Signed extension to refund relayer at the target chain (#1657)

* add utlity pallet to the Millau runtime

* RefundRelayerForMessagesDeliveryFromParachain prototype

* done with RefundRelayerForMessagesDeliveryFromParachain::post_dispatch

* parse calls

* check batch for obsolete headers/messages

* fmt

* shorten generic arg names + add parachain id generic arg

* check lane_id

* impl all state read functions

* fix typos from review

* renamed extension + reference issue from TODO

* tests for pre-dispaytch

* renamed extension source file

* tests for post-dispatch

* abstract fee calculation

* clippy

* actually fix clippy

* Update bin/runtime-common/src/refund_relayer_extension.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update bin/runtime-common/src/refund_relayer_extension.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update bin/runtime-common/src/refund_relayer_extension.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Update bin/runtime-common/src/refund_relayer_extension.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>

Co-authored-by: Adrian Catangiu <adrian@parity.io>

bridges/bin/millau/runtime/Cargo.toml

@@ -49,6 +49,7 @@ pallet-sudo = { git = "https://github.com/paritytech/substrate", branch = "maste
 pallet-timestamp = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 pallet-transaction-payment = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 pallet-transaction-payment-rpc-runtime-api = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+pallet-utility = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-api = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-block-builder = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-consensus-aura = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
@@ -113,6 +114,7 @@ std = [
 	"pallet-timestamp/std",
 	"pallet-transaction-payment-rpc-runtime-api/std",
 	"pallet-transaction-payment/std",
+	"pallet-utility/std",
 	"pallet-xcm/std",
 	"scale-info/std",
 	"sp-api/std",

bridges/bin/millau/runtime/src/lib.rs

@@ -550,6 +550,13 @@ impl pallet_bridge_parachains::Config<WithWestendParachainsInstance> for Runtime
 	type MaxParaHeadSize = MaxWestendParaHeadSize;
 }
 
+impl pallet_utility::Config for Runtime {
+	type RuntimeEvent = RuntimeEvent;
+	type RuntimeCall = RuntimeCall;
+	type PalletsOrigin = OriginCaller;
+	type WeightInfo = ();
+}
+
 construct_runtime!(
 	pub enum Runtime where
 		Block = Block,
@@ -558,6 +565,7 @@ construct_runtime!(
 	{
 		System: frame_system::{Pallet, Call, Config, Storage, Event<T>},
 		Sudo: pallet_sudo::{Pallet, Call, Config<T>, Storage, Event<T>},
+		Utility: pallet_utility,
 
 		// Must be before session.
 		Aura: pallet_aura::{Pallet, Config<T>},

bridges/bin/runtime-common/Cargo.toml

@@ -23,12 +23,15 @@ bp-runtime = { path = "../../primitives/runtime", default-features = false }
 pallet-bridge-grandpa = { path = "../../modules/grandpa", default-features = false }
 pallet-bridge-messages = { path = "../../modules/messages", default-features = false }
 pallet-bridge-parachains = { path = "../../modules/parachains", default-features = false }
+pallet-bridge-relayers = { path = "../../modules/relayers", default-features = false }
 
 # Substrate dependencies
 
 frame-support = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 frame-system = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 pallet-balances = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false, optional = true }
+pallet-transaction-payment = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+pallet-utility = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-api = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-core = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-io = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
@@ -43,6 +46,8 @@ xcm-builder = { git = "https://github.com/paritytech/polkadot", branch = "master
 xcm-executor = { git = "https://github.com/paritytech/polkadot", branch = "master", default-features = false }
 
 [dev-dependencies]
+bp-rialto = { path = "../../primitives/chain-rialto" }
+bp-test-utils = { path = "../../primitives/test-utils" }
 millau-runtime = { path = "../millau/runtime" }
 
 [features]
@@ -61,6 +66,9 @@ std = [
 	"pallet-bridge-grandpa/std",
 	"pallet-bridge-messages/std",
 	"pallet-bridge-parachains/std",
+	"pallet-bridge-relayers/std",
+	"pallet-transaction-payment/std",
+	"pallet-utility/std",
 	"pallet-xcm/std",
 	"scale-info/std",
 	"sp-api/std",

bridges/bin/runtime-common/src/lib.rs

@@ -27,6 +27,7 @@ pub mod messages_api;
 pub mod messages_benchmarking;
 pub mod messages_extension;
 pub mod parachains_benchmarking;
+pub mod refund_relayer_extension;
 
 mod messages_generation;
 
@@ -78,7 +79,7 @@ where
 #[macro_export]
 macro_rules! generate_bridge_reject_obsolete_headers_and_messages {
 	($call:ty, $account_id:ty, $($filter_call:ty),*) => {
-		#[derive(Clone, codec::Decode, codec::Encode, Eq, PartialEq, frame_support::RuntimeDebug, scale_info::TypeInfo)]
+		#[derive(Clone, codec::Decode, Default, codec::Encode, Eq, PartialEq, frame_support::RuntimeDebug, scale_info::TypeInfo)]
 		pub struct BridgeRejectObsoleteHeadersAndMessages;
 		impl sp_runtime::traits::SignedExtension for BridgeRejectObsoleteHeadersAndMessages {
 			const IDENTIFIER: &'static str = "BridgeRejectObsoleteHeadersAndMessages";

