babe: allow skipping over empty epochs

client/consensus/babe/src/lib.rs

@@ -1525,11 +1525,12 @@ where
 			if let Some(next_epoch_descriptor) = next_epoch_digest {
 				old_epoch_changes = Some((*epoch_changes).clone());
 
-				let viable_epoch = epoch_changes
+				let mut viable_epoch = epoch_changes
 					.viable_epoch(&epoch_descriptor, |slot| Epoch::genesis(&self.config, slot))
 					.ok_or_else(|| {
 						ConsensusError::ClientImport(Error::<Block>::FetchEpoch(parent_hash).into())
-					})?;
+					})?
+					.into_cloned();
 
 				let epoch_config = next_config_digest
 					.map(Into::into)
@@ -1542,6 +1543,35 @@ where
 					log::Level::Info
 				};
 
+				if viable_epoch.as_ref().end_slot() <= slot {
+					// some epochs must have been skipped as our current slot
+					// fits outside the current epoch. we will figure out
+					// which epoch it belongs to and we will re-use the same
+					// data for that epoch
+					let mut epoch_data = viable_epoch.as_mut();
+					let skipped_epochs = (*slot - *epoch_data.start_slot) / epoch_data.duration;
+
+					let original_epoch_index = epoch_data.epoch_index;
+
+					// NOTE: notice that we are only updating a local copy of the `Epoch`, this
+					// makes it so that when we insert the next epoch into `EpochChanges` below
+					// (after incrementing it), it will use the correct epoch index and start slot.
+					// we do not update the original epoch that will be re-used because there might
+					// be other forks (that we haven't imported) where the epoch isn't skipped, and
+					// to import those forks we want to keep the original epoch data. not updating
+					// the original epoch works because when we search the tree for which epoch to
+					// use for a given slot, we will search in-depth with the predicate
+					// `epoch.start_slot <= slot` which will still match correctly without updating
+					// `start_slot` to the correct value as below.
+					epoch_data.epoch_index += skipped_epochs;
+					epoch_data.start_slot =
+						Slot::from(*epoch_data.start_slot + skipped_epochs * epoch_data.duration);
+
+					warn!(target: "babe", "👶 Epoch(s) skipped: from {} to {}",
+						original_epoch_index, epoch_data.epoch_index,
+					);
+				}
+
 				log!(target: "babe",
 					 log_level,
 					 "👶 New epoch {} launching at block {} (block slot {} >= start slot {}).",

