// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

use crate::{
	communication::{
		notification::{
			BeefyBestBlockSender, BeefyBestBlockStream, BeefyVersionedFinalityProofSender,
			BeefyVersionedFinalityProofStream,
		},
		peers::KnownPeers,
		request_response::{
			outgoing_requests_engine::OnDemandJustificationsEngine, BeefyJustifsRequestHandler,
		},
	},
	error::Error,
	import::BeefyBlockImport,
	metrics::register_metrics,
};
use futures::{stream::Fuse, FutureExt, StreamExt};
use log::{debug, error, info, warn};
use parking_lot::Mutex;
use prometheus::Registry;
use sc_client_api::{Backend, BlockBackend, BlockchainEvents, FinalityNotifications, Finalizer};
use sc_consensus::BlockImport;
use sc_network::{NetworkRequest, NotificationService, ProtocolName};
use sc_network_gossip::{GossipEngine, Network as GossipNetwork, Syncing as GossipSyncing};
use sp_api::ProvideRuntimeApi;
use sp_blockchain::{Backend as BlockchainBackend, HeaderBackend};
use sp_consensus::{Error as ConsensusError, SyncOracle};
use sp_consensus_beefy::{
	ecdsa_crypto::AuthorityId, BeefyApi, ConsensusLog, MmrRootHash, PayloadProvider, ValidatorSet,
	BEEFY_ENGINE_ID,
};
use sp_keystore::KeystorePtr;
use sp_mmr_primitives::MmrApi;
use sp_runtime::traits::{Block, Header as HeaderT, NumberFor, Zero};
use std::{
	collections::{BTreeMap, VecDeque},
	marker::PhantomData,
	sync::Arc,
	time::Duration,
};

mod aux_schema;
mod error;
mod keystore;
mod metrics;
mod round;
mod worker;

pub mod communication;
pub mod import;
pub mod justification;

use crate::{
	communication::gossip::GossipValidator,
	justification::BeefyVersionedFinalityProof,
	keystore::BeefyKeystore,
	metrics::VoterMetrics,
	round::Rounds,
	worker::{BeefyWorker, PersistedState},
};
pub use communication::beefy_protocol_name::{
	gossip_protocol_name, justifications_protocol_name as justifs_protocol_name,
};
use sp_runtime::generic::OpaqueDigestItemId;

#[cfg(test)]
mod tests;

const LOG_TARGET: &str = "beefy";

const HEADER_SYNC_DELAY: Duration = Duration::from_secs(60);

/// A convenience BEEFY client trait that defines all the type bounds a BEEFY client
/// has to satisfy. Ideally that should actually be a trait alias. Unfortunately as
/// of today, Rust does not allow a type alias to be used as a trait bound. Tracking
/// issue is <https://github.com/rust-lang/rust/issues/41517>.
pub trait Client<B, BE>:
	BlockchainEvents<B> + HeaderBackend<B> + Finalizer<B, BE> + Send + Sync
where
	B: Block,
	BE: Backend<B>,
{
	// empty
}

impl<B, BE, T> Client<B, BE> for T
where
	B: Block,
	BE: Backend<B>,
	T: BlockchainEvents<B>
		+ HeaderBackend<B>
		+ Finalizer<B, BE>
		+ ProvideRuntimeApi<B>
		+ Send
		+ Sync,
{
	// empty
}

/// Links between the block importer, the background voter and the RPC layer,
/// to be used by the voter.
#[derive(Clone)]
pub struct BeefyVoterLinks<B: Block> {
	// BlockImport -> Voter links
	/// Stream of BEEFY signed commitments from block import to voter.
	pub from_block_import_justif_stream: BeefyVersionedFinalityProofStream<B>,

	// Voter -> RPC links
	/// Sends BEEFY signed commitments from voter to RPC.
	pub to_rpc_justif_sender: BeefyVersionedFinalityProofSender<B>,
	/// Sends BEEFY best block hashes from voter to RPC.
	pub to_rpc_best_block_sender: BeefyBestBlockSender<B>,
}

/// Links used by the BEEFY RPC layer, from the BEEFY background voter.
#[derive(Clone)]
pub struct BeefyRPCLinks<B: Block> {
	/// Stream of signed commitments coming from the voter.
	pub from_voter_justif_stream: BeefyVersionedFinalityProofStream<B>,
	/// Stream of BEEFY best block hashes coming from the voter.
	pub from_voter_best_beefy_stream: BeefyBestBlockStream<B>,
}

