Emit mortal transactions from relay (#1073)

* mortal relay transactions

* sign block hash for mortal transactions

* TransactionEraOf

* add explanation

* enable mortal transactions for Westend -> Millau headers relay

* clippy

* spellcheck

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

@@ -25,8 +25,14 @@ use relay_utils::{metrics::MetricsParams, BlockNumberBase};
 use sp_core::Bytes;
 use std::{fmt::Debug, marker::PhantomData, time::Duration};
 
-/// Default synchronization loop timeout.
-pub(crate) const STALL_TIMEOUT: Duration = Duration::from_secs(120);
+/// Default synchronization loop timeout. If transactions generated by relay are immortal, then
+/// this timeout is used.
+///
+/// There are no any strict requirements on block time in Substrate. But we assume here that all
+/// Substrate-based chains will be designed to produce relatively fast (compared to slowest
+/// blockchains) blocks. So 1 hour seems to be a good guess for (even congested) chains to mine
+/// transaction, or remove it from the pool.
+pub(crate) const STALL_TIMEOUT: Duration = Duration::from_secs(60 * 60);
 /// Default limit of recent finality proofs.
 ///
 /// Finality delay of 4096 blocks is unlikely to happen in practice in
@@ -62,6 +68,7 @@ pub trait SubstrateFinalitySyncPipeline: 'static + Clone + Debug + Send + Sync {
 	/// Make submit header transaction.
 	fn make_submit_finality_proof_transaction(
 		&self,
+		era: bp_runtime::TransactionEraOf<Self::TargetChain>,
 		transaction_nonce: <Self::TargetChain as Chain>::Index,
 		header: <Self::FinalitySyncPipeline as FinalitySyncPipeline>::Header,
 		proof: <Self::FinalitySyncPipeline as FinalitySyncPipeline>::FinalityProof,
@@ -123,6 +130,7 @@ pub async fn run<SourceChain, TargetChain, P>(
 	source_client: Client<SourceChain>,
 	target_client: Client<TargetChain>,
 	only_mandatory_headers: bool,
+	transactions_mortality: Option<u32>,
 	metrics_params: MetricsParams,
 ) -> anyhow::Result<()>
 where
@@ -146,11 +154,15 @@ where
 
 	finality_relay::run(
 		FinalitySource::new(source_client, None),
-		SubstrateFinalityTarget::new(target_client, pipeline),
+		SubstrateFinalityTarget::new(target_client, pipeline, transactions_mortality),
 		FinalitySyncParams {
 			tick: std::cmp::max(SourceChain::AVERAGE_BLOCK_INTERVAL, TargetChain::AVERAGE_BLOCK_INTERVAL),
 			recent_finality_proofs_limit: RECENT_FINALITY_PROOFS_LIMIT,
-			stall_timeout: STALL_TIMEOUT,
+			stall_timeout: relay_substrate_client::transaction_stall_timeout(
+				transactions_mortality,
+				TargetChain::AVERAGE_BLOCK_INTERVAL,
+			)
+			.unwrap_or(STALL_TIMEOUT),
 			only_mandatory_headers,
 		},
 		metrics_params,