client/consensus/babe/src/tests.rs

@@ -27,6 +27,7 @@ use rand_chacha::{rand_core::SeedableRng, ChaChaRng};
 use sc_block_builder::{BlockBuilder, BlockBuilderProvider};
 use sc_client_api::{backend::TransactionFor, BlockchainEvents, Finalizer};
 use sc_consensus::{BoxBlockImport, BoxJustificationImport};
+use sc_consensus_epochs::{EpochIdentifier, EpochIdentifierPosition};
 use sc_consensus_slots::BackoffAuthoringOnFinalizedHeadLagging;
 use sc_keystore::LocalKeystore;
 use sc_network_test::{Block as TestBlock, *};
@@ -1069,3 +1070,131 @@ fn obsolete_blocks_aux_data_cleanup() {
 	// Present C4, C5
 	assert!(aux_data_check(&fork3_hashes, true));
 }
+
+#[test]
+fn allows_skipping_epochs() {
+	let mut net = BabeTestNet::new(1);
+
+	let peer = net.peer(0);
+	let data = peer.data.as_ref().expect("babe link set up during initialization");
+
+	let client = peer.client().as_client();
+	let mut block_import = data.block_import.lock().take().expect("import set up during init");
+
+	let mut proposer_factory = DummyFactory {
+		client: client.clone(),
+		config: data.link.config.clone(),
+		epoch_changes: data.link.epoch_changes.clone(),
+		mutator: Arc::new(|_, _| ()),
+	};
+
+	let epoch_changes = data.link.epoch_changes.clone();
+	let epoch_length = data.link.config.epoch_length;
+
+	// we create all of the blocks in epoch 0 as well as a block in epoch 1
+	let blocks = propose_and_import_blocks(
+		&client,
+		&mut proposer_factory,
+		&mut block_import,
+		BlockId::Number(0),
+		epoch_length as usize + 1,
+	);
+
+	// the first block in epoch 0 (#1) announces both epoch 0 and 1 (this is a
+	// special genesis epoch)
+	let epoch0 = epoch_changes
+		.shared_data()
+		.epoch(&EpochIdentifier {
+			position: EpochIdentifierPosition::Genesis0,
+			hash: blocks[0],
+			number: 1,
+		})
+		.unwrap()
+		.clone();
+
+	assert_eq!(epoch0.epoch_index, 0);
+	assert_eq!(epoch0.start_slot, Slot::from(1));
+
+	let epoch1 = epoch_changes
+		.shared_data()
+		.epoch(&EpochIdentifier {
+			position: EpochIdentifierPosition::Genesis1,
+			hash: blocks[0],
+			number: 1,
+		})
+		.unwrap()
+		.clone();
+
+	assert_eq!(epoch1.epoch_index, 1);
+	assert_eq!(epoch1.start_slot, Slot::from(epoch_length + 1));
+
+	// the first block in epoch 1 (#7) announces epoch 2. we will be skipping
+	// this epoch and therefore re-using its data for epoch 3
+	let epoch2 = epoch_changes
+		.shared_data()
+		.epoch(&EpochIdentifier {
+			position: EpochIdentifierPosition::Regular,
+			hash: blocks[epoch_length as usize],
+			number: epoch_length + 1,
+		})
+		.unwrap()
+		.clone();
+
+	assert_eq!(epoch2.epoch_index, 2);
+	assert_eq!(epoch2.start_slot, Slot::from(epoch_length * 2 + 1));
+
+	// we now author a block that belongs to epoch 3, thereby skipping epoch 2
+	let last_block = client.expect_header(BlockId::Hash(*blocks.last().unwrap())).unwrap();
+	let block = propose_and_import_block(
+		&last_block,
+		Some((epoch_length * 3 + 1).into()),
+		&mut proposer_factory,
+		&mut block_import,
+	);
+
+	// and the first block in epoch 3 (#8) announces epoch 4
+	let epoch4 = epoch_changes
+		.shared_data()
+		.epoch(&EpochIdentifier {
+			position: EpochIdentifierPosition::Regular,
+			hash: block,
+			number: epoch_length + 2,
+		})
+		.unwrap()
+		.clone();
+
+	assert_eq!(epoch4.epoch_index, 4);
+	assert_eq!(epoch4.start_slot, Slot::from(epoch_length * 4 + 1));
+
+	// if we try to get the epoch data for a slot in epoch 3
+	let epoch3 = epoch_changes
+		.shared_data()
+		.epoch_data_for_child_of(
+			descendent_query(&*client),
+			&block,
+			epoch_length + 2,
+			(epoch_length * 3 + 2).into(),
+			|slot| Epoch::genesis(&data.link.config, slot),
+		)
+		.unwrap()
+		.unwrap();
+
+	// we get back the data for epoch 2
+	assert_eq!(epoch3, epoch2);
+
+	// but if we try to get the epoch data for a slot in epoch 4
+	let epoch4_ = epoch_changes
+		.shared_data()
+		.epoch_data_for_child_of(
+			descendent_query(&*client),
+			&block,
+			epoch_length + 2,
+			(epoch_length * 4 + 1).into(),
+			|slot| Epoch::genesis(&data.link.config, slot),
+		)
+		.unwrap()
+		.unwrap();
+
+	// we get epoch 4 as expected
+	assert_eq!(epoch4, epoch4_);
+}

frame/babe/src/lib.rs

@@ -575,10 +575,19 @@ impl<T: Config> Pallet<T> {
 		// by the session module to be called before this.
 		debug_assert!(Self::initialized().is_some());
 
-		// Update epoch index
-		let epoch_index = EpochIndex::<T>::get()
-			.checked_add(1)
-			.expect("epoch indices will never reach 2^64 before the death of the universe; qed");
+		// Update epoch index.
+		//
+		// NOTE: we figure out the epoch index from the slot, which may not
+		// necessarily be contiguous if the chain was offline for more than
+		// `T::EpochDuration` slots. When skipping from epoch N to e.g. N+4, we
+		// will be using the randomness and authorities for that epoch that had
+		// been previously announced for epoch N+1, and the randomness collected
+		// during the current epoch (N) will be used for epoch N+5.
+		let epoch_index = sp_consensus_babe::epoch_index(
+			CurrentSlot::<T>::get(),
+			GenesisSlot::<T>::get(),
+			T::EpochDuration::get(),
+		);
 
 		EpochIndex::<T>::put(epoch_index);
 		Authorities::<T>::put(authorities);
@@ -625,11 +634,16 @@ impl<T: Config> Pallet<T> {
 		}
 	}
 
