Stored conversion rate updater (#1005)

* update conversion rate: initial commit

* Rialto=Polkadot && Millau=Kusama + actually update conversion rates

* update deployment scripts and readme

* allow non-zero difference between stored and real rates

* dummy commit

* Revert "dummy commit"

This reverts commit a438198180a8385feeaaca60c9d2da0950465215.

* clippy

* #[allow(clippy::float_cmp)] in conversion rate update

* dummy

* Revert "dummy"

This reverts commit 90cd6e47cda56f655e94dbef76138e6cc58d664a.

* spell

* shared_value_ref() -> get()

* Revert "shared_value_ref() -> get()"

This reverts commit 20aa30de6a59b2099cfba3e9676e71200b7bb468.

bridges/bin/millau/node/src/chain_spec.rs

@@ -16,8 +16,8 @@
 
 use bp_millau::derive_account_from_rialto_id;
 use millau_runtime::{
-	AccountId, AuraConfig, BalancesConfig, BridgeWestendGrandpaConfig, GenesisConfig, GrandpaConfig, SessionConfig,
-	SessionKeys, Signature, SudoConfig, SystemConfig, WASM_BINARY,
+	AccountId, AuraConfig, BalancesConfig, BridgeRialtoMessagesConfig, BridgeWestendGrandpaConfig, GenesisConfig,
+	GrandpaConfig, SessionConfig, SessionKeys, Signature, SudoConfig, SystemConfig, WASM_BINARY,
 };
 use sp_consensus_aura::sr25519::AuthorityId as AuraId;
 use sp_core::{sr25519, Pair, Public};
@@ -134,6 +134,7 @@ impl Alternative {
 							get_account_id_from_seed::<sr25519::Public>("Ferdie//stash"),
 							get_account_id_from_seed::<sr25519::Public>("George//stash"),
 							get_account_id_from_seed::<sr25519::Public>("Harry//stash"),
+							get_account_id_from_seed::<sr25519::Public>("RialtoMessagesOwner"),
 							pallet_bridge_messages::Pallet::<
 								millau_runtime::Runtime,
 								millau_runtime::WithRialtoMessagesInstance,
@@ -208,6 +209,10 @@ fn testnet_genesis(
 			owner: Some(get_account_id_from_seed::<sr25519::Public>("George")),
 			..Default::default()
 		},
+		bridge_rialto_messages: BridgeRialtoMessagesConfig {
+			owner: Some(get_account_id_from_seed::<sr25519::Public>("RialtoMessagesOwner")),
+			..Default::default()
+		},
 	}
 }
 

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

@@ -407,7 +407,7 @@ construct_runtime!(
 		NodeBlock = opaque::Block,
 		UncheckedExtrinsic = UncheckedExtrinsic
 	{
-		BridgeRialtoMessages: pallet_bridge_messages::{Pallet, Call, Storage, Event<T>},
+		BridgeRialtoMessages: pallet_bridge_messages::{Pallet, Call, Storage, Event<T>, Config<T>},
 		BridgeDispatch: pallet_bridge_dispatch::{Pallet, Event<T>},
 		BridgeRialtoGrandpa: pallet_bridge_grandpa::{Pallet, Call, Storage},
 		BridgeWestendGrandpa: pallet_bridge_grandpa::<Instance1>::{Pallet, Call, Config<T>, Storage},