/// Make block importer and link half necessary to tie the background voter to it.
pub fn beefy_block_import_and_links<B, BE, RuntimeApi, I>(
	wrapped_block_import: I,
	backend: Arc<BE>,
	runtime: Arc<RuntimeApi>,
	prometheus_registry: Option<Registry>,
) -> (BeefyBlockImport<B, BE, RuntimeApi, I>, BeefyVoterLinks<B>, BeefyRPCLinks<B>)
where
	B: Block,
	BE: Backend<B>,
	I: BlockImport<B, Error = ConsensusError> + Send + Sync,
	RuntimeApi: ProvideRuntimeApi<B> + Send + Sync,
	RuntimeApi::Api: BeefyApi<B, AuthorityId>,
{
	// Voter -> RPC links
	let (to_rpc_justif_sender, from_voter_justif_stream) =
		BeefyVersionedFinalityProofStream::<B>::channel();
	let (to_rpc_best_block_sender, from_voter_best_beefy_stream) =
		BeefyBestBlockStream::<B>::channel();

	// BlockImport -> Voter links
	let (to_voter_justif_sender, from_block_import_justif_stream) =
		BeefyVersionedFinalityProofStream::<B>::channel();
	let metrics = register_metrics(prometheus_registry);

	// BlockImport
	let import = BeefyBlockImport::new(
		backend,
		runtime,
		wrapped_block_import,
		to_voter_justif_sender,
		metrics,
	);
	let voter_links = BeefyVoterLinks {
		from_block_import_justif_stream,
		to_rpc_justif_sender,
		to_rpc_best_block_sender,
	};
	let rpc_links = BeefyRPCLinks { from_voter_best_beefy_stream, from_voter_justif_stream };

	(import, voter_links, rpc_links)
}

/// BEEFY gadget network parameters.
pub struct BeefyNetworkParams<B: Block, N, S> {
	/// Network implementing gossip, requests and sync-oracle.
	pub network: Arc<N>,
	/// Syncing service implementing a sync oracle and an event stream for peers.
	pub sync: Arc<S>,
	/// Handle for receiving notification events.
	pub notification_service: Box<dyn NotificationService>,
	/// Chain specific BEEFY gossip protocol name. See
	/// [`communication::beefy_protocol_name::gossip_protocol_name`].
	pub gossip_protocol_name: ProtocolName,
	/// Chain specific BEEFY on-demand justifications protocol name. See
	/// [`communication::beefy_protocol_name::justifications_protocol_name`].
	pub justifications_protocol_name: ProtocolName,

	pub _phantom: PhantomData<B>,
}

/// BEEFY gadget initialization parameters.
pub struct BeefyParams<B: Block, BE, C, N, P, R, S> {
	/// BEEFY client
	pub client: Arc<C>,
	/// Client Backend
	pub backend: Arc<BE>,
	/// BEEFY Payload provider
	pub payload_provider: P,
	/// Runtime Api Provider
	pub runtime: Arc<R>,
	/// Local key store
	pub key_store: Option<KeystorePtr>,
	/// BEEFY voter network params
	pub network_params: BeefyNetworkParams<B, N, S>,
	/// Minimal delta between blocks, BEEFY should vote for
	pub min_block_delta: u32,
	/// Prometheus metric registry
	pub prometheus_registry: Option<Registry>,
	/// Links between the block importer, the background voter and the RPC layer.
	pub links: BeefyVoterLinks<B>,
	/// Handler for incoming BEEFY justifications requests from a remote peer.
	pub on_demand_justifications_handler: BeefyJustifsRequestHandler<B, C>,
	/// Whether running under "Authority" role.
	pub is_authority: bool,
}
/// Helper object holding BEEFY worker communication/gossip components.
///
/// These are created once, but will be reused if worker is restarted/reinitialized.
pub(crate) struct BeefyComms<B: Block, N> {
	pub gossip_engine: GossipEngine<B>,
	pub gossip_validator: Arc<GossipValidator<B, N>>,
	pub on_demand_justifications: OnDemandJustificationsEngine<B>,
}

