Select header that will be fully refunded in on-demand batch finality relay (#2729)

* select header that will be fully refunded for submission in on-demand **batch** finality relay

* added the only possible test

* spelling

* nl

* updated comment

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

@@ -376,9 +376,8 @@ impl ChainWithGrandpa for BridgedUnderlyingChain {
 	const WITH_CHAIN_GRANDPA_PALLET_NAME: &'static str = "";
 	const MAX_AUTHORITIES_COUNT: u32 = 16;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 8;
-	const MAX_HEADER_SIZE: u32 = 256;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = 256;
+	const AVERAGE_HEADER_SIZE: u32 = 64;
 }
 
 impl Chain for BridgedUnderlyingParachain {

bridges/modules/grandpa/src/call_ext.rs

@@ -15,7 +15,10 @@
 // along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
 
 use crate::{weights::WeightInfo, BridgedBlockNumber, BridgedHeader, Config, Error, Pallet};
-use bp_header_chain::{justification::GrandpaJustification, ChainWithGrandpa};
+use bp_header_chain::{
+	justification::GrandpaJustification, max_expected_submit_finality_proof_arguments_size,
+	ChainWithGrandpa, GrandpaConsensusLogReader,
+};
 use bp_runtime::{BlockNumberOf, OwnedBridgeModule};
 use codec::Encode;
 use frame_support::{dispatch::CallableCallFor, traits::IsSubType, weights::Weight};
@@ -169,28 +172,28 @@ pub(crate) fn submit_finality_proof_info_from_args<T: Config<I>, I: 'static>(
 			Weight::zero()
 		};
 
+	// check if the `finality_target` is a mandatory header. If so, we are ready to refund larger
+	// size
+	let is_mandatory_finality_target =
+		GrandpaConsensusLogReader::<BridgedBlockNumber<T, I>>::find_scheduled_change(
+			finality_target.digest(),
+		)
+		.is_some();
+
 	// we can estimate extra call size easily, without any additional significant overhead
 	let actual_call_size: u32 = finality_target
 		.encoded_size()
 		.saturating_add(justification.encoded_size())
 		.saturated_into();
-	let max_expected_call_size = max_expected_call_size::<T, I>(required_precommits);
+	let max_expected_call_size = max_expected_submit_finality_proof_arguments_size::<T::BridgedChain>(
+		is_mandatory_finality_target,
+		required_precommits,
+	);
 	let extra_size = actual_call_size.saturating_sub(max_expected_call_size);
 
 	SubmitFinalityProofInfo { block_number, extra_weight, extra_size }
 }
 
-/// Returns maximal expected size of `submit_finality_proof` call arguments.
-fn max_expected_call_size<T: Config<I>, I: 'static>(required_precommits: u32) -> u32 {
-	let max_expected_justification_size =
-		GrandpaJustification::<BridgedHeader<T, I>>::max_reasonable_size::<T::BridgedChain>(
-			required_precommits,
-		);
-
-	// call arguments are header and justification
-	T::BridgedChain::MAX_HEADER_SIZE.saturating_add(max_expected_justification_size)
-}
-
 #[cfg(test)]
 mod tests {
 	use crate::{

bridges/modules/grandpa/src/mock.rs

@@ -86,9 +86,8 @@ impl ChainWithGrandpa for TestBridgedChain {
 	const WITH_CHAIN_GRANDPA_PALLET_NAME: &'static str = "";
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_BRIDGED_AUTHORITIES;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 8;
-	const MAX_HEADER_SIZE: u32 = 256;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = 256;
+	const AVERAGE_HEADER_SIZE: u32 = 64;
 }
 
 /// Return test externalities to use in tests.

bridges/modules/parachains/src/mock.rs

@@ -252,9 +252,8 @@ impl ChainWithGrandpa for TestBridgedChain {
 	const WITH_CHAIN_GRANDPA_PALLET_NAME: &'static str = "";
 	const MAX_AUTHORITIES_COUNT: u32 = 16;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 8;
-	const MAX_HEADER_SIZE: u32 = 256;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = 256;
+	const AVERAGE_HEADER_SIZE: u32 = 64;
 }
 