bridges/bin/rialto/node/src/chain_spec.rs

@@ -16,8 +16,8 @@
 
 use bp_rialto::derive_account_from_millau_id;
 use rialto_runtime::{
-	AccountId, BabeConfig, BalancesConfig, BridgeKovanConfig, BridgeRialtoPoaConfig, GenesisConfig, GrandpaConfig,
-	SessionConfig, SessionKeys, Signature, SudoConfig, SystemConfig, WASM_BINARY,
+	AccountId, BabeConfig, BalancesConfig, BridgeKovanConfig, BridgeMillauMessagesConfig, BridgeRialtoPoaConfig,
+	GenesisConfig, GrandpaConfig, SessionConfig, SessionKeys, Signature, SudoConfig, SystemConfig, WASM_BINARY,
 };
 use serde_json::json;
 use sp_consensus_babe::AuthorityId as BabeId;
@@ -135,6 +135,7 @@ impl Alternative {
 							get_account_id_from_seed::<sr25519::Public>("Ferdie//stash"),
 							get_account_id_from_seed::<sr25519::Public>("George//stash"),
 							get_account_id_from_seed::<sr25519::Public>("Harry//stash"),
+							get_account_id_from_seed::<sr25519::Public>("MillauMessagesOwner"),
 							pallet_bridge_messages::Pallet::<
 								rialto_runtime::Runtime,
 								rialto_runtime::WithMillauMessagesInstance,
@@ -205,6 +206,10 @@ fn testnet_genesis(
 				.map(|x| (x.0.clone(), x.0.clone(), session_keys(x.1.clone(), x.2.clone())))
 				.collect::<Vec<_>>(),
 		},
+		bridge_millau_messages: BridgeMillauMessagesConfig {
+			owner: Some(get_account_id_from_seed::<sr25519::Public>("MillauMessagesOwner")),
+			..Default::default()
+		},
 	}
 }
 

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

@@ -559,7 +559,7 @@ construct_runtime!(
 		// Millau bridge modules.
 		BridgeMillauGrandpa: pallet_bridge_grandpa::{Pallet, Call, Storage},
 		BridgeDispatch: pallet_bridge_dispatch::{Pallet, Event<T>},
-		BridgeMillauMessages: pallet_bridge_messages::{Pallet, Call, Storage, Event<T>},
+		BridgeMillauMessages: pallet_bridge_messages::{Pallet, Call, Storage, Event<T>, Config<T>},
 	}
 );
 

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

@@ -245,3 +245,33 @@ pub(crate) fn add_standalone_metrics(
 		)),
 	)
 }
+
+/// Update Rialto -> Millau conversion rate, stored in Millau runtime storage.
+pub(crate) async fn update_rialto_to_millau_conversion_rate(
+	client: Client<Millau>,
+	signer: <Millau as TransactionSignScheme>::AccountKeyPair,
+	updated_rate: f64,
+) -> anyhow::Result<()> {
+	let genesis_hash = *client.genesis_hash();
+	let signer_id = (*signer.public().as_array_ref()).into();
+	client
+		.submit_signed_extrinsic(signer_id, move |transaction_nonce| {
+			Bytes(
+				Millau::sign_transaction(
+					genesis_hash,
+					&signer,
+					transaction_nonce,
+					millau_runtime::MessagesCall::update_pallet_parameter(
+						millau_runtime::rialto_messages::MillauToRialtoMessagesParameter::RialtoToMillauConversionRate(
+							sp_runtime::FixedU128::from_float(updated_rate),
+						),
+					)
+					.into(),
+				)
+				.encode(),
+			)
+		})
+		.await
+		.map(drop)
+		.map_err(|err| anyhow::format_err!("{:?}", err))
+}

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

@@ -37,9 +37,9 @@ mod wococo;
 // Rialto as BTC and Millau as wBTC (only in relayer).
 
 /// The identifier of token, which value is associated with Rialto token value by relayer.
-pub(crate) const RIALTO_ASSOCIATED_TOKEN_ID: &str = "bitcoin";
+pub(crate) const RIALTO_ASSOCIATED_TOKEN_ID: &str = "polkadot";
 /// The identifier of token, which value is associated with Millau token value by relayer.
-pub(crate) const MILLAU_ASSOCIATED_TOKEN_ID: &str = "wrapped-bitcoin";
+pub(crate) const MILLAU_ASSOCIATED_TOKEN_ID: &str = "kusama";
 
 use relay_utils::metrics::MetricsParams;
 