/// Helper builder object for building [worker::BeefyWorker].
///
/// It has to do it in two steps: initialization and build, because the first step can sleep waiting
/// for certain chain and backend conditions, and while sleeping we still need to pump the
/// GossipEngine. Once initialization is done, the GossipEngine (and other pieces) are added to get
/// the complete [worker::BeefyWorker] object.
pub(crate) struct BeefyWorkerBuilder<B: Block, BE, RuntimeApi> {
	// utilities
	backend: Arc<BE>,
	runtime: Arc<RuntimeApi>,
	key_store: BeefyKeystore<AuthorityId>,
	// voter metrics
	metrics: Option<VoterMetrics>,
	persisted_state: PersistedState<B>,
}

impl<B, BE, R> BeefyWorkerBuilder<B, BE, R>
where
	B: Block + codec::Codec,
	BE: Backend<B>,
	R: ProvideRuntimeApi<B>,
	R::Api: BeefyApi<B, AuthorityId>,
{
	/// This will wait for the chain to enable BEEFY (if not yet enabled) and also wait for the
	/// backend to sync all headers required by the voter to build a contiguous chain of mandatory
	/// justifications. Then it builds the initial voter state using a combination of previously
	/// persisted state in AUX DB and latest chain information/progress.
	///
	/// Returns a sane `BeefyWorkerBuilder` that can build the `BeefyWorker`.
	pub async fn async_initialize<N>(
		backend: Arc<BE>,
		runtime: Arc<R>,
		key_store: BeefyKeystore<AuthorityId>,
		metrics: Option<VoterMetrics>,
		min_block_delta: u32,
		gossip_validator: Arc<GossipValidator<B, N>>,
		finality_notifications: &mut Fuse<FinalityNotifications<B>>,
		is_authority: bool,
	) -> Result<Self, Error> {
		// Wait for BEEFY pallet to be active before starting voter.
		let (beefy_genesis, best_grandpa) =
			wait_for_runtime_pallet(&*runtime, finality_notifications).await?;

		let persisted_state = Self::load_or_init_state(
			beefy_genesis,
			best_grandpa,
			min_block_delta,
			backend.clone(),
			runtime.clone(),
			&key_store,
			&metrics,
			is_authority,
		)
		.await?;
		// Update the gossip validator with the right starting round and set id.
		persisted_state
			.gossip_filter_config()
			.map(|f| gossip_validator.update_filter(f))?;

		Ok(BeefyWorkerBuilder { backend, runtime, key_store, metrics, persisted_state })
	}

	/// Takes rest of missing pieces as params and builds the `BeefyWorker`.
	pub fn build<P, S, N>(
		self,
		payload_provider: P,
		sync: Arc<S>,
		comms: BeefyComms<B, N>,
		links: BeefyVoterLinks<B>,
		pending_justifications: BTreeMap<NumberFor<B>, BeefyVersionedFinalityProof<B>>,
		is_authority: bool,
	) -> BeefyWorker<B, BE, P, R, S, N> {
		BeefyWorker {
			backend: self.backend,
			runtime: self.runtime,
			key_store: self.key_store,
			metrics: self.metrics,
			persisted_state: self.persisted_state,
			payload_provider,
			sync,
			comms,
			links,
			pending_justifications,
			is_authority,
		}
	}

	// If no persisted state present, walk back the chain from first GRANDPA notification to either:
	//  - latest BEEFY finalized block, or if none found on the way,
	//  - BEEFY pallet genesis;
	// Enqueue any BEEFY mandatory blocks (session boundaries) found on the way, for voter to
	// finalize.
	async fn init_state(
		beefy_genesis: NumberFor<B>,
		best_grandpa: <B as Block>::Header,
		min_block_delta: u32,
		backend: Arc<BE>,
		runtime: Arc<R>,
	) -> Result<PersistedState<B>, Error> {
		let blockchain = backend.blockchain();

		let beefy_genesis = runtime
			.runtime_api()
			.beefy_genesis(best_grandpa.hash())
			.ok()
			.flatten()
			.filter(|genesis| *genesis == beefy_genesis)
			.ok_or_else(|| Error::Backend("BEEFY pallet expected to be active.".into()))?;
		// Walk back the imported blocks and initialize voter either, at the last block with
		// a BEEFY justification, or at pallet genesis block; voter will resume from there.
		let mut sessions = VecDeque::new();
		let mut header = best_grandpa.clone();
		let state = loop {
			if let Some(true) = blockchain
				.justifications(header.hash())
				.ok()
				.flatten()
				.map(|justifs| justifs.get(BEEFY_ENGINE_ID).is_some())
			{
				debug!(
					target: LOG_TARGET,
					"🥩 Initialize BEEFY voter at last BEEFY finalized block: {:?}.",
					*header.number()
				);
				let best_beefy = *header.number();
				// If no session boundaries detected so far, just initialize new rounds here.
				if sessions.is_empty() {
					let active_set =
						expect_validator_set(runtime.as_ref(), backend.as_ref(), &header).await?;
					let mut rounds = Rounds::new(best_beefy, active_set);
					// Mark the round as already finalized.
					rounds.conclude(best_beefy);
					sessions.push_front(rounds);
				}
				let state = PersistedState::checked_new(
					best_grandpa,
					best_beefy,
					sessions,
					min_block_delta,
					beefy_genesis,
				)
				.ok_or_else(|| Error::Backend("Invalid BEEFY chain".into()))?;
				break state
			}