 #[derive(Debug)]
@@ -284,9 +283,8 @@ impl ChainWithGrandpa for OtherBridgedChain {
 	const WITH_CHAIN_GRANDPA_PALLET_NAME: &'static str = "";
 	const MAX_AUTHORITIES_COUNT: u32 = 16;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 8;
-	const MAX_HEADER_SIZE: u32 = 256;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = 64;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = 256;
+	const AVERAGE_HEADER_SIZE: u32 = 64;
 }
 
 /// Return test externalities to use in tests.

bridges/primitives/chain-kusama/src/lib.rs

@@ -52,9 +52,8 @@ impl ChainWithGrandpa for Kusama {
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = AVERAGE_HEADER_SIZE;
 }
 
 // The SignedExtension used by Kusama.

bridges/primitives/chain-polkadot-bulletin/src/lib.rs

@@ -42,9 +42,8 @@ use sp_runtime::{traits::DispatchInfoOf, transaction_validity::TransactionValidi
 // This chain reuses most of Polkadot primitives.
 pub use bp_polkadot_core::{
 	AccountAddress, AccountId, Balance, Block, BlockNumber, Hash, Hasher, Header, Nonce, Signature,
-	SignedBlock, UncheckedExtrinsic, AVERAGE_HEADER_SIZE_IN_JUSTIFICATION,
-	EXTRA_STORAGE_PROOF_SIZE, MAX_HEADER_SIZE, REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY,
-	WORST_HEADER_SIZE_IN_JUSTIFICATION,
+	SignedBlock, UncheckedExtrinsic, AVERAGE_HEADER_SIZE, EXTRA_STORAGE_PROOF_SIZE,
+	MAX_MANDATORY_HEADER_SIZE, REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY,
 };
 
 /// Maximal number of GRANDPA authorities at Polkadot Bulletin chain.
@@ -208,9 +207,8 @@ impl ChainWithGrandpa for PolkadotBulletin {
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = AVERAGE_HEADER_SIZE;
 }
 
 decl_bridge_finality_runtime_apis!(polkadot_bulletin, grandpa);

bridges/primitives/chain-polkadot/src/lib.rs

@@ -52,9 +52,8 @@ impl ChainWithGrandpa for Polkadot {
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = AVERAGE_HEADER_SIZE;
 }
 
 /// The SignedExtension used by Polkadot.

bridges/primitives/chain-rococo/src/lib.rs

@@ -52,9 +52,8 @@ impl ChainWithGrandpa for Rococo {
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = AVERAGE_HEADER_SIZE;
 }
 
 parameter_types! {

bridges/primitives/chain-westend/src/lib.rs

@@ -52,9 +52,8 @@ impl ChainWithGrandpa for Westend {
 	const MAX_AUTHORITIES_COUNT: u32 = MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 = MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = AVERAGE_HEADER_SIZE;
 }
 
 parameter_types! {

bridges/primitives/header-chain/src/justification/mod.rs

@@ -82,8 +82,8 @@ impl<H: HeaderT> GrandpaJustification<H> {
 			.saturating_add(BlockNumberOf::<C>::max_encoded_len().saturated_into())
 			.saturating_add(HashOf::<C>::max_encoded_len().saturated_into());
 
-		let max_expected_votes_ancestries_size = C::REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY
-			.saturating_mul(C::AVERAGE_HEADER_SIZE_IN_JUSTIFICATION);
+		let max_expected_votes_ancestries_size =
+			C::REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY.saturating_mul(C::AVERAGE_HEADER_SIZE);
 
 		// justification is round number (u64=8b), a signed GRANDPA commit and the
 		// `votes_ancestries` vector

bridges/primitives/header-chain/src/lib.rs

@@ -266,36 +266,28 @@ pub trait ChainWithGrandpa: Chain {
 	/// to submitter.
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32;
 
-	/// Maximal size of the chain header. The header may be the header that enacts new GRANDPA
-	/// authorities set (so it has large digest inside).
+	/// Maximal size of the mandatory chain header. Mandatory header is the header that enacts new
+	/// GRANDPA authorities set (so it has large digest inside).
 	///
 	/// This isn't a strict limit. The relay may submit larger headers and the pallet will accept
 	/// the call. The limit is only used to compute maximal refund amount and doing calls which
 	/// exceed the limit, may be costly to submitter.
-	const MAX_HEADER_SIZE: u32;
+	const MAX_MANDATORY_HEADER_SIZE: u32;
 
