Relative slots (#2820)

* Initial work on relative slots for BABE

* More work

* Update core/consensus/babe/src/lib.rs

`Aura` → `Babe`

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* More work on relative slots

* Add missing field in test-runtime

* Bump `impl_version` and `authoring_version`

* Fix compile errors and warnings

* Upgrade dependencies

* Update dependencies more

* Revert some updates to dependencies

Somehow, those broke the build

* Fix compilation errors

* `Duration` → `u128` in calculations

* `slot_duration` is in milleseconds, not seconds

* Median algorithm: ignore blocks with slot_num < sl

* Fix silly compile error

* Store a duration, rather than an instant

It is more useful

* Fix compilation errors

* `INVERSE_NANO` → `NANOS_PER_SEC`

Also: `1000_000_000` → `1_000_000_000`

Suggested-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* Un-bump `authoring_version`

* Disable median algorithm when `median_required_blocks` is 0

Otherwise it would panic.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Simplify panic

* Fix build error

* Create `SignedDuration` struct

for signed `Duration` values.

Suggested-by: Bastian Köcher

* Refactor median algorithm into separate function

* Add issues for FIXMEs and respond to code review

* Fix minor warnings

substrate/core/client/db/src/utils.rs

@@ -189,7 +189,6 @@ pub fn block_id_to_lookup_key<Block>(
 
 /// Maps database error to client error
 pub fn db_err(err: io::Error) -> client::error::Error {
-	use std::error::Error;
 	client::error::Error::Backend(format!("{}", err))
 }
 

substrate/core/consensus/aura/src/lib.rs

@@ -63,7 +63,8 @@ use srml_aura::{
 };
 use substrate_telemetry::{telemetry, CONSENSUS_TRACE, CONSENSUS_DEBUG, CONSENSUS_WARN, CONSENSUS_INFO};
 
-use slots::{CheckedHeader, SlotData, SlotWorker, SlotInfo, SlotCompatible, slot_now, check_equivocation};
+use slots::{CheckedHeader, SlotData, SlotWorker, SlotInfo, SlotCompatible};
+use slots::{SignedDuration, check_equivocation};
 
 pub use aura_primitives::*;
 pub use consensus_common::SyncOracle;
@@ -283,8 +284,8 @@ impl<H, B, C, E, I, P, Error, SO> SlotWorker<B> for AuraWorker<C, E, I, P, SO> w
 		Box::new(proposal_work.map(move |b| {
 			// minor hack since we don't have access to the timestamp
 			// that is actually set by the proposer.
-			let slot_after_building = slot_now(slot_duration);
-			if slot_after_building != Some(slot_num) {
+			let slot_after_building = SignedDuration::default().slot_now(slot_duration);
+			if slot_after_building != slot_num {
 				info!(
 					"Discarding proposal for slot {}; block production took too long",
 					slot_num

substrate/core/consensus/babe/primitives/src/lib.rs

@@ -51,6 +51,15 @@ pub struct BabeConfiguration {
 	///
 	/// Dynamic thresholds may be supported in the future.
 	pub threshold: u64,
+
+	/// The minimum number of blocks that must be received before running the
+	/// median algorithm to compute the offset between the on-chain time and the
+	/// local time. Currently, only the value provided by this type at genesis
+	/// will be used, but this is subject to change.
+	///
+	/// Blocks less than `self.median_required_blocks` must be generated by an
+	/// *initial validator* ― that is, a node that was a validator at genesis.
+	pub median_required_blocks: u64,
 }
 
 #[cfg(feature = "std")]