bridges/bin/runtime-common/src/refund_relayer_extension.rs

@@ -0,0 +1,845 @@
+// Copyright 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/>.
+
+//! Signed extension that refunds relayer if he has delivered some new messages.
+//! It also refunds transaction cost if the transaction is an `utility.batchAll()`
+//! with calls that are: delivering new messsage and all necessary underlying headers
+//! (parachain or relay chain).
+
+// hack because we have circular (test-level) dependency between `millau-runtime`
+// and `bridge-runtime-common` crates
+#[cfg(not(test))]
+use crate::messages::target::FromBridgedChainMessagesProof;
+#[cfg(test)]
+use millau_runtime::bridge_runtime_common::messages::target::FromBridgedChainMessagesProof;
+
+use bp_messages::{target_chain::SourceHeaderChain, LaneId, MessageNonce};
+use bp_polkadot_core::parachains::ParaId;
+use bp_runtime::{Chain, HashOf};
+use codec::{Decode, Encode};
+use frame_support::{
+	dispatch::{CallableCallFor, DispatchInfo, Dispatchable, PostDispatchInfo},
+	traits::IsSubType,
+	CloneNoBound, EqNoBound, PartialEqNoBound, RuntimeDebugNoBound,
+};
+use pallet_bridge_grandpa::{
+	BridgedChain, Call as GrandpaCall, Config as GrandpaConfig, Pallet as GrandpaPallet,
+};
+use pallet_bridge_messages::{
+	Call as MessagesCall, Config as MessagesConfig, Pallet as MessagesPallet,
+};
+use pallet_bridge_parachains::{
+	Call as ParachainsCall, Config as ParachainsConfig, Pallet as ParachainsPallet, RelayBlockHash,
+	RelayBlockHasher, RelayBlockNumber,
+};
+use pallet_bridge_relayers::{Config as RelayersConfig, Pallet as RelayersPallet};
+use pallet_transaction_payment::{Config as TransactionPaymentConfig, OnChargeTransaction};
+use pallet_utility::{Call as UtilityCall, Config as UtilityConfig, Pallet as UtilityPallet};
+use scale_info::TypeInfo;
+use sp_runtime::{
+	traits::{DispatchInfoOf, Get, Header as HeaderT, PostDispatchInfoOf, SignedExtension, Zero},
+	transaction_validity::{TransactionValidity, TransactionValidityError, ValidTransaction},
+	DispatchResult, FixedPointOperand,
+};
+use sp_std::marker::PhantomData;
+
+// TODO (https://github.com/paritytech/parity-bridges-common/issues/1667):
+// support multiple bridges in this extension
+
+/// Transaction fee calculation.
+pub trait TransactionFeeCalculation<Balance> {
+	/// Compute fee that is paid for given transaction. The fee is later refunded to relayer.
+	fn compute_fee(
+		info: &DispatchInfo,
+		post_info: &PostDispatchInfo,
+		len: usize,
+		tip: Balance,
+	) -> Balance;
+}
+
+impl<R> TransactionFeeCalculation<BalanceOf<R>> for R
+where
+	R: TransactionPaymentConfig,
+	<R as frame_system::Config>::RuntimeCall:
+		Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
+	BalanceOf<R>: FixedPointOperand,
+{
+	fn compute_fee(
+		info: &DispatchInfo,
+		post_info: &PostDispatchInfo,
+		len: usize,
+		tip: BalanceOf<R>,
+	) -> BalanceOf<R> {
+		pallet_transaction_payment::Pallet::<R>::compute_actual_fee(len as _, info, post_info, tip)
+	}
+}
+/// Signed extension that refunds relayer for new messages coming from the parachain.
+///
+/// Also refunds relayer for successful finality delivery if it comes in batch (`utility.batchAll`)
+/// with message delivery transaction. Batch may deliver either both relay chain header and
+/// parachain head, or just parachain head. Corresponding headers must be used in messages
+/// proof verification.
+///
+/// Extension does not refund transaction tip due to security reasons.
+#[derive(
+	CloneNoBound, Decode, Encode, EqNoBound, PartialEqNoBound, RuntimeDebugNoBound, TypeInfo,
+)]
+#[scale_info(skip_type_params(RT, GI, PI, MI, BE, PID, LID, FEE))]
+#[allow(clippy::type_complexity)] // TODO: get rid of that in https://github.com/paritytech/parity-bridges-common/issues/1666
+pub struct RefundRelayerForMessagesFromParachain<RT, GI, PI, MI, BE, PID, LID, FEE>(
+	PhantomData<(RT, GI, PI, MI, BE, PID, LID, FEE)>,
+);
+
+/// Data that is crafted in `pre_dispatch` method and used at `post_dispatch`.
+#[derive(PartialEq)]
+#[cfg_attr(test, derive(Debug))]
+pub struct PreDispatchData<AccountId> {
+	/// Transaction submitter (relayer) account.
+	pub relayer: AccountId,
+	/// Type of the call.
+	pub call_type: CallType,
+}
+
+/// Type of the call that the extension recognizes.
+#[derive(Clone, Copy, PartialEq, RuntimeDebugNoBound)]
+pub enum CallType {
+	/// Relay chain finality + parachain finality + message delivery calls.
+	AllFinalityAndDelivery(ExpectedRelayChainState, ExpectedParachainState, MessagesState),
+	/// Parachain finality + message delivery calls.
+	ParachainFinalityAndDelivery(ExpectedParachainState, MessagesState),
