Parachains finality relay (#1199)

bridges/relays/bin-substrate/Cargo.toml

@@ -27,6 +27,7 @@ bp-kusama = { path = "../../primitives/chain-kusama" }
 bp-messages = { path = "../../primitives/messages" }
 bp-millau = { path = "../../primitives/chain-millau" }
 bp-polkadot = { path = "../../primitives/chain-polkadot" }
+bp-polkadot-core = { path = "../../primitives/polkadot-core" }
 bp-rialto = { path = "../../primitives/chain-rialto" }
 bp-rialto-parachain = { path = "../../primitives/chain-rialto-parachain" }
 bp-rococo = { path = "../../primitives/chain-rococo" }
@@ -39,6 +40,7 @@ messages-relay = { path = "../messages" }
 millau-runtime = { path = "../../bin/millau/runtime" }
 pallet-bridge-grandpa = { path = "../../modules/grandpa" }
 pallet-bridge-messages = { path = "../../modules/messages" }
+parachains-relay = { path = "../parachains" }
 relay-kusama-client = { path = "../client-kusama" }
 relay-millau-client = { path = "../client-millau" }
 relay-polkadot-client = { path = "../client-polkadot" }

bridges/relays/bin-substrate/src/chains/mod.rs

@@ -24,6 +24,7 @@ pub mod polkadot_headers_to_kusama;
 pub mod polkadot_messages_to_kusama;
 pub mod rialto_headers_to_millau;
 pub mod rialto_messages_to_millau;
+pub mod rialto_parachains_to_millau;
 pub mod rococo_headers_to_wococo;
 pub mod rococo_messages_to_wococo;
 pub mod westend_headers_to_millau;

bridges/relays/bin-substrate/src/chains/rialto_parachains_to_millau.rs

@@ -0,0 +1,39 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Rialto-to-Millau parachains sync entrypoint.
+
+use parachains_relay::ParachainsPipeline;
+use relay_millau_client::Millau;
+use relay_rialto_client::Rialto;
+use substrate_relay_helper::parachains_target::DirectSubmitParachainHeadsCallBuilder;
+
+/// Rialto-to-Millau parachains sync description.
+#[derive(Clone, Debug)]
+pub struct RialtoToMillauParachains;
+
+impl ParachainsPipeline for RialtoToMillauParachains {
+	type SourceChain = Rialto;
+	type TargetChain = Millau;
+}
+
+/// `submit_parachain_heads` call builder for Rialto-to-Millau parachains sync pipeline.
+pub type RialtoToMillauParachainsSubmitParachainHeadsCallBuilder =
+	DirectSubmitParachainHeadsCallBuilder<
+		RialtoToMillauParachains,
+		millau_runtime::Runtime,
+		millau_runtime::WitRialtoParachainsInstance,
+	>;

bridges/relays/bin-substrate/src/cli/mod.rs

@@ -36,6 +36,7 @@ mod reinit_bridge;
 mod relay_headers;
 mod relay_headers_and_messages;
 mod relay_messages;
+mod relay_parachains;
 mod resubmit_transactions;
 
 /// Parse relay CLI args.
@@ -85,6 +86,8 @@ pub enum Command {
 	ResubmitTransactions(resubmit_transactions::ResubmitTransactions),
 	/// Register parachain.
 	RegisterParachain(register_parachain::RegisterParachain),
+	///
+	RelayParachains(relay_parachains::RelayParachains),
 }
 
 impl Command {
@@ -118,6 +121,7 @@ impl Command {
 			Self::EstimateFee(arg) => arg.run().await?,
 			Self::ResubmitTransactions(arg) => arg.run().await?,
 			Self::RegisterParachain(arg) => arg.run().await?,
+			Self::RelayParachains(arg) => arg.run().await?,
 		}
 		Ok(())
 	}