			if *header.number() == beefy_genesis {
				// We've reached BEEFY genesis, initialize voter here.
				let genesis_set =
					expect_validator_set(runtime.as_ref(), backend.as_ref(), &header).await?;
				info!(
					target: LOG_TARGET,
					"🥩 Loading BEEFY voter state from genesis on what appears to be first startup. \
					Starting voting rounds at block {:?}, genesis validator set {:?}.",
					beefy_genesis,
					genesis_set,
				);

				sessions.push_front(Rounds::new(beefy_genesis, genesis_set));
				break PersistedState::checked_new(
					best_grandpa,
					Zero::zero(),
					sessions,
					min_block_delta,
					beefy_genesis,
				)
				.ok_or_else(|| Error::Backend("Invalid BEEFY chain".into()))?
			}

			if let Some(active) = find_authorities_change::<B>(&header) {
				debug!(
					target: LOG_TARGET,
					"🥩 Marking block {:?} as BEEFY Mandatory.",
					*header.number()
				);
				sessions.push_front(Rounds::new(*header.number(), active));
			}

			// Move up the chain.
			header = wait_for_parent_header(blockchain, header, HEADER_SYNC_DELAY).await?;
		};

		aux_schema::write_current_version(backend.as_ref())?;
		aux_schema::write_voter_state(backend.as_ref(), &state)?;
		Ok(state)
	}

	async fn load_or_init_state(
		beefy_genesis: NumberFor<B>,
		best_grandpa: <B as Block>::Header,
		min_block_delta: u32,
		backend: Arc<BE>,
		runtime: Arc<R>,
		key_store: &BeefyKeystore<AuthorityId>,
		metrics: &Option<VoterMetrics>,
		is_authority: bool,
	) -> Result<PersistedState<B>, Error> {
		// Initialize voter state from AUX DB if compatible.
		if let Some(mut state) = crate::aux_schema::load_persistent(backend.as_ref())?
			// Verify state pallet genesis matches runtime.
			.filter(|state| state.pallet_genesis() == beefy_genesis)
		{
			// Overwrite persisted state with current best GRANDPA block.
			state.set_best_grandpa(best_grandpa.clone());
			// Overwrite persisted data with newly provided `min_block_delta`.
			state.set_min_block_delta(min_block_delta);
			debug!(target: LOG_TARGET, "🥩 Loading BEEFY voter state from db: {:?}.", state);

			// Make sure that all the headers that we need have been synced.
			let mut new_sessions = vec![];
			let mut header = best_grandpa.clone();
			while *header.number() > state.best_beefy() {
				if state.voting_oracle().can_add_session(*header.number()) {
					if let Some(active) = find_authorities_change::<B>(&header) {
						new_sessions.push((active, *header.number()));
					}
				}
				header =
					wait_for_parent_header(backend.blockchain(), header, HEADER_SYNC_DELAY).await?;
			}

			// Make sure we didn't miss any sessions during node restart.
			for (validator_set, new_session_start) in new_sessions.drain(..).rev() {
				debug!(
					target: LOG_TARGET,
					"🥩 Handling missed BEEFY session after node restart: {:?}.",
					new_session_start
				);
				state.init_session_at(
					new_session_start,
					validator_set,
					key_store,
					metrics,
					is_authority,
				);
			}
			return Ok(state)
		}

		// No valid voter-state persisted, re-initialize from pallet genesis.
		Self::init_state(beefy_genesis, best_grandpa, min_block_delta, backend, runtime).await
	}
}