+	/// Standalone message delivery call.
+	Delivery(MessagesState),
+}
+
+impl CallType {
+	/// Returns the pre-dispatch messages pallet state.
+	fn pre_dispatch_messages_state(&self) -> MessagesState {
+		match *self {
+			Self::AllFinalityAndDelivery(_, _, messages_state) => messages_state,
+			Self::ParachainFinalityAndDelivery(_, messages_state) => messages_state,
+			Self::Delivery(messages_state) => messages_state,
+		}
+	}
+}
+
+/// Expected post-dispatch state of the relay chain pallet.
+#[derive(Clone, Copy, PartialEq, RuntimeDebugNoBound)]
+pub struct ExpectedRelayChainState {
+	/// Best known relay chain block number.
+	pub best_block_number: RelayBlockNumber,
+}
+
+/// Expected post-dispatch state of the parachain pallet.
+#[derive(Clone, Copy, PartialEq, RuntimeDebugNoBound)]
+pub struct ExpectedParachainState {
+	/// At which relay block the parachain head has been updated?
+	pub at_relay_block_number: RelayBlockNumber,
+}
+
+/// Pre-dispatch state of messages pallet.
+///
+/// This struct is for pre-dispatch state of the pallet, not the expected post-dispatch state.
+/// That's because message delivery transaction may deliver some of messages that it brings.
+/// If this happens, we consider it "helpful" and refund its cost. If transaction fails to
+/// deliver at least one message, it is considered wrong and is not refunded.
+#[derive(Clone, Copy, PartialEq, RuntimeDebugNoBound)]
+pub struct MessagesState {
+	/// Best delivered message nonce.
+	pub best_nonce: MessageNonce,
+}
+
+// without this typedef rustfmt fails with internal err
+type BalanceOf<R> =
+	<<R as TransactionPaymentConfig>::OnChargeTransaction as OnChargeTransaction<R>>::Balance;
+type CallOf<R> = <R as frame_system::Config>::RuntimeCall;
+
+impl<R, GI, PI, MI, BE, PID, LID, FEE> SignedExtension
+	for RefundRelayerForMessagesFromParachain<R, GI, PI, MI, BE, PID, LID, FEE>
+where
+	R: 'static
+		+ Send
+		+ Sync
+		+ frame_system::Config
+		+ UtilityConfig<RuntimeCall = CallOf<R>>
+		+ GrandpaConfig<GI>
+		+ ParachainsConfig<PI, BridgesGrandpaPalletInstance = GI>
+		+ MessagesConfig<MI>
+		+ RelayersConfig,
+	GI: 'static + Send + Sync,
+	PI: 'static + Send + Sync,
+	MI: 'static + Send + Sync,
+	BE: 'static
+		+ Send
+		+ Sync
+		+ Default
+		+ SignedExtension<AccountId = R::AccountId, Call = CallOf<R>>,
+	PID: 'static + Send + Sync + Get<u32>,
+	LID: 'static + Send + Sync + Get<LaneId>,
+	FEE: 'static + Send + Sync + TransactionFeeCalculation<<R as RelayersConfig>::Reward>,
+	<R as frame_system::Config>::RuntimeCall:
+		Dispatchable<Info = DispatchInfo, PostInfo = PostDispatchInfo>,
+	CallOf<R>: IsSubType<CallableCallFor<UtilityPallet<R>, R>>
+		+ IsSubType<CallableCallFor<GrandpaPallet<R, GI>, R>>
+		+ IsSubType<CallableCallFor<ParachainsPallet<R, PI>, R>>
+		+ IsSubType<CallableCallFor<MessagesPallet<R, MI>, R>>,
+	<R as GrandpaConfig<GI>>::BridgedChain:
+		Chain<BlockNumber = RelayBlockNumber, Hash = RelayBlockHash, Hasher = RelayBlockHasher>,
+	<R as MessagesConfig<MI>>::SourceHeaderChain: SourceHeaderChain<
+		MessagesProof = FromBridgedChainMessagesProof<HashOf<BridgedChain<R, GI>>>,
+	>,
+{
+	const IDENTIFIER: &'static str = "RefundRelayerForMessagesFromParachain";
+	type AccountId = R::AccountId;
+	type Call = CallOf<R>;
+	type AdditionalSigned = ();
+	type Pre = Option<PreDispatchData<R::AccountId>>;
+
+	fn additional_signed(&self) -> Result<(), TransactionValidityError> {
+		Ok(())
+	}
+
+	fn validate(
+		&self,
+		_who: &Self::AccountId,
+		_call: &Self::Call,
+		_info: &DispatchInfoOf<Self::Call>,
+		_len: usize,
+	) -> TransactionValidity {
+		Ok(ValidTransaction::default())
+	}
+
+	fn pre_dispatch(
+		self,
+		who: &Self::AccountId,
+		call: &Self::Call,
+		post_info: &DispatchInfoOf<Self::Call>,
+		len: usize,
+	) -> Result<Self::Pre, TransactionValidityError> {
+		// reject batch transactions with obsolete headers
+		if let Some(UtilityCall::<R>::batch_all { ref calls }) = call.is_sub_type() {
+			for nested_call in calls {
+				let reject_obsolete_transactions = BE::default();
+				reject_obsolete_transactions.pre_dispatch(who, nested_call, post_info, len)?;
+			}
+		}
+
+		// now try to check if tx matches one of types we support
+		let parse_call_type = || {
+			if let Some(UtilityCall::<R>::batch_all { ref calls }) = call.is_sub_type() {
+				if calls.len() == 3 {
+					return Some(CallType::AllFinalityAndDelivery(
+						extract_expected_relay_chain_state::<R, GI>(&calls[0])?,
+						extract_expected_parachain_state::<R, GI, PI, PID>(&calls[1])?,
+						extract_messages_state::<R, GI, MI, LID>(&calls[2])?,
+					))
+				}
+				if calls.len() == 2 {