bridges/relays/bin-substrate/src/cli/relay_parachains.rs

@@ -0,0 +1,119 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use bp_polkadot_core::parachains::ParaId;
+use parachains_relay::{parachains_loop::ParachainSyncParams, ParachainsPipeline};
+use relay_utils::metrics::{GlobalMetrics, StandaloneMetric};
+use structopt::StructOpt;
+use strum::{EnumString, EnumVariantNames, VariantNames};
+use substrate_relay_helper::{
+	parachains_source::ParachainsSource, parachains_target::ParachainsTarget, TransactionParams,
+};
+
+use crate::cli::{
+	PrometheusParams, SourceConnectionParams, TargetConnectionParams, TargetSigningParams,
+};
+
+/// Start parachain heads relayer process.
+#[derive(StructOpt)]
+pub struct RelayParachains {
+	/// A bridge instance to relay parachains heads for.
+	#[structopt(possible_values = RelayParachainsBridge::VARIANTS, case_insensitive = true)]
+	bridge: RelayParachainsBridge,
+	#[structopt(flatten)]
+	source: SourceConnectionParams,
+	#[structopt(flatten)]
+	target: TargetConnectionParams,
+	#[structopt(flatten)]
+	target_sign: TargetSigningParams,
+	#[structopt(flatten)]
+	prometheus_params: PrometheusParams,
+}
+
+/// Parachain heads relay bridge.
+#[derive(Debug, EnumString, EnumVariantNames)]
+#[strum(serialize_all = "kebab_case")]
+pub enum RelayParachainsBridge {
+	RialtoToMillau,
+}
+
+macro_rules! select_bridge {
+	($bridge: expr, $generic: tt) => {
+		match $bridge {
+			RelayParachainsBridge::RialtoToMillau => {
+				use crate::chains::rialto_parachains_to_millau::{
+					RialtoToMillauParachains as Pipeline,
+					RialtoToMillauParachainsSubmitParachainHeadsCallBuilder as SubmitParachainHeadsCallBuilder,
+				};
+
+				use bp_millau::BRIDGE_PARAS_PALLET_NAME as BRIDGE_PARAS_PALLET_NAME_AT_TARGET;
+				use bp_rialto::PARAS_PALLET_NAME as PARAS_PALLET_NAME_AT_SOURCE;
+
+				use relay_millau_client::Millau as TargetTransactionSignScheme;
+
+				$generic
+			},
+		}
+	};
+}
+
+impl RelayParachains {
+	/// Run the command.
+	pub async fn run(self) -> anyhow::Result<()> {
+		select_bridge!(self.bridge, {
+			type SourceChain = <Pipeline as ParachainsPipeline>::SourceChain;
+			type TargetChain = <Pipeline as ParachainsPipeline>::TargetChain;
+
+			let source_client = self.source.to_client::<SourceChain>().await?;
+			let source_client = ParachainsSource::<Pipeline>::new(
+				source_client,
+				PARAS_PALLET_NAME_AT_SOURCE.into(),
+			);
+
+			let taret_transaction_params = TransactionParams {
+				signer: self.target_sign.to_keypair::<TargetChain>()?,
+				mortality: self.target_sign.target_transactions_mortality,
+			};
+			let target_client = self.target.to_client::<TargetChain>().await?;
+			let target_client = ParachainsTarget::<
+				Pipeline,
+				TargetTransactionSignScheme,
+				SubmitParachainHeadsCallBuilder,
+			>::new(
+				target_client.clone(),
+				taret_transaction_params,
+				BRIDGE_PARAS_PALLET_NAME_AT_TARGET.into(),
+			);
+
+			let metrics_params: relay_utils::metrics::MetricsParams = self.prometheus_params.into();
+			GlobalMetrics::new()?.register_and_spawn(&metrics_params.registry)?;
+
+			parachains_relay::parachains_loop::run(
+				source_client,
+				target_client,
+				ParachainSyncParams {
+					parachains: vec![ParaId(2000)],
+					stall_timeout: std::time::Duration::from_secs(60),
+					strategy: parachains_relay::parachains_loop::ParachainSyncStrategy::Any,
+				},
+				metrics_params,
+				futures::future::pending(),
+			)
+			.await
+			.map_err(|e| anyhow::format_err!("{}", e))
+		})
+	}
+}

bridges/relays/client-substrate/Cargo.toml

@@ -9,6 +9,7 @@ license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 async-std = { version = "1.6.5", features = ["attributes"] }
 async-trait = "0.1"
 codec = { package = "parity-scale-codec", version = "3.0.0" }
+futures = "0.3.7"
 jsonrpsee = { version = "0.8", features = ["macros", "ws-client"] }
 log = "0.4.11"
 num-traits = "0.2"
@@ -44,5 +45,6 @@ sp-storage = { git = "https://github.com/paritytech/substrate", branch = "master
 sp-trie = { git = "https://github.com/paritytech/substrate", branch = "master" }
 sp-version = { git = "https://github.com/paritytech/substrate", branch = "master" }
 