-	/// Average size of the chain header from justification ancestry. We don't expect to see there
-	/// headers that change GRANDPA authorities set (GRANDPA will probably be able to finalize at
-	/// least one additional header per session on non test chains), so this is average size of
-	/// headers that aren't changing the set.
+	/// Average size of the chain header. We don't expect to see there headers that change GRANDPA
+	/// authorities set (GRANDPA will probably be able to finalize at least one additional header
+	/// per session on non test chains), so this is average size of headers that aren't changing the
+	/// set.
 	///
-	/// This isn't a strict limit. The relay may submit justifications with larger headers in its
-	/// ancestry and the pallet will accept the call. The limit is only used to compute fee, paid
-	/// by the user at the sending chain. It covers most of cases, but if the actual header,
-	/// submitted with the messages transaction will be larger than the
-	/// `AVERAGE_HEADER_SIZE_IN_JUSTIFICATION`, the difference (`WORST_HEADER_SIZE_IN_JUSTIFICATION`
-	/// - `AVERAGE_HEADER_SIZE_IN_JUSTIFICATION`) must be covered by the sending chain.
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32;
-
-	/// Worst-case size of the chain header from justification ancestry. We don't expect to see
-	/// there headers that change GRANDPA authorities set (GRANDPA will probably be able to finalize
-	/// at least one additional header per session on non test chains), so this is the worst-case
-	/// size of headers that aren't changing the set.
+	/// This isn't a strict limit. The relay may submit justifications with larger headers and the
+	/// pallet will accept the call. However, if the total size of all `submit_finality_proof`
+	/// arguments exceeds the maximal size, computed using this average size, relayer will only get
+	/// partial refund.
 	///
-	/// This isn't a strict limit. The relay may submit justifications with larger headers in its
-	/// ancestry and the pallet will accept the call. The limit is only used to compute maximal
-	/// refund amount and doing calls which exceed the limit, may be costly to submitter.
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32;
+	/// We expect some headers on production chains that are above this size. But they are rare and
+	/// if rellayer cares about its profitability, we expect it'll select other headers for
+	/// submission.
+	const AVERAGE_HEADER_SIZE: u32;
 }
 
 impl<T> ChainWithGrandpa for T
@@ -308,9 +300,67 @@ where
 	const MAX_AUTHORITIES_COUNT: u32 = <T::Chain as ChainWithGrandpa>::MAX_AUTHORITIES_COUNT;
 	const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 =
 		<T::Chain as ChainWithGrandpa>::REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY;
-	const MAX_HEADER_SIZE: u32 = <T::Chain as ChainWithGrandpa>::MAX_HEADER_SIZE;
-	const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 =
-		<T::Chain as ChainWithGrandpa>::AVERAGE_HEADER_SIZE_IN_JUSTIFICATION;
-	const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 =
-		<T::Chain as ChainWithGrandpa>::WORST_HEADER_SIZE_IN_JUSTIFICATION;
+	const MAX_MANDATORY_HEADER_SIZE: u32 =
+		<T::Chain as ChainWithGrandpa>::MAX_MANDATORY_HEADER_SIZE;
+	const AVERAGE_HEADER_SIZE: u32 = <T::Chain as ChainWithGrandpa>::AVERAGE_HEADER_SIZE;
+}
+
+/// Returns maximal expected size of `submit_finality_proof` call arguments.
+pub fn max_expected_submit_finality_proof_arguments_size<C: ChainWithGrandpa>(
+	is_mandatory_finality_target: bool,
+	precommits: u32,
+) -> u32 {
+	let max_expected_justification_size =
+		GrandpaJustification::<HeaderOf<C>>::max_reasonable_size::<C>(precommits);
+
+	// call arguments are header and justification
+	let max_expected_finality_target_size = if is_mandatory_finality_target {
+		C::MAX_MANDATORY_HEADER_SIZE
+	} else {
+		C::AVERAGE_HEADER_SIZE
+	};
+	max_expected_finality_target_size.saturating_add(max_expected_justification_size)
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use frame_support::weights::Weight;
+	use sp_runtime::{testing::H256, traits::BlakeTwo256, MultiSignature};
+
+	struct TestChain;
+
+	impl Chain for TestChain {
+		type BlockNumber = u32;
+		type Hash = H256;
+		type Hasher = BlakeTwo256;
+		type Header = sp_runtime::generic::Header<u32, BlakeTwo256>;
+		type AccountId = u64;
+		type Balance = u64;
+		type Nonce = u64;
+		type Signature = MultiSignature;
+
+		fn max_extrinsic_size() -> u32 {
+			0
+		}
+		fn max_extrinsic_weight() -> Weight {
+			Weight::zero()
+		}
+	}
+
+	impl ChainWithGrandpa for TestChain {
+		const WITH_CHAIN_GRANDPA_PALLET_NAME: &'static str = "Test";
+		const MAX_AUTHORITIES_COUNT: u32 = 128;
+		const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 2;
+		const MAX_MANDATORY_HEADER_SIZE: u32 = 100_000;
+		const AVERAGE_HEADER_SIZE: u32 = 1_024;
+	}
+
+	#[test]
+	fn max_expected_submit_finality_proof_arguments_size_respects_mandatory_argument() {
+		assert!(
+			max_expected_submit_finality_proof_arguments_size::<TestChain>(true, 100) >
+				max_expected_submit_finality_proof_arguments_size::<TestChain>(false, 100),
+		);
+	}
 }