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

@@ -244,3 +244,33 @@ pub(crate) fn add_standalone_metrics(
 		)),
 	)
 }
+
+/// Update Millau -> Rialto conversion rate, stored in Rialto runtime storage.
+pub(crate) async fn update_millau_to_rialto_conversion_rate(
+	client: Client<Rialto>,
+	signer: <Rialto as TransactionSignScheme>::AccountKeyPair,
+	updated_rate: f64,
+) -> anyhow::Result<()> {
+	let genesis_hash = *client.genesis_hash();
+	let signer_id = (*signer.public().as_array_ref()).into();
+	client
+		.submit_signed_extrinsic(signer_id, move |transaction_nonce| {
+			Bytes(
+				Rialto::sign_transaction(
+					genesis_hash,
+					&signer,
+					transaction_nonce,
+					rialto_runtime::MessagesCall::update_pallet_parameter(
+						rialto_runtime::millau_messages::RialtoToMillauMessagesParameter::MillauToRialtoConversionRate(
+							sp_runtime::FixedU128::from_float(updated_rate),
+						),
+					)
+					.into(),
+				)
+				.encode(),
+			)
+		})
+		.await
+		.map(drop)
+		.map_err(|err| anyhow::format_err!("{:?}", err))
+}

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

@@ -391,6 +391,17 @@ macro_rules! declare_chain_options {
 				pub [<$chain_prefix _signer_password_file>]: Option<std::path::PathBuf>,
 			}
 
+			#[doc = "Parameters required to sign transaction on behalf of owner of the messages pallet at " $chain "."]
+			#[derive(StructOpt, Debug, PartialEq, Eq)]
+			pub struct [<$chain MessagesPalletOwnerSigningParams>] {
+				#[doc = "The SURI of secret key to use when transactions are submitted to the " $chain " node."]
+				#[structopt(long)]
+				pub [<$chain_prefix _messages_pallet_owner>]: Option<String>,
+				#[doc = "The password for the SURI of secret key to use when transactions are submitted to the " $chain " node."]
+				#[structopt(long)]
+				pub [<$chain_prefix _messages_pallet_owner_password>]: Option<String>,
+			}
+
 			impl [<$chain SigningParams>] {
 				/// Parse signing params into chain-specific KeyPair.
 				pub fn to_keypair<Chain: CliChain>(&self) -> anyhow::Result<Chain::KeyPair> {
@@ -433,6 +444,23 @@ macro_rules! declare_chain_options {
 				}
 			}
 
+			#[allow(dead_code)]
+			impl [<$chain MessagesPalletOwnerSigningParams>] {
+				/// Parse signing params into chain-specific KeyPair.
+				pub fn to_keypair<Chain: CliChain>(&self) -> anyhow::Result<Option<Chain::KeyPair>> {
+					use sp_core::crypto::Pair;
+
+					let [<$chain_prefix _messages_pallet_owner>] = match self.[<$chain_prefix _messages_pallet_owner>] {
+						Some(ref messages_pallet_owner) => messages_pallet_owner,
+						None => return Ok(None),
+					};
+					Chain::KeyPair::from_string(
+						[<$chain_prefix _messages_pallet_owner>],
+						self.[<$chain_prefix _messages_pallet_owner_password>].as_deref()
+					).map_err(|e| anyhow::format_err!("{:?}", e)).map(Some)
+				}
+			}
+
 			impl [<$chain ConnectionParams>] {
 				/// Convert connection params into Substrate client.
 				pub async fn to_client<Chain: CliChain>(

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

@@ -26,6 +26,7 @@ use futures::{FutureExt, TryFutureExt};
 use structopt::StructOpt;
 use strum::VariantNames;
 
+use relay_substrate_client::{Chain, Client, TransactionSignScheme};
 use relay_utils::metrics::MetricsParams;
 use substrate_relay_helper::messages_lane::{MessagesRelayParams, SubstrateMessageLane};
 use substrate_relay_helper::on_demand_headers::OnDemandHeadersRelay;
@@ -33,6 +34,14 @@ use substrate_relay_helper::on_demand_headers::OnDemandHeadersRelay;
 use crate::cli::{relay_messages::RelayerMode, CliChain, HexLaneId, PrometheusParams};
 use crate::declare_chain_options;
 