-	/// Finds the start slot of the current epoch. only guaranteed to
-	/// give correct results after `initialize` of the first block
-	/// in the chain (as its result is based off of `GenesisSlot`).
+	/// Finds the start slot of the current epoch.
+	///
+	/// Only guaranteed to give correct results after `initialize` of the first
+	/// block in the chain (as its result is based off of `GenesisSlot`).
 	pub fn current_epoch_start() -> Slot {
-		Self::epoch_start(EpochIndex::<T>::get())
+		sp_consensus_babe::epoch_start_slot(
+			EpochIndex::<T>::get(),
+			GenesisSlot::<T>::get(),
+			T::EpochDuration::get(),
+		)
 	}
 
 	/// Produces information about the current epoch.
@@ -653,9 +667,15 @@ impl<T: Config> Pallet<T> {
 			 if u64 is not enough we should crash for safety; qed.",
 		);
 
+		let start_slot = sp_consensus_babe::epoch_start_slot(
+			next_epoch_index,
+			GenesisSlot::<T>::get(),
+			T::EpochDuration::get(),
+		);
+
 		Epoch {
 			epoch_index: next_epoch_index,
-			start_slot: Self::epoch_start(next_epoch_index),
+			start_slot,
 			duration: T::EpochDuration::get(),
 			authorities: NextAuthorities::<T>::get().to_vec(),
 			randomness: NextRandomness::<T>::get(),
@@ -667,17 +687,6 @@ impl<T: Config> Pallet<T> {
 		}
 	}
 
-	fn epoch_start(epoch_index: u64) -> Slot {
-		// (epoch_index * epoch_duration) + genesis_slot
-
-		const PROOF: &str = "slot number is u64; it should relate in some way to wall clock time; \
-							 if u64 is not enough we should crash for safety; qed.";
-
-		let epoch_start = epoch_index.checked_mul(T::EpochDuration::get()).expect(PROOF);
-
-		epoch_start.checked_add(*GenesisSlot::<T>::get()).expect(PROOF).into()
-	}
-
 	fn deposit_consensus<U: Encode>(new: U) {
 		let log = DigestItem::Consensus(BABE_ENGINE_ID, new.encode());
 		<frame_system::Pallet<T>>::deposit_log(log)

frame/babe/src/tests.rs

@@ -948,3 +948,34 @@ fn generate_equivocation_report_blob() {
 		println!("equivocation_proof.encode(): {:?}", equivocation_proof.encode());
 	});
 }
+
+#[test]
+fn skipping_over_epochs_works() {
+	let mut ext = new_test_ext(3);
+
+	ext.execute_with(|| {
+		let epoch_duration: u64 = <Test as Config>::EpochDuration::get();
+
+		// this sets the genesis slot to 100;
+		let genesis_slot = 100;
+		go_to_block(1, genesis_slot);
+
+		// we will author all blocks from epoch #0 and arrive at a point where
+		// we are in epoch #1. we should already have the randomness ready that
+		// will be used in epoch #2
+		progress_to_block(epoch_duration + 1);
+		assert_eq!(EpochIndex::<Test>::get(), 1);
+
+		// genesis randomness is an array of zeros
+		let randomness_for_epoch_2 = NextRandomness::<Test>::get();
+		assert!(randomness_for_epoch_2 != [0; 32]);
+
+		// we will now create a block for a slot that is part of epoch #4.
+		// we should appropriately increment the epoch index as well as re-use
+		// the randomness from epoch #2 on epoch #4
+		go_to_block(System::block_number() + 1, genesis_slot + epoch_duration * 4);
+
+		assert_eq!(EpochIndex::<Test>::get(), 4);
+		assert_eq!(Randomness::<Test>::get(), randomness_for_epoch_2);
+	});
+}

primitives/consensus/babe/src/lib.rs

@@ -360,6 +360,25 @@ pub struct Epoch {
 	pub config: BabeEpochConfiguration,
 }
 
+/// Returns the epoch index the given slot belongs to.
+pub fn epoch_index(slot: Slot, genesis_slot: Slot, epoch_duration: u64) -> u64 {
+	*slot.saturating_sub(genesis_slot) / epoch_duration
+}
+
+/// Returns the first slot at the given epoch index.
+pub fn epoch_start_slot(epoch_index: u64, genesis_slot: Slot, epoch_duration: u64) -> Slot {
+	// (epoch_index * epoch_duration) + genesis_slot
+
+	const PROOF: &str = "slot number is u64; it should relate in some way to wall clock time; \
+						 if u64 is not enough we should crash for safety; qed.";
+
+	epoch_index
+		.checked_mul(epoch_duration)
+		.and_then(|slot| slot.checked_add(*genesis_slot))
+		.expect(PROOF)
+		.into()
+}
+
 sp_api::decl_runtime_apis! {
 	/// API necessary for block authorship with BABE.
 	#[api_version(2)]