+					return Some(CallType::ParachainFinalityAndDelivery(
+						extract_expected_parachain_state::<R, GI, PI, PID>(&calls[0])?,
+						extract_messages_state::<R, GI, MI, LID>(&calls[1])?,
+					))
+				}
+				return None
+			}
+
+			Some(CallType::Delivery(extract_messages_state::<R, GI, MI, LID>(call)?))
+		};
+
+		Ok(parse_call_type().map(|call_type| PreDispatchData { relayer: who.clone(), call_type }))
+	}
+
+	fn post_dispatch(
+		pre: Option<Self::Pre>,
+		info: &DispatchInfoOf<Self::Call>,
+		post_info: &PostDispatchInfoOf<Self::Call>,
+		len: usize,
+		result: &DispatchResult,
+	) -> Result<(), TransactionValidityError> {
+		// we never refund anything if it is not bridge transaction or if it is a bridge
+		// transaction that we do not support here
+		let (relayer, call_type) = match pre {
+			Some(Some(pre)) => (pre.relayer, pre.call_type),
+			_ => return Ok(()),
+		};
+
+		// we never refund anything if transaction has failed
+		if result.is_err() {
+			return Ok(())
+		}
+
+		// check if relay chain state has been updated
+		if let CallType::AllFinalityAndDelivery(expected_relay_chain_state, _, _) = call_type {
+			let actual_relay_chain_state = relay_chain_state::<R, GI>();
+			if actual_relay_chain_state != Some(expected_relay_chain_state) {
+				// we only refund relayer if all calls have updated chain state
+				return Ok(())
+			}
+
+			// there's a conflict between how bridge GRANDPA pallet works and the
+			// `AllFinalityAndDelivery` transaction. If relay chain header is mandatory, the GRANDPA
+			// pallet returns `Pays::No`, because such transaction is mandatory for operating the
+			// bridge. But `utility.batchAll` transaction always requires payment. But in both cases
+			// we'll refund relayer - either explicitly here, or using `Pays::No` if he's choosing
+			// to submit dedicated transaction.
+		}
+
+		// check if parachain state has been updated
+		match call_type {
+			CallType::AllFinalityAndDelivery(_, expected_parachain_state, _) |
+			CallType::ParachainFinalityAndDelivery(expected_parachain_state, _) => {
+				let actual_parachain_state = parachain_state::<R, PI, PID>();
+				if actual_parachain_state != Some(expected_parachain_state) {
+					// we only refund relayer if all calls have updated chain state
+					return Ok(())
+				}
+			},
+			_ => (),
+		}
+
+		// check if messages have been delivered
+		let actual_messages_state = messages_state::<R, MI, LID>();
+		let pre_dispatch_messages_state = call_type.pre_dispatch_messages_state();
+		if actual_messages_state == Some(pre_dispatch_messages_state) {
+			// we only refund relayer if all calls have updated chain state
+			return Ok(())
+		}
+
+		// regarding the tip - refund that happens here (at this side of the bridge) isn't the whole
+		// relayer compensation. He'll receive some amount at the other side of the bridge. It shall
+		// (in theory) cover the tip here. Otherwise, if we'll be compensating tip here, some
+		// malicious relayer may use huge tips, effectively depleting account that pay rewards. The
+		// cost of this attack is nothing. Hence we use zero as tip here.
+		let tip = Zero::zero();
+
+		// compute the relayer reward
+		let reward = FEE::compute_fee(info, post_info, len, tip);
+
+		// finally - register reward in relayers pallet
+		RelayersPallet::<R>::register_relayer_reward(LID::get(), &relayer, reward);
+
+		Ok(())
+	}
+}
+
+/// Extracts expected relay chain state from the call.
+fn extract_expected_relay_chain_state<R, GI>(call: &CallOf<R>) -> Option<ExpectedRelayChainState>
+where
+	R: GrandpaConfig<GI>,
+	GI: 'static,
+	<R as GrandpaConfig<GI>>::BridgedChain: Chain<BlockNumber = RelayBlockNumber>,
+	CallOf<R>: IsSubType<CallableCallFor<GrandpaPallet<R, GI>, R>>,
+{
+	if let Some(GrandpaCall::<R, GI>::submit_finality_proof { ref finality_target, .. }) =
+		call.is_sub_type()
+	{
+		return Some(ExpectedRelayChainState { best_block_number: *finality_target.number() })
+	}
+	None
+}
+
+/// Extracts expected parachain state from the call.
+fn extract_expected_parachain_state<R, GI, PI, PID>(
+	call: &CallOf<R>,
+) -> Option<ExpectedParachainState>
+where
+	R: GrandpaConfig<GI> + ParachainsConfig<PI, BridgesGrandpaPalletInstance = GI>,
+	GI: 'static,
+	PI: 'static,
+	PID: Get<u32>,
+	<R as GrandpaConfig<GI>>::BridgedChain:
+		Chain<BlockNumber = RelayBlockNumber, Hash = RelayBlockHash, Hasher = RelayBlockHasher>,
+	CallOf<R>: IsSubType<CallableCallFor<ParachainsPallet<R, PI>, R>>,
+{
+	if let Some(ParachainsCall::<R, PI>::submit_parachain_heads {
+		ref at_relay_block,
+		ref parachains,
+		..
+	}) = call.is_sub_type()
+	{
