde::Deserialize)]
+pub struct RococoGenesisExt {
+	/// The runtime genesis config.
+	runtime_genesis_config: rococo::GenesisConfig,
+	/// The session length in blocks.
+	///
+	/// If `None` is supplied, the default value is used.
+	session_length_in_blocks: Option<u32>,
+}
+
+impl sp_runtime::BuildStorage for RococoGenesisExt {
+	fn assimilate_storage(
+		&self,
+		storage: &mut sp_core::storage::Storage,
+	) -> Result<(), String> {
+		sp_state_machine::BasicExternalities::execute_with_storage(storage, || {
+			if let Some(length) = self.session_length_in_blocks.as_ref() {
+				rococo::constants::time::EpochDurationInBlocks::set(length);
+			}
+		});
+		self.runtime_genesis_config.assimilate_storage(storage)
+	}
+}
 
 pub fn polkadot_config() -> Result<PolkadotChainSpec, String> {
 	PolkadotChainSpec::from_json_bytes(&include_bytes!("../res/polkadot.json")[..])
@@ -925,7 +950,10 @@ pub fn rococo_staging_testnet_config() -> Result<RococoChainSpec, String> {
 		"Rococo Staging Testnet",
 		"rococo_staging_testnet",
 		ChainType::Live,
-		move || rococo_staging_testnet_config_genesis(wasm_binary),
+		move || RococoGenesisExt {
+			runtime_genesis_config: rococo_staging_testnet_config_genesis(wasm_binary),
+			session_length_in_blocks: None,
+		},
 		boot_nodes,
 		Some(
 			TelemetryEndpoints::new(vec![(ROCOCO_STAGING_TELEMETRY_URL.to_string(), 0)])
@@ -1542,7 +1570,11 @@ pub fn rococo_local_testnet_config() -> Result<RococoChainSpec, String> {
 		"Rococo Local Testnet",
 		"rococo_local_testnet",
 		ChainType::Local,
-		move || rococo_local_testnet_genesis(wasm_binary),
+		move || RococoGenesisExt {
+			runtime_genesis_config: rococo_local_testnet_genesis(wasm_binary),
+			// Use 1 minute session length.
+			session_length_in_blocks: Some(10),
+		},
 		vec![],
 		None,
 		Some(DEFAULT_PROTOCOL_ID),

polkadot/runtime/rococo/src/constants.rs

@@ -33,7 +33,9 @@ pub mod time {
 	use primitives::v0::{Moment, BlockNumber};
 	pub const MILLISECS_PER_BLOCK: Moment = 6000;
 	pub const SLOT_DURATION: Moment = MILLISECS_PER_BLOCK;
-	pub const EPOCH_DURATION_IN_BLOCKS: BlockNumber = 1 * HOURS;
+	frame_support::parameter_types! {
+		pub storage EpochDurationInBlocks: BlockNumber = 1 * HOURS;
+	}
 
 	// These time units are defined in number of blocks.
 	pub const MINUTES: BlockNumber = 60_000 / (MILLISECS_PER_BLOCK as BlockNumber);

polkadot/runtime/rococo/src/lib.rs

@@ -328,13 +328,13 @@ parameter_types! {
 	pub const RewardCurve: &'static PiecewiseLinear<'static> = &REWARD_CURVE;
 	pub const MaxNominatorRewardedPerValidator: u32 = 64;
 	// quarter of the last session will be for election.
-	pub const ElectionLookahead: BlockNumber = EPOCH_DURATION_IN_BLOCKS / 4;
+	pub ElectionLookahead: BlockNumber = EpochDurationInBlocks::get() / 4;
 	pub const MaxIterations: u32 = 10;
 	pub MinSolutionScoreBump: Perbill = Perbill::from_rational_approximation(5u32, 10_000);
 }
 
 parameter_types! {
-	pub const SessionDuration: BlockNumber = EPOCH_DURATION_IN_BLOCKS as _;
+	pub SessionDuration: BlockNumber = EpochDurationInBlocks::get() as _;
 }
 
 parameter_types! {
@@ -457,12 +457,11 @@ impl pallet_session::Config for Runtime {
 }
 
 parameter_types! {
-	pub const EpochDuration: u64 = EPOCH_DURATION_IN_BLOCKS as u64;
 	pub const ExpectedBlockTime: Moment = MILLISECS_PER_BLOCK;
 }
 
 impl pallet_babe::Config for Runtime {
-	type EpochDuration = EpochDuration;
+	type EpochDuration = EpochDurationInBlocks;
 	type ExpectedBlockTime = ExpectedBlockTime;
 
 	// session module is the trigger
@@ -798,7 +797,7 @@ sp_api::impl_runtime_apis! {
 			// <https://research.web3.foundation/en/latest/polkadot/BABE/Babe/#6-practical-results>
 			babe_primitives::BabeGenesisConfiguration {
 				slot_duration: Babe::slot_duration(),
-				epoch_length: EpochDuration::get(),
+				epoch_length: EpochDurationInBlocks::get().into(),
 				c: PRIMARY_PROBABILITY,
 				genesis_authorities: Babe::authorities(),
 				randomness: Babe::randomness(),