-#[dev-dependencies]
-futures = "0.3.7"
+[features]
+default = []
+test-helpers = []

bridges/relays/client-substrate/src/guard.rs

@@ -196,7 +196,7 @@ impl<C: ChainWithBalances> Environment<C> for Client<C> {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use frame_support::weights::{IdentityFee, Weight};
+	use crate::test_chain::TestChain;
 	use futures::{
 		channel::mpsc::{unbounded, UnboundedReceiver, UnboundedSender},
 		future::FutureExt,
@@ -204,49 +204,6 @@ mod tests {
 		SinkExt,
 	};
 
-	#[derive(Debug, Clone)]
-	struct TestChain;
-
-	impl bp_runtime::Chain for TestChain {
-		type BlockNumber = u32;
-		type Hash = sp_core::H256;
-		type Hasher = sp_runtime::traits::BlakeTwo256;
-		type Header = sp_runtime::generic::Header<u32, sp_runtime::traits::BlakeTwo256>;
-
-		type AccountId = u32;
-		type Balance = u32;
-		type Index = u32;
-		type Signature = sp_runtime::testing::TestSignature;
-
-		fn max_extrinsic_size() -> u32 {
-			unreachable!()
-		}
-		fn max_extrinsic_weight() -> Weight {
-			unreachable!()
-		}
-	}
-
-	impl Chain for TestChain {
-		const NAME: &'static str = "Test";
-		const TOKEN_ID: Option<&'static str> = None;
-		const BEST_FINALIZED_HEADER_ID_METHOD: &'static str = "BestTestHeader";
-		const AVERAGE_BLOCK_INTERVAL: Duration = Duration::from_millis(1);
-		const STORAGE_PROOF_OVERHEAD: u32 = 0;
-		const MAXIMAL_ENCODED_ACCOUNT_ID_SIZE: u32 = 0;
-
-		type SignedBlock = sp_runtime::generic::SignedBlock<
-			sp_runtime::generic::Block<Self::Header, sp_runtime::OpaqueExtrinsic>,
-		>;
-		type Call = ();
-		type WeightToFee = IdentityFee<u32>;
-	}
-
-	impl ChainWithBalances for TestChain {
-		fn account_info_storage_key(_account_id: &u32) -> sp_core::storage::StorageKey {
-			unreachable!()
-		}
-	}
-
 	struct TestEnvironment {
 		runtime_version_rx: UnboundedReceiver<RuntimeVersion>,
 		free_native_balance_rx: UnboundedReceiver<u32>,

bridges/relays/client-substrate/src/lib.rs

@@ -26,6 +26,7 @@ mod sync_header;
 
 pub mod guard;
 pub mod metrics;
+pub mod test_chain;
 
 use std::time::Duration;
 