+		if parachains.len() != 1 || parachains[0].0 != ParaId(PID::get()) {
+			return None
+		}
+
+		return Some(ExpectedParachainState { at_relay_block_number: at_relay_block.0 })
+	}
+	None
+}
+
+/// Extracts messages state from the call.
+fn extract_messages_state<R, GI, MI, LID>(call: &CallOf<R>) -> Option<MessagesState>
+where
+	R: GrandpaConfig<GI> + MessagesConfig<MI>,
+	GI: 'static,
+	MI: 'static,
+	LID: Get<LaneId>,
+	CallOf<R>: IsSubType<CallableCallFor<MessagesPallet<R, MI>, R>>,
+	<R as MessagesConfig<MI>>::SourceHeaderChain: SourceHeaderChain<
+		MessagesProof = FromBridgedChainMessagesProof<HashOf<BridgedChain<R, GI>>>,
+	>,
+{
+	if let Some(MessagesCall::<R, MI>::receive_messages_proof { ref proof, .. }) =
+		call.is_sub_type()
+	{
+		if LID::get() != proof.lane {
+			return None
+		}
+
+		return Some(MessagesState {
+			best_nonce: MessagesPallet::<R, MI>::inbound_lane_data(proof.lane)
+				.last_delivered_nonce(),
+		})
+	}
+	None
+}
+
+/// Returns relay chain state that we are interested in.
+fn relay_chain_state<R, GI>() -> Option<ExpectedRelayChainState>
+where
+	R: GrandpaConfig<GI>,
+	GI: 'static,
+	<R as GrandpaConfig<GI>>::BridgedChain: Chain<BlockNumber = RelayBlockNumber>,
+{
+	GrandpaPallet::<R, GI>::best_finalized_number()
+		.map(|best_block_number| ExpectedRelayChainState { best_block_number })
+}
+
+/// Returns parachain state that we are interested in.
+fn parachain_state<R, PI, PID>() -> Option<ExpectedParachainState>
+where
+	R: ParachainsConfig<PI>,
+	PI: 'static,
+	PID: Get<u32>,
+{
+	ParachainsPallet::<R, PI>::best_parachain_info(ParaId(PID::get())).map(|para_info| {
+		ExpectedParachainState {
+			at_relay_block_number: para_info.best_head_hash.at_relay_block_number,
+		}
+	})
+}
+
+/// Returns messages state that we are interested in.
+fn messages_state<R, MI, LID>() -> Option<MessagesState>
+where
+	R: MessagesConfig<MI>,
+	MI: 'static,
+	LID: Get<LaneId>,
+{
+	Some(MessagesState {
+		best_nonce: MessagesPallet::<R, MI>::inbound_lane_data(LID::get()).last_delivered_nonce(),
+	})
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use bp_messages::InboundLaneData;
+	use bp_parachains::{BestParaHeadHash, ParaInfo};
+	use bp_polkadot_core::parachains::ParaHeadsProof;
+	use bp_runtime::HeaderId;
+	use bp_test_utils::make_default_justification;
+	use frame_support::{assert_storage_noop, parameter_types, weights::Weight};
+	use millau_runtime::{
+		RialtoGrandpaInstance, Runtime, RuntimeCall, WithRialtoParachainMessagesInstance,
+		WithRialtoParachainsInstance,
+	};
+	use sp_runtime::{transaction_validity::InvalidTransaction, DispatchError};
+
+	parameter_types! {
+		pub TestParachain: u32 = 1000;
+		pub TestLaneId: LaneId = [0, 0, 0, 0];
+	}
+
+	type TestExtension = RefundRelayerForMessagesFromParachain<
+		millau_runtime::Runtime,
+		RialtoGrandpaInstance,
+		WithRialtoParachainsInstance,
+		WithRialtoParachainMessagesInstance,
+		millau_runtime::BridgeRejectObsoleteHeadersAndMessages,
+		TestParachain,
+		TestLaneId,
+		millau_runtime::Runtime,
+	>;
+
+	fn relayer_account() -> millau_runtime::AccountId {
+		[0u8; 32].into()
+	}
+
+	fn initialize_environment(
+		best_relay_header_number: RelayBlockNumber,
+		parachain_head_at_relay_header_number: RelayBlockNumber,
+		best_delivered_message: MessageNonce,
+	) {
+		let best_relay_header = HeaderId(best_relay_header_number, RelayBlockHash::default());
+		pallet_bridge_grandpa::BestFinalized::<Runtime, RialtoGrandpaInstance>::put(
+			best_relay_header,
+		);
+
+		let para_id = ParaId(TestParachain::get());
+		let para_info = ParaInfo {
+			best_head_hash: BestParaHeadHash {
+				at_relay_block_number: parachain_head_at_relay_header_number,
+				head_hash: Default::default(),
+			},
+			next_imported_hash_position: 0,
+		};
+		pallet_bridge_parachains::ParasInfo::<Runtime, WithRialtoParachainsInstance>::insert(
+			para_id, para_info,
+		);
+
+		let lane_id = TestLaneId::get();
+		let lane_data =
+			InboundLaneData { last_confirmed_nonce: best_delivered_message, ..Default::default() };
+		pallet_bridge_messages::InboundLanes::<Runtime, WithRialtoParachainMessagesInstance>::insert(lane_id, lane_data);
+	}
+
+	fn submit_relay_header_call(relay_header_number: RelayBlockNumber) -> RuntimeCall {
+		let relay_header = bp_rialto::Header::new(
+			relay_header_number,
+			Default::default(),
+			Default::default(),
+			Default::default(),
+			Default::default(),
+		);
+		let relay_justification = make_default_justification(&relay_header);
+
+		RuntimeCall::BridgeRialtoGrandpa(GrandpaCall::submit_finality_proof {
+			finality_target: Box::new(relay_header),
+			justification: relay_justification,
+		})
+	}
+