+/// Maximal allowed conversion rate error ratio (abs(real - stored) / stored) that we allow.
+///
+/// If it is zero, then transaction will be submitted every time we see difference between
+/// stored and real conversion rates. If it is large enough (e.g. > than 10 percents, which is 0.1),
+/// then rational relayers may stop relaying messages because they were submitted using
+/// lesser conversion rate.
+const CONVERSION_RATE_ALLOWED_DIFFERENCE_RATIO: f64 = 0.05;
+
 /// Start headers+messages relayer process.
 #[derive(StructOpt)]
 pub enum RelayHeadersAndMessages {
@@ -68,9 +77,13 @@ macro_rules! declare_bridge_options {
 				#[structopt(flatten)]
 				left_sign: [<$chain1 SigningParams>],
 				#[structopt(flatten)]
+				left_messages_pallet_owner: [<$chain1 MessagesPalletOwnerSigningParams>],
+				#[structopt(flatten)]
 				right: [<$chain2 ConnectionParams>],
 				#[structopt(flatten)]
 				right_sign: [<$chain2 SigningParams>],
+				#[structopt(flatten)]
+				right_messages_pallet_owner: [<$chain2 MessagesPalletOwnerSigningParams>],
 			}
 
 			#[allow(unreachable_patterns)]
@@ -106,9 +119,11 @@ macro_rules! select_bridge {
 
 				use crate::chains::millau_messages_to_rialto::{
 					add_standalone_metrics as add_left_to_right_standalone_metrics, run as left_to_right_messages,
+					update_rialto_to_millau_conversion_rate as update_right_to_left_conversion_rate,
 				};
 				use crate::chains::rialto_messages_to_millau::{
 					add_standalone_metrics as add_right_to_left_standalone_metrics, run as right_to_left_messages,
+					update_millau_to_rialto_conversion_rate as update_left_to_right_conversion_rate,
 				};
 
 				$generic
@@ -135,6 +150,22 @@ macro_rules! select_bridge {
 					add_standalone_metrics as add_right_to_left_standalone_metrics, run as right_to_left_messages,
 				};
 
+				async fn update_right_to_left_conversion_rate(
+					_client: Client<Left>,
+					_signer: <Left as TransactionSignScheme>::AccountKeyPair,
+					_updated_rate: f64,
+				) -> anyhow::Result<()> {
+					Err(anyhow::format_err!("Conversion rate is not supported by this bridge"))
+				}
+
+				async fn update_left_to_right_conversion_rate(
+					_client: Client<Right>,
+					_signer: <Right as TransactionSignScheme>::AccountKeyPair,
+					_updated_rate: f64,
+				) -> anyhow::Result<()> {
+					Err(anyhow::format_err!("Conversion rate is not supported by this bridge"))
+				}
+
 				$generic
 			}
 		}
@@ -158,16 +189,86 @@ impl RelayHeadersAndMessages {
 
 			let left_client = params.left.to_client::<Left>().await?;
 			let left_sign = params.left_sign.to_keypair::<Left>()?;
+			let left_messages_pallet_owner = params.left_messages_pallet_owner.to_keypair::<Left>()?;
 			let right_client = params.right.to_client::<Right>().await?;
 			let right_sign = params.right_sign.to_keypair::<Right>()?;
+			let right_messages_pallet_owner = params.right_messages_pallet_owner.to_keypair::<Right>()?;
 
 			let lanes = params.shared.lane;
 			let relayer_mode = params.shared.relayer_mode.into();
 
