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 +1543,47 @@ 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.saturating_sub(epoch_data.start_slot) / epoch_data.duration;
+
+					// 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.
+					let epoch_index = epoch_data.epoch_index.checked_add(skipped_epochs).expect(
+						"epoch number is u64; it should be strictly smaller than number of slots; \
+						slots relate in some way to wall clock time; \
+						if u64 is not enough we should crash for safety; qed.",
+					);
+
+					let start_slot = skipped_epochs
+						.checked_mul(epoch_data.duration)
+						.and_then(|skipped_slots| epoch_data.start_slot.checked_add(skipped_slots))
+						.expect(
+							"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.",
+						);
+
+					warn!(target: "babe", "👶 Epoch(s) skipped: from {} to {}",
+						epoch_data.epoch_index, epoch_index,
+					);
+
+					epoch_data.epoch_index = epoch_index;
+					epoch_data.start_slot = Slot::from(start_slot);
+				}
+
 				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 +1070,258 @@ 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_);
+}
+
+#[test]
+fn allows_skipping_epochs_on_some_forks() {
+	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 two blocks in epoch 1
+	let blocks = propose_and_import_blocks(
+		&client,
+		&mut proposer_factory,
+		&mut block_import,
+		BlockId::Number(0),
+		epoch_length as usize + 1,
+	);
+
+	// we now author a block that belongs to epoch 2, built on top of the last
+	// authored block in epoch 1.
+	let last_block = client.expect_header(BlockId::Hash(*blocks.last().unwrap())).unwrap();
+
+	let epoch2_block = propose_and_import_block(
+		&last_block,
+		Some((epoch_length * 2 + 1).into()),
+		&mut proposer_factory,
+		&mut block_import,
+	);
+
+	// if we try to get the epoch data for a slot in epoch 2, we get the data that
+	// was previously announced when epoch 1 started
+	let epoch2 = epoch_changes
+		.shared_data()
+		.epoch_data_for_child_of(
+			descendent_query(&*client),
+			&epoch2_block,
+			epoch_length + 2,
+			(epoch_length * 2 + 2).into(),
+			|slot| Epoch::genesis(&data.link.config, slot),
+		)
+		.unwrap()
+		.unwrap();
+
+	// we now author a block that belongs to epoch 3, built on top of the last
+	// authored block in epoch 1. authoring this block means we're skipping epoch 2
+	// entirely on this fork
+	let epoch3_block = propose_and_import_block(
+		&last_block,
+		Some((epoch_length * 3 + 1).into()),
+		&mut proposer_factory,
+		&mut block_import,
+	);
+
+	// 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),
+			&epoch3_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);
+
+	// if we try to get the epoch data for a slot in epoch 4 in the fork
+	// where we skipped epoch 2, we should get the epoch data for epoch 4
+	// that was announced at the beginning of epoch 3
+	let epoch_data = epoch_changes
+		.shared_data()
+		.epoch_data_for_child_of(
+			descendent_query(&*client),
+			&epoch3_block,
+			epoch_length + 2,
+			(epoch_length * 4 + 1).into(),
+			|slot| Epoch::genesis(&data.link.config, slot),
+		)
+		.unwrap()
+		.unwrap();
+
+	assert!(epoch_data != epoch3_);
+
+	// if we try to get the epoch data for a slot in epoch 4 in the fork
+	// where we didn't skip epoch 2, we should get back the data for epoch 3,
+	// that was announced when epoch 2 started in that fork
+	let epoch_data = epoch_changes
+		.shared_data()
+		.epoch_data_for_child_of(
+			descendent_query(&*client),
+			&epoch2_block,
+			epoch_length + 2,
+			(epoch_length * 4 + 1).into(),
+			|slot| Epoch::genesis(&data.link.config, slot),
+		)
+		.unwrap()
+		.unwrap();
+
+	assert!(epoch_data != epoch3_);
+
+	let epoch3 = epoch_changes
+		.shared_data()
+		.epoch(&EpochIdentifier {
+			position: EpochIdentifierPosition::Regular,
+			hash: epoch2_block,
+			number: epoch_length + 2,
+		})
+		.unwrap()
+		.clone();
+
+	assert_eq!(epoch_data, epoch3);
+}