+	fn submit_parachain_head_call(
+		parachain_head_at_relay_header_number: RelayBlockNumber,
+	) -> RuntimeCall {
+		RuntimeCall::BridgeRialtoParachains(ParachainsCall::submit_parachain_heads {
+			at_relay_block: (parachain_head_at_relay_header_number, RelayBlockHash::default()),
+			parachains: vec![(ParaId(TestParachain::get()), [1u8; 32].into())],
+			parachain_heads_proof: ParaHeadsProof(vec![]),
+		})
+	}
+
+	fn message_delivery_call(best_message: MessageNonce) -> RuntimeCall {
+		RuntimeCall::BridgeRialtoParachainMessages(MessagesCall::receive_messages_proof {
+			relayer_id_at_bridged_chain: relayer_account(),
+			proof: millau_runtime::bridge_runtime_common::messages::target::FromBridgedChainMessagesProof {
+				bridged_header_hash: Default::default(),
+				storage_proof: vec![],
+				lane: TestLaneId::get(),
+				nonces_start: best_message,
+				nonces_end: best_message,
+			},
+			messages_count: 1,
+			dispatch_weight: Weight::zero(),
+		})
+	}
+
+	fn parachain_finality_and_delivery_batch_call(
+		parachain_head_at_relay_header_number: RelayBlockNumber,
+		best_message: MessageNonce,
+	) -> RuntimeCall {
+		RuntimeCall::Utility(UtilityCall::batch_all {
+			calls: vec![
+				submit_parachain_head_call(parachain_head_at_relay_header_number),
+				message_delivery_call(best_message),
+			],
+		})
+	}
+
+	fn all_finality_and_delivery_batch_call(
+		relay_header_number: RelayBlockNumber,
+		parachain_head_at_relay_header_number: RelayBlockNumber,
+		best_message: MessageNonce,
+	) -> RuntimeCall {
+		RuntimeCall::Utility(UtilityCall::batch_all {
+			calls: vec![
+				submit_relay_header_call(relay_header_number),
+				submit_parachain_head_call(parachain_head_at_relay_header_number),
+				message_delivery_call(best_message),
+			],
+		})
+	}
+
+	fn all_finality_pre_dispatch_data() -> PreDispatchData<millau_runtime::AccountId> {
+		PreDispatchData {
+			relayer: relayer_account(),
+			call_type: CallType::AllFinalityAndDelivery(
+				ExpectedRelayChainState { best_block_number: 200 },
+				ExpectedParachainState { at_relay_block_number: 200 },
+				MessagesState { best_nonce: 100 },
+			),
+		}
+	}
+
+	fn parachain_finality_pre_dispatch_data() -> PreDispatchData<millau_runtime::AccountId> {
+		PreDispatchData {
+			relayer: relayer_account(),
+			call_type: CallType::ParachainFinalityAndDelivery(
+				ExpectedParachainState { at_relay_block_number: 200 },
+				MessagesState { best_nonce: 100 },
+			),
+		}
+	}
+
+	fn delivery_pre_dispatch_data() -> PreDispatchData<millau_runtime::AccountId> {
+		PreDispatchData {
+			relayer: relayer_account(),
+			call_type: CallType::Delivery(MessagesState { best_nonce: 100 }),
+		}
+	}
+
+	fn run_test(test: impl FnOnce()) {
+		sp_io::TestExternalities::new(Default::default()).execute_with(|| test())
+	}
+
+	fn run_pre_dispatch(
+		call: RuntimeCall,
+	) -> Result<Option<PreDispatchData<millau_runtime::AccountId>>, TransactionValidityError> {
+		let extension: TestExtension = RefundRelayerForMessagesFromParachain(PhantomData);
+		extension.pre_dispatch(&relayer_account(), &call, &DispatchInfo::default(), 0)
+	}
+
+	fn dispatch_info() -> DispatchInfo {
+		DispatchInfo {
+			weight: frame_support::weights::constants::WEIGHT_PER_SECOND,
+			class: frame_support::dispatch::DispatchClass::Normal,
+			pays_fee: frame_support::dispatch::Pays::Yes,
+		}
+	}
+
+	fn post_dispatch_info() -> PostDispatchInfo {
+		PostDispatchInfo { actual_weight: None, pays_fee: frame_support::dispatch::Pays::Yes }
+	}
+
+	fn run_post_dispatch(
+		pre_dispatch_data: Option<PreDispatchData<millau_runtime::AccountId>>,
+		dispatch_result: DispatchResult,
+	) {
+		let post_dispatch_result = TestExtension::post_dispatch(
+			Some(pre_dispatch_data),
+			&dispatch_info(),
+			&post_dispatch_info(),
+			1024,
+			&dispatch_result,
+		);
+		assert_eq!(post_dispatch_result, Ok(()));
+	}
+
+	fn expected_reward() -> millau_runtime::Balance {
+		pallet_transaction_payment::Pallet::<Runtime>::compute_actual_fee(
+			1024,
+			&dispatch_info(),
+			&post_dispatch_info(),
+			Zero::zero(),
+		)
+	}
+
+	#[test]
+	fn pre_dispatch_rejects_batch_with_obsolete_relay_chain_header() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(all_finality_and_delivery_batch_call(100, 200, 200)),
+				Err(TransactionValidityError::Invalid(InvalidTransaction::Stale)),
+			);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_rejects_batch_with_obsolete_parachain_head() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(all_finality_and_delivery_batch_call(101, 100, 200)),
+				Err(TransactionValidityError::Invalid(InvalidTransaction::Stale)),
+			);
+
+			assert_eq!(