+			const METRIC_IS_SOME_PROOF: &str = "it is `None` when metric has been already registered; \
+				this is the command entrypoint, so nothing has been registered yet; \
+				qed";
+
 			let metrics_params: MetricsParams = params.shared.prometheus_params.into();
 			let metrics_params = relay_utils::relay_metrics(None, metrics_params).into_params();
-			let (metrics_params, _) = add_left_to_right_standalone_metrics(None, metrics_params, left_client.clone())?;
-			let (metrics_params, _) = add_right_to_left_standalone_metrics(None, metrics_params, right_client.clone())?;
+			let (metrics_params, left_to_right_metrics) =
+				add_left_to_right_standalone_metrics(None, metrics_params, left_client.clone())?;
+			let (metrics_params, right_to_left_metrics) =
+				add_right_to_left_standalone_metrics(None, metrics_params, right_client.clone())?;
+			if let Some(left_messages_pallet_owner) = left_messages_pallet_owner {
+				let left_client = left_client.clone();
+				substrate_relay_helper::conversion_rate_update::run_conversion_rate_update_loop(
+					left_to_right_metrics
+						.target_to_source_conversion_rate
+						.expect(METRIC_IS_SOME_PROOF),
+					left_to_right_metrics
+						.target_to_base_conversion_rate
+						.clone()
+						.expect(METRIC_IS_SOME_PROOF),
+					left_to_right_metrics
+						.source_to_base_conversion_rate
+						.clone()
+						.expect(METRIC_IS_SOME_PROOF),
+					CONVERSION_RATE_ALLOWED_DIFFERENCE_RATIO,
+					move |new_rate| {
+						log::info!(
+							target: "bridge",
+							"Going to update {} -> {} (on {}) conversion rate to {}.",
+							Right::NAME,
+							Left::NAME,
+							Left::NAME,
+							new_rate,
+						);
+						update_right_to_left_conversion_rate(
+							left_client.clone(),
+							left_messages_pallet_owner.clone(),
+							new_rate,
+						)
+					},
+				);
+			}
+			if let Some(right_messages_pallet_owner) = right_messages_pallet_owner {
+				let right_client = right_client.clone();
+				substrate_relay_helper::conversion_rate_update::run_conversion_rate_update_loop(
+					right_to_left_metrics
+						.target_to_source_conversion_rate
+						.expect(METRIC_IS_SOME_PROOF),
+					left_to_right_metrics
+						.source_to_base_conversion_rate
+						.expect(METRIC_IS_SOME_PROOF),
+					left_to_right_metrics
+						.target_to_base_conversion_rate
+						.expect(METRIC_IS_SOME_PROOF),
+					CONVERSION_RATE_ALLOWED_DIFFERENCE_RATIO,
+					move |new_rate| {
+						log::info!(
+							target: "bridge",
+							"Going to update {} -> {} (on {}) conversion rate to {}.",
+							Left::NAME,
+							Right::NAME,
+							Right::NAME,
+							new_rate,
+						);
+						update_left_to_right_conversion_rate(
+							right_client.clone(),
+							right_messages_pallet_owner.clone(),
+							new_rate,
+						)
+					},
+				);
+			}
 
 			let left_to_right_on_demand_headers = OnDemandHeadersRelay::new(
 				left_client.clone(),

bridges/relays/client-substrate/src/metrics/float_storage_value.rs

@@ -17,9 +17,12 @@
 use crate::chain::Chain;
 use crate::client::Client;
 
+use async_std::sync::{Arc, RwLock};
 use async_trait::async_trait;
 use codec::Decode;
-use relay_utils::metrics::{metric_name, register, Gauge, PrometheusError, Registry, StandaloneMetrics, F64};
+use relay_utils::metrics::{
+	metric_name, register, F64SharedRef, Gauge, PrometheusError, Registry, StandaloneMetrics, F64,
+};
 use sp_core::storage::StorageKey;
 use sp_runtime::{traits::UniqueSaturatedInto, FixedPointNumber};
 use std::time::Duration;
@@ -34,6 +37,7 @@ pub struct FloatStorageValueMetric<C: Chain, T: Clone> {
 	storage_key: StorageKey,
 	maybe_default_value: Option<T>,
 	metric: Gauge<F64>,
+	shared_value_ref: F64SharedRef,
 }
 
 impl<C: Chain, T: Decode + FixedPointNumber> FloatStorageValueMetric<C, T> {
@@ -47,13 +51,20 @@ impl<C: Chain, T: Decode + FixedPointNumber> FloatStorageValueMetric<C, T> {
 		name: String,
 		help: String,
 	) -> Result<Self, PrometheusError> {
+		let shared_value_ref = Arc::new(RwLock::new(None));
 		Ok(FloatStorageValueMetric {
 			client,
 			storage_key,
 			maybe_default_value,
 			metric: register(Gauge::new(metric_name(prefix, &name), help)?, registry)?,
+			shared_value_ref,
 		})
 	}
+
+	/// Get shared reference to metric value.
+	pub fn shared_value_ref(&self) -> F64SharedRef {
+		self.shared_value_ref.clone()
+	}
 }
 
