give polkadot control over round proposer based on random seed

polkadot/api/src/lib.rs

@@ -31,9 +31,6 @@ extern crate substrate_state_machine as state_machine;
 #[macro_use]
 extern crate error_chain;
 
-#[macro_use]
-extern crate log;
-
 #[cfg(test)]
 extern crate substrate_keyring as keyring;
 
@@ -42,7 +39,7 @@ use client::Client;
 use polkadot_executor::Executor as LocalDispatch;
 use substrate_executor::{NativeExecutionDispatch, NativeExecutor};
 use state_machine::OverlayedChanges;
-use primitives::{AccountId, BlockId, Index, SessionKey, Timestamp};
+use primitives::{AccountId, BlockId, Hash, Index, SessionKey, Timestamp};
 use primitives::parachain::DutyRoster;
 use runtime::{Block, Header, UncheckedExtrinsic, Extrinsic, Call, TimestampCall};
 
@@ -126,6 +123,9 @@ pub trait PolkadotApi {
 	/// Get validators at a given block.
 	fn validators(&self, at: &Self::CheckedBlockId) -> Result<Vec<AccountId>>;
 
+	/// Get the value of the randomness beacon at a given block.
+	fn random_seed(&self, at: &Self::CheckedBlockId) -> Result<Hash>;
+
 	/// Get the authority duty roster at a block.
 	fn duty_roster(&self, at: &Self::CheckedBlockId) -> Result<DutyRoster>;
 
@@ -191,6 +191,10 @@ impl<B: Backend> PolkadotApi for Client<B, NativeExecutor<LocalDispatch>>
 		with_runtime!(self, at, ::runtime::Session::validators)
 	}
 
+	fn random_seed(&self, at: &CheckedId) -> Result<Hash> {
+		with_runtime!(self, at, ::runtime::System::random_seed)
+	}
+
 	fn duty_roster(&self, at: &CheckedId) -> Result<DutyRoster> {
 		// duty roster can only be queried at the start of a block,
 		// so we create a dummy.

polkadot/consensus/src/lib.rs

@@ -495,6 +495,7 @@ impl<C: PolkadotApi, N: Network> bft::ProposerFactory for ProposerFactory<C, N>
 
 		let checked_id = self.client.check_id(BlockId::Hash(parent_hash))?;
 		let duty_roster = self.client.duty_roster(&checked_id)?;
+		let random_seed = self.client.random_seed(&checked_id)?;
 
 		let group_info = make_group_info(duty_roster, authorities)?;
 		let table = Arc::new(SharedTable::new(group_info, sign_with.clone(), parent_hash));
@@ -510,6 +511,7 @@ impl<C: PolkadotApi, N: Network> bft::ProposerFactory for ProposerFactory<C, N>
 			parent_hash,
 			parent_number: parent_header.number,
 			parent_id: checked_id,
+			random_seed,
 			local_key: sign_with,
 			client: self.client.clone(),
 			transaction_pool: self.transaction_pool.clone(),
@@ -533,6 +535,7 @@ pub struct Proposer<C: PolkadotApi, R> {
 	parent_hash: HeaderHash,
 	parent_number: BlockNumber,
 	parent_id: C::CheckedBlockId,
+	random_seed: Hash,
 	client: Arc<C>,
 	local_key: Arc<ed25519::Pair>,
 	transaction_pool: Arc<Mutex<TransactionPool>>,
@@ -625,6 +628,16 @@ impl<C: PolkadotApi, R: TableRouter> bft::Proposer for Proposer<C, R> {
 		Box::new(self.delay.clone().map_err(Error::from).and_then(move |_| evaluated))
 	}
 
+	fn round_proposer(&self, round_number: usize, authorities: &[AuthorityId]) -> AuthorityId {
+		use primitives::uint::U256;
+
+		let len: U256 = authorities.len().into();
+		let offset = U256::from_big_endian(&self.random_seed.0) % len;
+		let offset = offset.low_u64() as usize + round_number;
+
+		authorities[offset % authorities.len()].clone()
+	}
+
 	fn import_misbehavior(&self, misbehavior: Vec<(AuthorityId, bft::Misbehavior)>) {
 		use bft::generic::Misbehavior as GenericMisbehavior;
 		use primitives::bft::{MisbehaviorKind, MisbehaviorReport};

substrate/bft/src/lib.rs

@@ -133,11 +133,16 @@ pub trait Proposer {
 
 	/// Create a proposal.
 	fn propose(&self) -> Self::Create;
+
 	/// Evaluate proposal. True means valid.
-	// TODO: change this to a future.
 	fn evaluate(&self, proposal: &Block) -> Self::Evaluate;
+
 	/// Import witnessed misbehavior.
 	fn import_misbehavior(&self, misbehavior: Vec<(AuthorityId, Misbehavior)>);
+
+	/// Determine the proposer for a given round. This should be a deterministic function
+	/// with consistent results across all authorities.
+	fn round_proposer(&self, round_number: usize, authorities: &[AuthorityId]) -> AuthorityId;
 }
 
 /// Block import trait.
@@ -189,20 +194,7 @@ impl<P: Proposer> generic::Context for BftInstance<P> {
 	}
 
 	fn round_proposer(&self, round: usize) -> AuthorityId {
-		use primitives::hashing::blake2_256;
-
-		// repeat blake2_256 on parent hash round + 1 times.
-		// use as index into authorities vec.
-		// TODO: parent hash is really insecure as a randomness beacon as
-		// the prior can easily influence the block hash.
-		let hashed = (0..round + 1).fold(self.parent_hash.0, |a, _| {
-			blake2_256(&a[..])
-		});
-
-		let index = u32::decode(&mut &hashed[..])
-			.expect("there are more than 4 bytes in a 32 byte hash; qed");
-
-		self.authorities[(index as usize) % self.authorities.len()]
+		self.proposer.round_proposer(round, &self.authorities[..])
 	}
 
 	fn proposal_valid(&self, proposal: &Block) -> Self::EvaluateProposal {
@@ -649,6 +641,10 @@ mod tests {
 		}
 
 		fn import_misbehavior(&self, _misbehavior: Vec<(AuthorityId, Misbehavior)>) {}
+
+		fn round_proposer(&self, round_number: usize, authorities: &[AuthorityId]) -> AuthorityId {
+			authorities[round_number % authorities.len()].clone()
+		}
 	}
 
 	fn make_service(client: FakeClient)