+				run_pre_dispatch(parachain_finality_and_delivery_batch_call(100, 200)),
+				Err(TransactionValidityError::Invalid(InvalidTransaction::Stale)),
+			);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_rejects_batch_with_obsolete_messages() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(all_finality_and_delivery_batch_call(200, 200, 100)),
+				Err(TransactionValidityError::Invalid(InvalidTransaction::Stale)),
+			);
+
+			assert_eq!(
+				run_pre_dispatch(parachain_finality_and_delivery_batch_call(200, 100)),
+				Err(TransactionValidityError::Invalid(InvalidTransaction::Stale)),
+			);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_parses_batch_with_relay_chain_and_parachain_headers() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(all_finality_and_delivery_batch_call(200, 200, 200)),
+				Ok(Some(all_finality_pre_dispatch_data())),
+			);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_parses_batch_with_parachain_header() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(parachain_finality_and_delivery_batch_call(200, 200)),
+				Ok(Some(parachain_finality_pre_dispatch_data())),
+			);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_fails_to_parse_batch_with_multiple_parachain_headers() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			let call = RuntimeCall::Utility(UtilityCall::batch_all {
+				calls: vec![
+					RuntimeCall::BridgeRialtoParachains(ParachainsCall::submit_parachain_heads {
+						at_relay_block: (100, RelayBlockHash::default()),
+						parachains: vec![
+							(ParaId(TestParachain::get()), [1u8; 32].into()),
+							(ParaId(TestParachain::get() + 1), [1u8; 32].into()),
+						],
+						parachain_heads_proof: ParaHeadsProof(vec![]),
+					}),
+					message_delivery_call(200),
+				],
+			});
+
+			assert_eq!(run_pre_dispatch(call), Ok(None),);
+		});
+	}
+
+	#[test]
+	fn pre_dispatch_parses_message_delivery_transaction() {
+		run_test(|| {
+			initialize_environment(100, 100, 100);
+
+			assert_eq!(
+				run_pre_dispatch(message_delivery_call(200)),
+				Ok(Some(delivery_pre_dispatch_data())),
+			);
+		});
+	}
+
+	#[test]
+	fn post_dispatch_ignores_unknown_transaction() {
+		run_test(|| {
+			assert_storage_noop!(run_post_dispatch(None, Ok(())));
+		});
+	}
+
+	#[test]
+	fn post_dispatch_ignores_failed_transaction() {
+		run_test(|| {
+			assert_storage_noop!(run_post_dispatch(
+				Some(all_finality_pre_dispatch_data()),
+				Err(DispatchError::BadOrigin)
+			));
+		});
+	}
+
+	#[test]
+	fn post_dispatch_ignores_transaction_that_has_not_updated_relay_chain_state() {
+		run_test(|| {
+			initialize_environment(100, 200, 200);
+
+			assert_storage_noop!(run_post_dispatch(Some(all_finality_pre_dispatch_data()), Ok(())));
+		});
+	}
+
+	#[test]
+	fn post_dispatch_ignores_transaction_that_has_not_updated_parachain_state() {
+		run_test(|| {
+			initialize_environment(200, 100, 200);
+
+			assert_storage_noop!(run_post_dispatch(Some(all_finality_pre_dispatch_data()), Ok(())));
+			assert_storage_noop!(run_post_dispatch(
+				Some(parachain_finality_pre_dispatch_data()),
+				Ok(())
+			));
+		});
+	}
+
+	#[test]
+	fn post_dispatch_ignores_transaction_that_has_not_delivered_any_messages() {
+		run_test(|| {
+			initialize_environment(200, 200, 100);
+
+			assert_storage_noop!(run_post_dispatch(Some(all_finality_pre_dispatch_data()), Ok(())));
+			assert_storage_noop!(run_post_dispatch(
+				Some(parachain_finality_pre_dispatch_data()),
+				Ok(())
+			));
+			assert_storage_noop!(run_post_dispatch(Some(delivery_pre_dispatch_data()), Ok(())));
+		});
+	}
+
+	#[test]
+	fn post_dispatch_refunds_relayer_in_all_finality_batch() {
+		run_test(|| {
+			initialize_environment(200, 200, 200);
+
+			run_post_dispatch(Some(all_finality_pre_dispatch_data()), Ok(()));
+			assert_eq!(
+				RelayersPallet::<Runtime>::relayer_reward(relayer_account(), TestLaneId::get()),
+				Some(expected_reward()),
+			);
+		});
+	}
+
+	#[test]
+	fn post_dispatch_refunds_relayer_in_parachain_finality_batch() {
+		run_test(|| {
+			initialize_environment(200, 200, 200);
+
+			run_post_dispatch(Some(parachain_finality_pre_dispatch_data()), Ok(()));
+			assert_eq!(
+				RelayersPallet::<Runtime>::relayer_reward(relayer_account(), TestLaneId::get()),
+				Some(expected_reward()),
+			);
+		});
+	}
+
+	#[test]
+	fn post_dispatch_refunds_relayer_in_message_delivery_transaction() {
+		run_test(|| {
+			initialize_environment(200, 200, 200);
+
+			run_post_dispatch(Some(delivery_pre_dispatch_data()), Ok(()));
+			assert_eq!(
+				RelayersPallet::<Runtime>::relayer_reward(relayer_account(), TestLaneId::get()),
+				Some(expected_reward()),
+			);
+		});
+	}
+}