/// Start the BEEFY gadget.
///
/// This is a thin shim around running and awaiting a BEEFY worker.
pub async fn start_beefy_gadget<B, BE, C, N, P, R, S>(
	beefy_params: BeefyParams<B, BE, C, N, P, R, S>,
) where
	B: Block,
	BE: Backend<B>,
	C: Client<B, BE> + BlockBackend<B>,
	P: PayloadProvider<B> + Clone,
	R: ProvideRuntimeApi<B>,
	R::Api: BeefyApi<B, AuthorityId> + MmrApi<B, MmrRootHash, NumberFor<B>>,
	N: GossipNetwork<B> + NetworkRequest + Send + Sync + 'static,
	S: GossipSyncing<B> + SyncOracle + 'static,
{
	let BeefyParams {
		client,
		backend,
		payload_provider,
		runtime,
		key_store,
		network_params,
		min_block_delta,
		prometheus_registry,
		links,
		mut on_demand_justifications_handler,
		is_authority,
	} = beefy_params;

	let BeefyNetworkParams {
		network,
		sync,
		notification_service,
		gossip_protocol_name,
		justifications_protocol_name,
		..
	} = network_params;

	let metrics = register_metrics(prometheus_registry.clone());

	// Subscribe to finality notifications and justifications before waiting for runtime pallet and
	// reuse the streams, so we don't miss notifications while waiting for pallet to be available.
	let mut finality_notifications = client.finality_notification_stream().fuse();
	let mut block_import_justif = links.from_block_import_justif_stream.subscribe(100_000).fuse();

	let known_peers = Arc::new(Mutex::new(KnownPeers::new()));
	// Default votes filter is to discard everything.
	// Validator is updated later with correct starting round and set id.
	let gossip_validator =
		communication::gossip::GossipValidator::new(known_peers.clone(), network.clone());
	let gossip_validator = Arc::new(gossip_validator);
	let gossip_engine = GossipEngine::new(
		network.clone(),
		sync.clone(),
		notification_service,
		gossip_protocol_name.clone(),
		gossip_validator.clone(),
		None,
	);

	// The `GossipValidator` adds and removes known peers based on valid votes and network
	// events.
	let on_demand_justifications = OnDemandJustificationsEngine::new(
		network.clone(),
		justifications_protocol_name.clone(),
		known_peers,
		prometheus_registry.clone(),
	);
	let mut beefy_comms = BeefyComms { gossip_engine, gossip_validator, on_demand_justifications };

	// We re-create and re-run the worker in this loop in order to quickly reinit and resume after
	// select recoverable errors.
	loop {
		// Make sure to pump gossip engine while waiting for initialization conditions.
		let worker_builder = futures::select! {
			builder_init_result = BeefyWorkerBuilder::async_initialize(
				backend.clone(),
				runtime.clone(),
				key_store.clone().into(),
				metrics.clone(),
				min_block_delta,
				beefy_comms.gossip_validator.clone(),
				&mut finality_notifications,
				is_authority,
			).fuse() => {
				match builder_init_result {
					Ok(builder) => builder,
					Err(e) => {
						error!(target: LOG_TARGET, "🥩 Error: {:?}. Terminating.", e);
						return
					},
				}
			},
			// Pump gossip engine.
			_ = &mut beefy_comms.gossip_engine => {
				error!(target: LOG_TARGET, "🥩 Gossip engine has unexpectedly terminated.");
				return
			}
		};

		let worker = worker_builder.build(
			payload_provider.clone(),
			sync.clone(),
			beefy_comms,
			links.clone(),
			BTreeMap::new(),
			is_authority,
		);

		match futures::future::select(
			Box::pin(worker.run(&mut block_import_justif, &mut finality_notifications)),
			Box::pin(on_demand_justifications_handler.run()),
		)
		.await
		{
			// On `ConsensusReset` error, just reinit and restart voter.
			futures::future::Either::Left(((error::Error::ConsensusReset, reuse_comms), _)) => {
				error!(target: LOG_TARGET, "🥩 Error: {:?}. Restarting voter.", error::Error::ConsensusReset);
				beefy_comms = reuse_comms;
				continue
			},
			// On other errors, bring down / finish the task.
			futures::future::Either::Left(((worker_err, _), _)) =>
				error!(target: LOG_TARGET, "🥩 Error: {:?}. Terminating.", worker_err),
			futures::future::Either::Right((odj_handler_err, _)) =>
				error!(target: LOG_TARGET, "🥩 Error: {:?}. Terminating.", odj_handler_err),
		};
		return
	}
}