substrate/core/consensus/babe/src/lib.rs

@@ -30,15 +30,20 @@ use digest::CompatibleDigestItem;
 pub use digest::{BabePreDigest, BABE_VRF_PREFIX};
 pub use babe_primitives::*;
 pub use consensus_common::SyncOracle;
+use consensus_common::import_queue::{
+	SharedBlockImport, SharedJustificationImport, SharedFinalityProofImport,
+	SharedFinalityProofRequestBuilder,
+};
 use consensus_common::well_known_cache_keys::Id as CacheKeyId;
 use runtime_primitives::{generic, generic::{BlockId, OpaqueDigestItemId}, Justification};
 use runtime_primitives::traits::{
 	Block, Header, DigestItemFor, ProvideRuntimeApi,
-	SimpleBitOps,
+	SimpleBitOps, Zero,
 };
-use std::{sync::Arc, u64, fmt::{Debug, Display}};
+use std::{sync::Arc, u64, fmt::{Debug, Display}, time::{Instant, Duration}};
 use runtime_support::serde::{Serialize, Deserialize};
 use parity_codec::{Decode, Encode};
+use parking_lot::Mutex;
 use primitives::{crypto::Pair, sr25519};
 use merlin::Transcript;
 use inherents::{InherentDataProviders, InherentData};
@@ -77,7 +82,7 @@ use futures::{Future, IntoFuture, future};
 use tokio_timer::Timeout;
 use log::{error, warn, debug, info, trace};
 
-use slots::{SlotWorker, SlotData, SlotInfo, SlotCompatible, slot_now};
+use slots::{SlotWorker, SlotData, SlotInfo, SlotCompatible, SignedDuration};
 
 pub use babe_primitives::AuthorityId;
 
@@ -332,8 +337,8 @@ impl<Hash, H, B, C, E, I, Error, SO> SlotWorker<B> for BabeWorker<C, E, I, SO> w
 		Box::new(proposal_work.map(move |b| {
 			// minor hack since we don't have access to the timestamp
 			// that is actually set by the proposer.
-			let slot_after_building = slot_now(slot_duration);
-			if slot_after_building != Some(slot_num) {
+			let slot_after_building = SignedDuration::default().slot_now(slot_duration);
+			if slot_after_building != slot_num {
 				info!(
 					target: "babe",
 					"Discarding proposal for slot {}; block production took too long",
@@ -512,7 +517,8 @@ fn check_header<B: Block + Sized, C: AuxStore>(
 pub struct BabeVerifier<C> {
 	client: Arc<C>,
 	inherent_data_providers: inherents::InherentDataProviders,
-	threshold: u64,
+	config: Config,
+	timestamps: Mutex<(Option<Duration>, Vec<(Instant, u64)>)>,
 }
 
 impl<C> BabeVerifier<C> {
@@ -540,6 +546,38 @@ impl<C> BabeVerifier<C> {
 	}
 }
 
+fn median_algorithm(
+	median_required_blocks: u64,
+	slot_duration: u64,
+	slot_num: u64,
+	slot_now: u64,
+	timestamps: &mut (Option<Duration>, Vec<(Instant, u64)>),
+) {
+	let num_timestamps = timestamps.1.len();
+	if num_timestamps as u64 >= median_required_blocks && median_required_blocks > 0 {
+		let mut new_list: Vec<_> = timestamps.1.iter().map(|&(t, sl)| {
+				let offset: u128 = u128::from(slot_duration)
+					.checked_mul(1_000_000u128) // self.config.get() returns *milliseconds*
+					.and_then(|x| x.checked_mul(u128::from(slot_num).saturating_sub(u128::from(sl))))
+					.expect("we cannot have timespans long enough for this to overflow; qed");
+				const NANOS_PER_SEC: u32 = 1_000_000_000;
+				let nanos = (offset % u128::from(NANOS_PER_SEC)) as u32;
+				let secs = (offset / u128::from(NANOS_PER_SEC)) as u64;
+				t + Duration::new(secs, nanos)
+			}).collect();
+		// FIXME #2926: use a selection algorithm instead of a full sorting algorithm.
+		new_list.sort_unstable();
+		let &median = new_list
+			.get(num_timestamps / 2)
+			.expect("we have at least one timestamp, so this is a valid index; qed");
+		timestamps.1.clear();
+		// FIXME #2927: pass this to the block authoring logic somehow
+		timestamps.0.replace(Instant::now() - median);
+	} else {
+		timestamps.1.push((Instant::now(), slot_now))
+	}
+}
+
 impl<B: Block, C> Verifier<B> for BabeVerifier<C> where
 	C: ProvideRuntimeApi + Send + Sync + AuxStore + ProvideCache<B>,
 	C::Api: BlockBuilderApi<B> + BabeApi<B>,
@@ -582,7 +620,7 @@ impl<B: Block, C> Verifier<B> for BabeVerifier<C> where
 			header,
 			hash,
 			&authorities[..],
-			self.threshold,
+			self.config.threshold(),
 		)?;
 		match checked_header {
 			CheckedHeader::Checked(pre_header, (pre_digest, seal)) => {
@@ -629,7 +667,13 @@ impl<B: Block, C> Verifier<B> for BabeVerifier<C> where
 					auxiliary: Vec::new(),
 					fork_choice: ForkChoiceStrategy::LongestChain,
 				};
-
+				median_algorithm(
+					self.config.0.median_required_blocks,
+					self.config.get(),
+					slot_num,
+					slot_now,
+					&mut *self.timestamps.lock(),
+				);
 				// FIXME #1019 extract authorities
 				Ok((import_block, maybe_keys))
 			}
@@ -739,6 +783,72 @@ fn claim_slot(
 	get_keypair(key).vrf_sign_n_check(transcript, |inout| check(inout, threshold))
 }
 