bridges/relays/client-substrate/src/test_chain.rs

@@ -0,0 +1,71 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Pallet provides a set of guard functions that are running in background threads
+//! and are aborting process if some condition fails.
+
+//! Test chain implementation to use in tests.
+
+#![cfg(any(feature = "test-helpers", test))]
+
+use crate::{Chain, ChainWithBalances};
+use frame_support::weights::{IdentityFee, Weight};
+use std::time::Duration;
+
+/// Chain that may be used in tests.
+#[derive(Clone, Debug, PartialEq)]
+pub struct TestChain;
+
+impl bp_runtime::Chain for TestChain {
+	type BlockNumber = u32;
+	type Hash = sp_core::H256;
+	type Hasher = sp_runtime::traits::BlakeTwo256;
+	type Header = sp_runtime::generic::Header<u32, sp_runtime::traits::BlakeTwo256>;
+
+	type AccountId = u32;
+	type Balance = u32;
+	type Index = u32;
+	type Signature = sp_runtime::testing::TestSignature;
+
+	fn max_extrinsic_size() -> u32 {
+		unreachable!()
+	}
+
+	fn max_extrinsic_weight() -> Weight {
+		unreachable!()
+	}
+}
+
+impl Chain for TestChain {
+	const NAME: &'static str = "Test";
+	const TOKEN_ID: Option<&'static str> = None;
+	const BEST_FINALIZED_HEADER_ID_METHOD: &'static str = "TestMethod";
+	const AVERAGE_BLOCK_INTERVAL: Duration = Duration::from_millis(0);
+	const STORAGE_PROOF_OVERHEAD: u32 = 0;
+	const MAXIMAL_ENCODED_ACCOUNT_ID_SIZE: u32 = 0;
+
+	type SignedBlock = sp_runtime::generic::SignedBlock<
+		sp_runtime::generic::Block<Self::Header, sp_runtime::OpaqueExtrinsic>,
+	>;
+	type Call = ();
+	type WeightToFee = IdentityFee<u32>;
+}
+
+impl ChainWithBalances for TestChain {
+	fn account_info_storage_key(_account_id: &u32) -> sp_core::storage::StorageKey {
+		unreachable!()
+	}
+}

bridges/relays/lib-substrate-relay/Cargo.toml

@@ -18,16 +18,20 @@ log = "0.4.14"
 # Bridge dependencies
 
 bp-header-chain = { path = "../../primitives/header-chain" }
+bp-parachains = { path = "../../primitives/parachains" }
+bp-polkadot-core = { path = "../../primitives/polkadot-core" }
 bridge-runtime-common = { path = "../../bin/runtime-common" }
 
 finality-grandpa = { version = "0.15.0" }
 finality-relay = { path = "../finality" }
+parachains-relay = { path = "../parachains" }
 relay-utils = { path = "../utils" }
 messages-relay = { path = "../messages" }
 relay-substrate-client = { path = "../client-substrate" }
 
 pallet-bridge-grandpa = { path = "../../modules/grandpa" }
 pallet-bridge-messages = { path = "../../modules/messages" }
+pallet-bridge-parachains = { path = "../../modules/parachains" }
 
 bp-runtime = { path = "../../primitives/runtime" }
 bp-messages = { path = "../../primitives/messages" }

bridges/relays/lib-substrate-relay/src/lib.rs

@@ -29,6 +29,8 @@ pub mod messages_metrics;
 pub mod messages_source;
 pub mod messages_target;
 pub mod on_demand_headers;
+pub mod parachains_source;
+pub mod parachains_target;
 
 /// Default relay loop stall timeout. If transactions generated by relay are immortal, then
 /// this timeout is used.