 #[async_trait]
@@ -66,17 +77,17 @@ where
 	}
 
 	async fn update(&self) {
-		relay_utils::metrics::set_gauge_value(
-			&self.metric,
-			self.client
+		let value = self
+			.client
 			.storage_value::<T>(self.storage_key.clone())
 			.await
 			.map(|maybe_storage_value| {
 				maybe_storage_value.or(self.maybe_default_value).map(|storage_value| {
-						storage_value.into_inner().unique_saturated_into() as f64
-							/ T::DIV.unique_saturated_into() as f64
+					storage_value.into_inner().unique_saturated_into() as f64 / T::DIV.unique_saturated_into() as f64
 				})
-				}),
-		);
+			})
+			.map_err(drop);
+		relay_utils::metrics::set_gauge_value(&self.metric, value);
+		*self.shared_value_ref.write().await = value.ok().and_then(|x| x);
 	}
 }

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

@@ -0,0 +1,210 @@
+// 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/>.
+
+//! Tools for updating conversion rate that is stored in the runtime storage.
+
+use relay_utils::metrics::F64SharedRef;
+use std::{future::Future, time::Duration};
+
+/// Duration between updater wakeups.
+const SLEEP_DURATION: Duration = Duration::from_secs(60);
+
+/// Update-conversion-rate transaction status.
+#[derive(Debug, Clone, Copy, PartialEq)]
+enum TransactionStatus {
+	/// We have not submitted any transaction recently.
+	Idle,
+	/// We have recently submitted transaction that should update conversion rate.
+	Submitted(f64),
+}
+
+/// Run infinite conversion rate updater loop.
+///
+/// The loop is maintaining the Left -> Right conversion rate, used as `RightTokens = LeftTokens * Rate`.
+pub fn run_conversion_rate_update_loop<
+	SubmitConversionRateFuture: Future<Output = anyhow::Result<()>> + Send + 'static,
+>(
+	left_to_right_stored_conversion_rate: F64SharedRef,
+	left_to_base_conversion_rate: F64SharedRef,
+	right_to_base_conversion_rate: F64SharedRef,
+	max_difference_ratio: f64,
+	submit_conversion_rate: impl Fn(f64) -> SubmitConversionRateFuture + Send + 'static,
+) {
+	async_std::task::spawn(async move {
+		let mut transaction_status = TransactionStatus::Idle;
+		loop {
+			async_std::task::sleep(SLEEP_DURATION).await;
+			let maybe_new_conversion_rate = maybe_select_new_conversion_rate(
+				&mut transaction_status,
+				&left_to_right_stored_conversion_rate,
+				&left_to_base_conversion_rate,
+				&right_to_base_conversion_rate,
+				max_difference_ratio,
+			)
+			.await;
+			if let Some((prev_conversion_rate, new_conversion_rate)) = maybe_new_conversion_rate {
+				let submit_conversion_rate_future = submit_conversion_rate(new_conversion_rate);
+				match submit_conversion_rate_future.await {
+					Ok(()) => {
+						transaction_status = TransactionStatus::Submitted(prev_conversion_rate);
+					}
+					Err(error) => {
+						log::trace!(target: "bridge", "Failed to submit conversion rate update transaction: {:?}", error);
+					}
+				}
+			}
+		}
+	});
+}
+
+/// Select new conversion rate to submit to the node.
+async fn maybe_select_new_conversion_rate(
+	transaction_status: &mut TransactionStatus,
+	left_to_right_stored_conversion_rate: &F64SharedRef,
+	left_to_base_conversion_rate: &F64SharedRef,
+	right_to_base_conversion_rate: &F64SharedRef,
+	max_difference_ratio: f64,
+) -> Option<(f64, f64)> {
+	let left_to_right_stored_conversion_rate = (*left_to_right_stored_conversion_rate.read().await)?;
+	match *transaction_status {
+		TransactionStatus::Idle => (),
+		TransactionStatus::Submitted(previous_left_to_right_stored_conversion_rate) => {
+			// we can't compare float values from different sources directly, so we only care whether the
+			// stored rate has been changed or not. If it has been changed, then we assume that our proposal
+			// has been accepted.
+			//
+			// float comparison is ok here, because we compare same-origin (stored in runtime storage) values
+			// and if they are different, it means that the value has actually been updated