/// Waits until the parent header of `current` is available and returns it.
///
/// When the node uses GRANDPA warp sync it initially downloads only the mandatory GRANDPA headers.
/// The rest of the headers (gap sync) are lazily downloaded later. But the BEEFY voter also needs
/// the headers in range `[beefy_genesis..=best_grandpa]` to be available. This helper method
/// enables us to wait until these headers have been synced.
async fn wait_for_parent_header<B, BC>(
	blockchain: &BC,
	current: <B as Block>::Header,
	delay: Duration,
) -> Result<<B as Block>::Header, Error>
where
	B: Block,
	BC: BlockchainBackend<B>,
{
	if *current.number() == Zero::zero() {
		let msg = format!("header {} is Genesis, there is no parent for it", current.hash());
		warn!(target: LOG_TARGET, "{}", msg);
		return Err(Error::Backend(msg));
	}
	loop {
		match blockchain
			.header(*current.parent_hash())
			.map_err(|e| Error::Backend(e.to_string()))?
		{
			Some(parent) => return Ok(parent),
			None => {
				info!(
					target: LOG_TARGET,
					"🥩 Parent of header number {} not found. \
					BEEFY gadget waiting for header sync to finish ...",
					current.number()
				);
				tokio::time::sleep(delay).await;
			},
		}
	}
}

/// Wait for BEEFY runtime pallet to be available, return active validator set.
/// Should be called only once during worker initialization.
async fn wait_for_runtime_pallet<B, R>(
	runtime: &R,
	finality: &mut Fuse<FinalityNotifications<B>>,
) -> Result<(NumberFor<B>, <B as Block>::Header), Error>
where
	B: Block,
	R: ProvideRuntimeApi<B>,
	R::Api: BeefyApi<B, AuthorityId>,
{
	info!(target: LOG_TARGET, "🥩 BEEFY gadget waiting for BEEFY pallet to become available...");
	loop {
		let notif = finality.next().await.ok_or_else(|| {
			let err_msg = "🥩 Finality stream has unexpectedly terminated.".into();
			error!(target: LOG_TARGET, "{}", err_msg);
			Error::Backend(err_msg)
		})?;
		let at = notif.header.hash();
		if let Some(start) = runtime.runtime_api().beefy_genesis(at).ok().flatten() {
			if *notif.header.number() >= start {
				// Beefy pallet available, return header for best grandpa at the time.
				info!(
					target: LOG_TARGET,
					"🥩 BEEFY pallet available: block {:?} beefy genesis {:?}",
					notif.header.number(), start
				);
				return Ok((start, notif.header))
			}
		}
	}
}

/// Provides validator set active `at_header`. It tries to get it from state, otherwise falls
/// back to walk up the chain looking the validator set enactment in header digests.
///
/// Note: function will `async::sleep()` when walking back the chain if some needed header hasn't
/// been synced yet (as it happens when warp syncing when headers are synced in the background).
async fn expect_validator_set<B, BE, R>(
	runtime: &R,
	backend: &BE,
	at_header: &B::Header,
) -> Result<ValidatorSet<AuthorityId>, Error>
where
	B: Block,
	BE: Backend<B>,
	R: ProvideRuntimeApi<B>,
	R::Api: BeefyApi<B, AuthorityId>,
{
	let blockchain = backend.blockchain();
	// Walk up the chain looking for the validator set active at 'at_header'. Process both state and
	// header digests.
	debug!(
		target: LOG_TARGET,
		"🥩 Trying to find validator set active at header(number {:?}, hash {:?})",
		at_header.number(),
		at_header.hash()
	);
	let mut header = at_header.clone();
	loop {
		debug!(target: LOG_TARGET, "🥩 Looking for auth set change at block number: {:?}", *header.number());
		if let Ok(Some(active)) = runtime.runtime_api().validator_set(header.hash()) {
			return Ok(active)
		} else {
			match find_authorities_change::<B>(&header) {
				Some(active) => return Ok(active),
				// Move up the chain. Ultimately we'll get it from chain genesis state, or error out
				// there.
				None =>
					header = wait_for_parent_header(blockchain, header, HEADER_SYNC_DELAY)
						.await
						.map_err(|e| Error::Backend(e.to_string()))?,
			}
		}
	}
}

/// Scan the `header` digest log for a BEEFY validator set change. Return either the new
/// validator set or `None` in case no validator set change has been signaled.
pub(crate) fn find_authorities_change<B>(header: &B::Header) -> Option<ValidatorSet<AuthorityId>>
where
	B: Block,
{
	let id = OpaqueDigestItemId::Consensus(&BEEFY_ENGINE_ID);

	let filter = |log: ConsensusLog<AuthorityId>| match log {
		ConsensusLog::AuthoritiesChange(validator_set) => Some(validator_set),
		_ => None,
	};
	header.digest().convert_first(|l| l.try_to(id).and_then(filter))
}