bridges/relays/lib-substrate-relay/src/parachains_source.rs

@@ -0,0 +1,91 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Parachain heads source.
+
+use async_trait::async_trait;
+use bp_parachains::parachain_head_storage_key_at_source;
+use bp_polkadot_core::parachains::{ParaHash, ParaHead, ParaHeadsProof, ParaId};
+use codec::Decode;
+use parachains_relay::{parachains_loop::SourceClient, ParachainsPipeline};
+use relay_substrate_client::{Client, Error as SubstrateError, HeaderIdOf};
+use relay_utils::relay_loop::Client as RelayClient;
+
+/// Substrate client as parachain heads source.
+#[derive(Clone)]
+pub struct ParachainsSource<P: ParachainsPipeline> {
+	client: Client<P::SourceChain>,
+	paras_pallet_name: String,
+}
+
+impl<P: ParachainsPipeline> ParachainsSource<P> {
+	/// Creates new parachains source client.
+	pub fn new(client: Client<P::SourceChain>, paras_pallet_name: String) -> Self {
+		ParachainsSource { client, paras_pallet_name }
+	}
+}
+
+#[async_trait]
+impl<P: ParachainsPipeline> RelayClient for ParachainsSource<P> {
+	type Error = SubstrateError;
+
+	async fn reconnect(&mut self) -> Result<(), SubstrateError> {
+		self.client.reconnect().await
+	}
+}
+
+#[async_trait]
+impl<P: ParachainsPipeline> SourceClient<P> for ParachainsSource<P> {
+	async fn ensure_synced(&self) -> Result<bool, Self::Error> {
+		match self.client.ensure_synced().await {
+			Ok(_) => Ok(true),
+			Err(SubstrateError::ClientNotSynced(_)) => Ok(false),
+			Err(e) => Err(e),
+		}
+	}
+
+	async fn parachain_head(
+		&self,
+		at_block: HeaderIdOf<P::SourceChain>,
+		para_id: ParaId,
+	) -> Result<Option<ParaHash>, Self::Error> {
+		let storage_key = parachain_head_storage_key_at_source(&self.paras_pallet_name, para_id);
+		let para_head = self.client.raw_storage_value(storage_key, Some(at_block.1)).await?;
+		let para_head = para_head.map(|h| ParaHead::decode(&mut &h.0[..])).transpose()?;
+		let para_hash = para_head.map(|h| h.hash());
+
+		Ok(para_hash)
+	}
+
+	async fn prove_parachain_heads(
+		&self,
+		at_block: HeaderIdOf<P::SourceChain>,
+		parachains: &[ParaId],
+	) -> Result<ParaHeadsProof, Self::Error> {
+		let storage_keys = parachains
+			.iter()
+			.map(|para_id| parachain_head_storage_key_at_source(&self.paras_pallet_name, *para_id))
+			.collect();
+		let parachain_heads_proof = self
+			.client
+			.prove_storage(storage_keys, at_block.1)
+			.await?
+			.iter_nodes()
+			.collect();
+
+		Ok(parachain_heads_proof)
+	}
+}