+			#[allow(clippy::float_cmp)]
+			if previous_left_to_right_stored_conversion_rate == left_to_right_stored_conversion_rate {
+				// the rate has not been changed => we won't submit any transactions until it is accepted,
+				// or the rate is changed by someone else
+				return None;
+			}
+
+			*transaction_status = TransactionStatus::Idle;
+		}
+	}
+
+	let left_to_base_conversion_rate = (*left_to_base_conversion_rate.read().await)?;
+	let right_to_base_conversion_rate = (*right_to_base_conversion_rate.read().await)?;
+	let actual_left_to_right_conversion_rate = right_to_base_conversion_rate / left_to_base_conversion_rate;
+
+	let rate_difference = (actual_left_to_right_conversion_rate - left_to_right_stored_conversion_rate).abs();
+	let rate_difference_ratio = rate_difference / left_to_right_stored_conversion_rate;
+	if rate_difference_ratio < max_difference_ratio {
+		return None;
+	}
+
+	Some((
+		left_to_right_stored_conversion_rate,
+		actual_left_to_right_conversion_rate,
+	))
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use async_std::sync::{Arc, RwLock};
+
+	fn test_maybe_select_new_conversion_rate(
+		mut transaction_status: TransactionStatus,
+		stored_conversion_rate: Option<f64>,
+		left_to_base_conversion_rate: Option<f64>,
+		right_to_base_conversion_rate: Option<f64>,
+		max_difference_ratio: f64,
+	) -> (Option<(f64, f64)>, TransactionStatus) {
+		let stored_conversion_rate = Arc::new(RwLock::new(stored_conversion_rate));
+		let left_to_base_conversion_rate = Arc::new(RwLock::new(left_to_base_conversion_rate));
+		let right_to_base_conversion_rate = Arc::new(RwLock::new(right_to_base_conversion_rate));
+		let result = async_std::task::block_on(maybe_select_new_conversion_rate(
+			&mut transaction_status,
+			&stored_conversion_rate,
+			&left_to_base_conversion_rate,
+			&right_to_base_conversion_rate,
+			max_difference_ratio,
+		));
+		(result, transaction_status)
+	}
+
+	#[test]
+	fn rate_is_not_updated_when_transaction_is_submitted() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(
+				TransactionStatus::Submitted(10.0),
+				Some(10.0),
+				Some(1.0),
+				Some(1.0),
+				0.0
+			),
+			(None, TransactionStatus::Submitted(10.0)),
+		);
+	}
+
+	#[test]
+	fn transaction_state_is_changed_to_idle_when_stored_rate_shanges() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(
+				TransactionStatus::Submitted(1.0),
+				Some(10.0),
+				Some(1.0),
+				Some(1.0),
+				100.0
+			),
+			(None, TransactionStatus::Idle),
+		);
+	}
+
+	#[test]
+	fn transaction_is_not_submitted_when_left_to_base_rate_is_unknown() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(TransactionStatus::Idle, Some(10.0), None, Some(1.0), 0.0),
+			(None, TransactionStatus::Idle),
+		);
+	}
+
+	#[test]
+	fn transaction_is_not_submitted_when_right_to_base_rate_is_unknown() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(TransactionStatus::Idle, Some(10.0), Some(1.0), None, 0.0),
+			(None, TransactionStatus::Idle),
+		);
+	}
+
+	#[test]
+	fn transaction_is_not_submitted_when_stored_rate_is_unknown() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(TransactionStatus::Idle, None, Some(1.0), Some(1.0), 0.0),
+			(None, TransactionStatus::Idle),
+		);
+	}
+
+	#[test]
+	fn transaction_is_not_submitted_when_difference_is_below_threshold() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(TransactionStatus::Idle, Some(1.0), Some(1.0), Some(1.01), 0.02),
+			(None, TransactionStatus::Idle),
+		);
+	}
+
+	#[test]
+	fn transaction_is_submitted_when_difference_is_above_threshold() {
+		assert_eq!(
+			test_maybe_select_new_conversion_rate(TransactionStatus::Idle, Some(1.0), Some(1.0), Some(1.03), 0.02),
+			(Some((1.0, 1.03)), TransactionStatus::Idle),
+		);
+	}
+}

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

@@ -18,6 +18,7 @@
 
 #![warn(missing_docs)]
 
+pub mod conversion_rate_update;
 pub mod finality_pipeline;
 pub mod finality_target;
 pub mod headers_initialize;