+fn initialize_authorities_cache<B, C>(client: &C) -> Result<(), ConsensusError> where
+	B: Block,
+	C: ProvideRuntimeApi + ProvideCache<B>,
+	C::Api: BabeApi<B>,
+{
+	// no cache => no initialization
+	let cache = match client.cache() {
+		Some(cache) => cache,
+		None => return Ok(()),
+	};
+
+	// check if we already have initialized the cache
+	let genesis_id = BlockId::Number(Zero::zero());
+	let genesis_authorities: Option<Vec<AuthorityId>> = cache
+		.get_at(&well_known_cache_keys::AUTHORITIES, &genesis_id)
+		.and_then(|v| Decode::decode(&mut &v[..]));
+	if genesis_authorities.is_some() {
+		return Ok(());
+	}
+
+	let map_err = |error| consensus_common::Error::from(consensus_common::Error::ClientImport(
+		format!(
+			"Error initializing authorities cache: {}",
+			error,
+		)));
+	let genesis_authorities = authorities(client, &genesis_id)?;
+	cache.initialize(&well_known_cache_keys::AUTHORITIES, genesis_authorities.encode())
+		.map_err(map_err)
+}
+
+/// Start an import queue for the Babe consensus algorithm.
+pub fn import_queue<B, C, E>(
+	config: Config,
+	block_import: SharedBlockImport<B>,
+	justification_import: Option<SharedJustificationImport<B>>,
+	finality_proof_import: Option<SharedFinalityProofImport<B>>,
+	finality_proof_request_builder: Option<SharedFinalityProofRequestBuilder<B>>,
+	client: Arc<C>,
+	inherent_data_providers: InherentDataProviders,
+) -> Result<BabeImportQueue<B>, consensus_common::Error> where
+	B: Block,
+	C: 'static + ProvideRuntimeApi + ProvideCache<B> + Send + Sync + AuxStore,
+	C::Api: BlockBuilderApi<B> + BabeApi<B>,
+	DigestItemFor<B>: CompatibleDigestItem,
+	E: 'static,
+{
+	register_babe_inherent_data_provider(&inherent_data_providers, config.get())?;
+	initialize_authorities_cache(&*client)?;
+
+	let verifier = Arc::new(
+		BabeVerifier {
+			client: client,
+			inherent_data_providers,
+			timestamps: Default::default(),
+			config,
+		}
+	);
+	Ok(BasicQueue::new(
+		verifier,
+		block_import,
+		justification_import,
+		finality_proof_import,
+		finality_proof_request_builder,
+	))
+}
+
 #[cfg(test)]
 #[allow(dead_code, unused_imports, deprecated)]
 // FIXME #2532: need to allow deprecated until refactor is done
@@ -753,7 +863,6 @@ mod tests {
 	use network::test::{Block as TestBlock, PeersClient};
 	use runtime_primitives::traits::{Block as BlockT, DigestFor};
 	use network::config::ProtocolConfig;
-	use parking_lot::Mutex;
 	use tokio::runtime::current_thread;
 	use keyring::sr25519::Keyring;
 	use super::generic::DigestItem;
@@ -837,7 +946,8 @@ mod tests {
 			Arc::new(BabeVerifier {
 				client,
 				inherent_data_providers,
-				threshold: config.threshold(),
+				config,
+				timestamps: Default::default(),
 			})
 		}
 

substrate/core/consensus/slots/src/lib.rs

@@ -25,7 +25,7 @@
 mod slots;
 mod aux_schema;
 