bridges/modules/grandpa/src/lib.rs

@@ -540,6 +540,13 @@ pub mod pallet {
 	}
 }
 
+impl<T: Config<I>, I: 'static> Pallet<T, I> {
+	/// Get the best finalized block number.
+	pub fn best_finalized_number() -> Option<BridgedBlockNumber<T, I>> {
+		BestFinalized::<T, I>::get().map(|id| id.number())
+	}
+}
+
 /// Bridge GRANDPA pallet as header chain.
 pub type GrandpaChainHeaders<T, I> = Pallet<T, I>;
 

bridges/modules/messages/src/lib.rs

@@ -168,10 +168,10 @@ pub mod pallet {
 	}
 
 	/// Shortcut to messages proof type for Config.
-	type MessagesProofOf<T, I> =
+	pub type MessagesProofOf<T, I> =
 		<<T as Config<I>>::SourceHeaderChain as SourceHeaderChain>::MessagesProof;
 	/// Shortcut to messages delivery proof type for Config.
-	type MessagesDeliveryProofOf<T, I> =
+	pub type MessagesDeliveryProofOf<T, I> =
 		<<T as Config<I>>::TargetHeaderChain as TargetHeaderChain<
 			<T as Config<I>>::OutboundPayload,
 			<T as frame_system::Config>::AccountId,
@@ -631,6 +631,11 @@ pub mod pallet {
 				dispatch_weight: T::MessageDispatch::dispatch_weight(&mut dispatch_message),
 			}
 		}
+
+		/// Return inbound lane data.
+		pub fn inbound_lane_data(lane: LaneId) -> InboundLaneData<T::InboundRelayer> {
+			InboundLanes::<T, I>::get(lane).0
+		}
 	}
 }
 

bridges/modules/parachains/src/lib.rs

@@ -401,6 +401,11 @@ pub mod pallet {
 	}
 
 	impl<T: Config<I>, I: 'static> Pallet<T, I> {
+		/// Get stored parachain info.
+		pub fn best_parachain_info(parachain: ParaId) -> Option<ParaInfo> {
+			ParasInfo::<T, I>::get(parachain)
+		}
+
 		/// Get best finalized header of the given parachain.
 		pub fn best_parachain_head(parachain: ParaId) -> Option<ParaHead> {
 			let best_para_head_hash = ParasInfo::<T, I>::get(parachain)?.best_head_hash.head_hash;

bridges/modules/relayers/Cargo.toml

@@ -22,6 +22,7 @@ frame-benchmarking = { git = "https://github.com/paritytech/substrate", branch =
 frame-support = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 frame-system = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-arithmetic = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
+sp-runtime = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 sp-std = { git = "https://github.com/paritytech/substrate", branch = "master", default-features = false }
 
 [dev-dependencies]
@@ -42,6 +43,7 @@ std = [
 	"log/std",
 	"scale-info/std",
 	"sp-arithmetic/std",
+	"sp-runtime/std",
 	"sp-std/std",
 ]
 runtime-benchmarks = [

bridges/modules/relayers/src/lib.rs

@@ -109,8 +109,9 @@ pub mod pallet {
 
 				log::trace!(
 					target: crate::LOG_TARGET,
-					"Relayer {:?} can now claim reward: {:?}",
+					"Relayer {:?} can now claim reward for serving lane {:?}: {:?}",
 					relayer,
+					lane_id,
 					new_reward,
 				);
 			});
@@ -141,6 +142,7 @@ pub mod pallet {
 
 	/// Map of the relayer => accumulated reward.
 	#[pallet::storage]
+	#[pallet::getter(fn relayer_reward)]
 	pub type RelayerRewards<T: Config> = StorageDoubleMap<
 		_,
 		Blake2_128Concat,