bridges/relays/lib-substrate-relay/src/parachains_target.rs

@@ -0,0 +1,211 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Parachain heads target.
+
+use crate::TransactionParams;
+
+use async_trait::async_trait;
+use bp_parachains::{parachain_head_storage_key_at_target, BestParaHeadHash};
+use bp_polkadot_core::parachains::{ParaHeadsProof, ParaId};
+use codec::{Decode, Encode};
+use pallet_bridge_parachains::{
+	Call as BridgeParachainsCall, Config as BridgeParachainsConfig, RelayBlockHash,
+	RelayBlockHasher, RelayBlockNumber,
+};
+use parachains_relay::{parachains_loop::TargetClient, ParachainsPipeline};
+use relay_substrate_client::{
+	AccountIdOf, AccountKeyPairOf, BlockNumberOf, CallOf, Chain, Client, Error as SubstrateError,
+	HashOf, HeaderIdOf, SignParam, TransactionEra, TransactionSignScheme, UnsignedTransaction,
+};
+use relay_utils::{relay_loop::Client as RelayClient, HeaderId};
+use sp_core::{Bytes, Pair};
+use sp_runtime::traits::Header as HeaderT;
+use std::marker::PhantomData;
+
+/// Different ways of building `submit_parachain_heads` calls.
+pub trait SubmitParachainHeadsCallBuilder<P: ParachainsPipeline>: 'static + Send + Sync {
+	/// Given parachains and their heads proof, build call of `submit_parachain_heads`
+	/// function of bridge parachains module at the target chain.
+	fn build_submit_parachain_heads_call(
+		relay_block_hash: HashOf<P::SourceChain>,
+		parachains: Vec<ParaId>,
+		parachain_heads_proof: ParaHeadsProof,
+	) -> CallOf<P::TargetChain>;
+}
+
+/// Building `submit_parachain_heads` call when you have direct access to the target
+/// chain runtime.
+pub struct DirectSubmitParachainHeadsCallBuilder<P, R, I> {
+	_phantom: PhantomData<(P, R, I)>,
+}
+
+impl<P, R, I> SubmitParachainHeadsCallBuilder<P> for DirectSubmitParachainHeadsCallBuilder<P, R, I>
+where
+	P: ParachainsPipeline,
+	P::SourceChain: Chain<Hash = RelayBlockHash>,
+	R: BridgeParachainsConfig<I> + Send + Sync,
+	I: 'static + Send + Sync,
+	R::BridgedChain: bp_runtime::Chain<
+		BlockNumber = RelayBlockNumber,
+		Hash = RelayBlockHash,
+		Hasher = RelayBlockHasher,
+	>,
+	CallOf<P::TargetChain>: From<BridgeParachainsCall<R, I>>,
+{
+	fn build_submit_parachain_heads_call(
+		relay_block_hash: HashOf<P::SourceChain>,
+		parachains: Vec<ParaId>,
+		parachain_heads_proof: ParaHeadsProof,
+	) -> CallOf<P::TargetChain> {
+		BridgeParachainsCall::<R, I>::submit_parachain_heads {
+			relay_block_hash,
+			parachains,
+			parachain_heads_proof,
+		}
+		.into()
+	}
+}
+
+/// Substrate client as parachain heads source.
+pub struct ParachainsTarget<P: ParachainsPipeline, S: TransactionSignScheme, CB> {
+	client: Client<P::TargetChain>,
+	transaction_params: TransactionParams<AccountKeyPairOf<S>>,
+	bridge_paras_pallet_name: String,
+	_phantom: PhantomData<CB>,
+}
+
+impl<P: ParachainsPipeline, S: TransactionSignScheme, CB> ParachainsTarget<P, S, CB> {
+	/// Creates new parachains target client.
+	pub fn new(
+		client: Client<P::TargetChain>,
+		transaction_params: TransactionParams<AccountKeyPairOf<S>>,
+		bridge_paras_pallet_name: String,
+	) -> Self {
+		ParachainsTarget {
+			client,
+			transaction_params,
+			bridge_paras_pallet_name,
+			_phantom: Default::default(),
+		}
+	}
+}
+
+impl<P: ParachainsPipeline, S: TransactionSignScheme, CB> Clone for ParachainsTarget<P, S, CB> {
+	fn clone(&self) -> Self {
+		ParachainsTarget {
+			client: self.client.clone(),
+			transaction_params: self.transaction_params.clone(),
+			bridge_paras_pallet_name: self.bridge_paras_pallet_name.clone(),
+			_phantom: Default::default(),
+		}
+	}
+}
+
+#[async_trait]
+impl<
+		P: ParachainsPipeline,
+		S: 'static + TransactionSignScheme,
+		CB: SubmitParachainHeadsCallBuilder<P>,
+	> RelayClient for ParachainsTarget<P, S, CB>
+{
+	type Error = SubstrateError;
+
+	async fn reconnect(&mut self) -> Result<(), SubstrateError> {
+		self.client.reconnect().await
+	}
+}
+
+#[async_trait]
+impl<P, S, CB> TargetClient<P> for ParachainsTarget<P, S, CB>
+where
+	P: ParachainsPipeline,
+	S: 'static + TransactionSignScheme<Chain = P::TargetChain>,
+	CB: SubmitParachainHeadsCallBuilder<P>,
+	AccountIdOf<P::TargetChain>: From<<AccountKeyPairOf<S> as Pair>::Public>,
+{
+	async fn best_block(&self) -> Result<HeaderIdOf<P::TargetChain>, Self::Error> {
+		let best_header = self.client.best_header().await?;
+		let best_hash = best_header.hash();
+		let best_id = HeaderId(*best_header.number(), best_hash);
+
+		Ok(best_id)
+	}
+
+	async fn best_finalized_source_block(
+		&self,
+		at_block: &HeaderIdOf<P::TargetChain>,
+	) -> Result<HeaderIdOf<P::SourceChain>, Self::Error> {
+		let encoded_best_finalized_source_block = self
+			.client
+			.state_call(
+				P::SourceChain::BEST_FINALIZED_HEADER_ID_METHOD.into(),
+				Bytes(Vec::new()),
+				Some(at_block.1),
+			)
+			.await?;
+		let decoded_best_finalized_source_block: (
+			BlockNumberOf<P::SourceChain>,
+			HashOf<P::SourceChain>,
+		) = Decode::decode(&mut &encoded_best_finalized_source_block.0[..])
+			.map_err(SubstrateError::ResponseParseFailed)?;
+		Ok(HeaderId(decoded_best_finalized_source_block.0, decoded_best_finalized_source_block.1))
+	}
+
+	async fn parachain_head(
+		&self,
+		at_block: HeaderIdOf<P::TargetChain>,
+		para_id: ParaId,
+	) -> Result<Option<BestParaHeadHash>, Self::Error> {
+		let storage_key =
+			parachain_head_storage_key_at_target(&self.bridge_paras_pallet_name, para_id);
+		let para_head = self.client.storage_value(storage_key, Some(at_block.1)).await?;
+
+		Ok(para_head)
+	}
+
+	async fn submit_parachain_heads_proof(
+		&self,
+		at_relay_block: HeaderIdOf<P::SourceChain>,
+		updated_parachains: Vec<ParaId>,
+		proof: ParaHeadsProof,
+	) -> Result<(), Self::Error> {
+		let genesis_hash = *self.client.genesis_hash();
+		let transaction_params = self.transaction_params.clone();
+		let (spec_version, transaction_version) = self.client.simple_runtime_version().await?;
+		let call =
+			CB::build_submit_parachain_heads_call(at_relay_block.1, updated_parachains, proof);
+		self.client
+			.submit_signed_extrinsic(
+				self.transaction_params.signer.public().into(),
+				move |best_block_id, transaction_nonce| {
+					Ok(Bytes(
+						S::sign_transaction(SignParam {
+							spec_version,
+							transaction_version,
+							genesis_hash,
+							signer: transaction_params.signer,
+							era: TransactionEra::new(best_block_id, transaction_params.mortality),
+							unsigned: UnsignedTransaction::new(call.into(), transaction_nonce),
+						})?
+						.encode(),
+					))
+				},
+			)
+			.await
+			.map(drop)
+	}
+}