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

@@ -20,6 +20,7 @@ use crate::messages_source::SubstrateMessagesProof;
 use crate::messages_target::SubstrateMessagesReceivingProof;
 use crate::on_demand_headers::OnDemandHeadersRelay;
 
+use async_trait::async_trait;
 use bp_messages::{LaneId, MessageNonce};
 use frame_support::weights::Weight;
 use messages_relay::message_lane::{MessageLane, SourceHeaderIdOf, TargetHeaderIdOf};
@@ -58,6 +59,7 @@ pub struct MessagesRelayParams<SC: Chain, SS, TC: Chain, TS> {
 }
 
 /// Message sync pipeline for Substrate <-> Substrate relays.
+#[async_trait]
 pub trait SubstrateMessageLane: 'static + Clone + Send + Sync {
 	/// Underlying generic message lane.
 	type MessageLane: MessageLane;
@@ -211,6 +213,8 @@ pub struct StandaloneMessagesMetrics {
 	pub target_to_base_conversion_rate: Option<F64SharedRef>,
 	/// Shared reference to the actual source -> <base> chain token conversion rate.
 	pub source_to_base_conversion_rate: Option<F64SharedRef>,
+	/// Shared reference to the stored (in the source chain runtime storage) target -> source chain conversion rate.
+	pub target_to_source_conversion_rate: Option<F64SharedRef>,
 }
 
 impl StandaloneMessagesMetrics {
@@ -218,7 +222,7 @@ impl StandaloneMessagesMetrics {
 	pub async fn target_to_source_conversion_rate(&self) -> Option<f64> {
 		let target_to_base_conversion_rate = (*self.target_to_base_conversion_rate.as_ref()?.read().await)?;
 		let source_to_base_conversion_rate = (*self.source_to_base_conversion_rate.as_ref()?.read().await)?;
-		Some(target_to_base_conversion_rate / source_to_base_conversion_rate)
+		Some(source_to_base_conversion_rate / target_to_base_conversion_rate)
 	}
 }
 
@@ -231,6 +235,7 @@ pub fn add_standalone_metrics<P: SubstrateMessageLane>(
 	target_chain_token_id: Option<&str>,
 	target_to_source_conversion_rate_params: Option<(StorageKey, FixedU128)>,
 ) -> anyhow::Result<(MetricsParams, StandaloneMessagesMetrics)> {
+	let mut target_to_source_conversion_rate = None;
 	let mut source_to_base_conversion_rate = None;
 	let mut target_to_base_conversion_rate = None;
 	let mut metrics_params =
@@ -266,6 +271,7 @@ pub fn add_standalone_metrics<P: SubstrateMessageLane>(
 					P::SourceChain::NAME
 				),
 			)?;
+			target_to_source_conversion_rate = Some(metric.shared_value_ref());
 			Ok(metric)
 		})?;
 	}
@@ -288,6 +294,7 @@ pub fn add_standalone_metrics<P: SubstrateMessageLane>(
 		StandaloneMessagesMetrics {
 			target_to_base_conversion_rate,
 			source_to_base_conversion_rate,
+			target_to_source_conversion_rate,
 		},
 	))
 }
@@ -295,6 +302,7 @@ pub fn add_standalone_metrics<P: SubstrateMessageLane>(
 #[cfg(test)]
 mod tests {
 	use super::*;
+	use async_std::sync::{Arc, RwLock};
 
 	type RialtoToMillauMessagesWeights = pallet_bridge_messages::weights::RialtoWeight<rialto_runtime::Runtime>;
 
@@ -314,4 +322,15 @@ mod tests {
 			(782, 216_583_333_334),
 		);
 	}
+
+	#[async_std::test]
+	async fn target_to_source_conversion_rate_works() {
+		let metrics = StandaloneMessagesMetrics {
+			target_to_base_conversion_rate: Some(Arc::new(RwLock::new(Some(183.15)))),
+			source_to_base_conversion_rate: Some(Arc::new(RwLock::new(Some(12.32)))),
+			target_to_source_conversion_rate: None, // we don't care
+		};
+
+		assert_eq!(metrics.target_to_source_conversion_rate().await, Some(12.32 / 183.15),);
+	}
 }