-pub use slots::{slot_now, SlotInfo, Slots};
+pub use slots::{SignedDuration, SlotInfo, Slots};
 pub use aux_schema::{check_equivocation, MAX_SLOT_CAPACITY, PRUNING_BOUND};
 
 use codec::{Decode, Encode};

substrate/core/consensus/slots/src/slots.rs

@@ -23,29 +23,44 @@ use consensus_common::Error;
 use futures::prelude::*;
 use futures::try_ready;
 use inherents::{InherentData, InherentDataProviders};
-use log::warn;
+
 use std::marker::PhantomData;
 use std::time::{Duration, Instant};
 use tokio_timer::Delay;
 
 /// Returns current duration since unix epoch.
-pub fn duration_now() -> Option<Duration> {
+pub fn duration_now() -> Duration {
 	use std::time::SystemTime;
-
 	let now = SystemTime::now();
-	now.duration_since(SystemTime::UNIX_EPOCH)
-		.map_err(|e| {
-			warn!(
-				"Current time {:?} is before unix epoch. Something is wrong: {:?}",
-				now, e
-			);
-		})
-		.ok()
+	now.duration_since(SystemTime::UNIX_EPOCH).unwrap_or_else(|e| panic!(
+		"Current time {:?} is before unix epoch. Something is wrong: {:?}",
+		now,
+		e,
+	))
+}
+
+
+/// A `Duration` with a sign (before or after).  Immutable.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
+pub struct SignedDuration {
+	offset: Duration,
+	is_positive: bool,
 }
 
-/// Get the slot for now.
-pub fn slot_now(slot_duration: u64) -> Option<u64> {
-	duration_now().map(|s| s.as_secs() / slot_duration)
+impl SignedDuration {
+	/// Construct a `SignedDuration`
+	pub fn new(offset: Duration, is_positive: bool) -> Self {
+		Self { offset, is_positive }
+	}
+
+	/// Get the slot for now.  Panics if `slot_duration` is 0.
+	pub fn slot_now(&self, slot_duration: u64) -> u64 {
+		if self.is_positive {
+			duration_now() + self.offset
+		} else {
+			duration_now() - self.offset
+		}.as_secs() / slot_duration
+	}
 }
 
 /// Returns the duration until the next slot, based on current duration since
@@ -112,11 +127,7 @@ impl<SC: SlotCompatible> Stream for Slots<SC> {
 		self.inner_delay = match self.inner_delay.take() {
 			None => {
 				// schedule wait.
-				let wait_until = match duration_now() {
-					None => return Ok(Async::Ready(None)),
-					Some(now) => Instant::now() + time_until_next(now, slot_duration),
-				};
-
+				let wait_until = Instant::now() + time_until_next(duration_now(), slot_duration);
 				Some(Delay::new(wait_until))
 			}
 			Some(d) => Some(d),

substrate/core/network/src/test/mod.rs

@@ -517,7 +517,7 @@ impl<D, S: NetworkSpecialization<Block>> Peer<D, S> {
 		use_tokio: bool,
 		network_to_protocol_sender: mpsc::UnboundedSender<FromNetworkMsg<Block>>,
 		protocol_sender: mpsc::UnboundedSender<ProtocolMsg<Block, S>>,
-		network_sender: mpsc::UnboundedSender<NetworkMsg<Block>>,
+		_network_sender: mpsc::UnboundedSender<NetworkMsg<Block>>,
 		network_port: mpsc::UnboundedReceiver<NetworkMsg<Block>>,
 		data: D,
 	) -> Self {

substrate/core/test-runtime/src/lib.rs

@@ -470,6 +470,7 @@ cfg_if! {
 						slot_duration: 1,
 						expected_block_time: 1,
 						threshold: std::u64::MAX,
+						median_required_blocks: 100,
 					}
 				}
 				fn authorities() -> Vec<BabeId> { system::authorities() }
@@ -611,6 +612,7 @@ cfg_if! {
 			impl consensus_babe::BabeApi<Block> for Runtime {
 				fn startup_data() -> consensus_babe::BabeConfiguration {
 					consensus_babe::BabeConfiguration {
+						median_required_blocks: 0,
 						slot_duration: 1,
 						expected_block_time: 1,
 						threshold: core::u64::MAX,