bridges/relays/parachains/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "parachains-relay"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+async-std = "1.6.5"
+async-trait = "0.1.40"
+backoff = "0.2"
+futures = "0.3.5"
+linked-hash-map = "0.5.3"
+log = "0.4.11"
+num-traits = "0.2"
+parking_lot = "0.11.0"
+relay-utils = { path = "../utils" }
+
+# Bridge dependencies
+
+bp-parachains = { path = "../../primitives/parachains" }
+bp-polkadot-core = { path = "../../primitives/polkadot-core" }
+relay-substrate-client = { path = "../client-substrate" }
+
+[dev-dependencies]
+codec = { package = "parity-scale-codec", version = "3.0.0" }
+relay-substrate-client = { path = "../client-substrate", features = ["test-helpers"] }
+sp-core = { git = "https://github.com/paritytech/substrate", branch = "master" }

bridges/relays/parachains/src/lib.rs

@@ -0,0 +1,30 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use std::fmt::Debug;
+
+use relay_substrate_client::Chain;
+
+pub mod parachains_loop;
+pub mod parachains_loop_metrics;
+
+/// Finality proofs synchronization pipeline.
+pub trait ParachainsPipeline: 'static + Clone + Debug + Send + Sync {
+	/// Relay chain which is storing parachain heads in its `paras` module.
+	type SourceChain: Chain;
+	/// Target chain (either relay or para) which wants to know about new parachain heads.
+	type TargetChain: Chain;
+}

bridges/relays/parachains/src/parachains_loop_metrics.rs

@@ -0,0 +1,34 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use relay_utils::metrics::{Metric, PrometheusError, Registry};
+
+/// Parachains sync metrics.
+#[derive(Clone)]
+pub struct ParachainsLoopMetrics;
+
+impl ParachainsLoopMetrics {
+	/// Create and register parachains loop metrics.
+	pub fn new(_prefix: Option<&str>) -> Result<Self, PrometheusError> {
+		Ok(ParachainsLoopMetrics)
+	}
+}
+
+impl Metric for ParachainsLoopMetrics {
+	fn register(&self, _registry: &Registry) -> Result<(), PrometheusError> {
+		Ok(())
+	}
+}