bridges/primitives/polkadot-core/src/lib.rs

@@ -64,7 +64,7 @@ pub const MAX_AUTHORITIES_COUNT: u32 = 1_256;
 ///
 /// See [`bp-header-chain::ChainWithGrandpa`] for more details.
 ///
-/// This value comes from recent (February, 2023) Kusama and Polkadot headers. There are no
+/// This value comes from recent (December, 2023) Kusama and Polkadot headers. There are no
 /// justifications with any additional headers in votes ancestry, so reasonable headers may
 /// be set to zero. But we assume that there may be small GRANDPA lags, so we're leaving some
 /// reserve here.
@@ -75,28 +75,17 @@ pub const REASONABLE_HEADERS_IN_JUSTIFICATON_ANCESTRY: u32 = 2;
 ///
 /// See [`bp-header-chain::ChainWithGrandpa`] for more details.
 ///
-/// This value comes from recent (February, 2023) Kusama headers. Average is `336` there, but let's
-/// have some reserve and make it 1024.
-pub const AVERAGE_HEADER_SIZE_IN_JUSTIFICATION: u32 = 1024;
-
-/// Worst-case header size in `votes_ancestries` field of justification on Polkadot-like
-/// chains.
-///
-/// See [`bp-header-chain::ChainWithGrandpa`] for more details.
-///
-/// This value comes from recent (February, 2023) Kusama headers. Average is `336` there, but some
-/// non-mandatory headers has size `40kb` (they contain the BABE epoch descriptor with all
-/// authorities - just like our mandatory header). Since we assume `2` headers in justification
-/// votes ancestry, let's set average header to `40kb / 2`.
-pub const WORST_HEADER_SIZE_IN_JUSTIFICATION: u32 = 20 * 1024;
+/// This value comes from recent (December, 2023) Kusama headers. Most of headers are `327` bytes
+/// there, but let's have some reserve and make it 1024.
+pub const AVERAGE_HEADER_SIZE: u32 = 1024;
 
 /// Approximate maximal header size on Polkadot-like chains.
 ///
 /// See [`bp-header-chain::ChainWithGrandpa`] for more details.
 ///
-/// This value comes from recent (February, 2023) Kusama headers. Maximal header is a mandatory
-/// header. In its SCALE-encoded form it is `80348` bytes. Let's have some reserve here.
-pub const MAX_HEADER_SIZE: u32 = 90_000;
+/// This value comes from recent (December, 2023) Kusama headers. Maximal header is a mandatory
+/// header. In its SCALE-encoded form it is `113407` bytes. Let's have some reserve here.
+pub const MAX_MANDATORY_HEADER_SIZE: u32 = 120 * 1024;
 
 /// Number of extra bytes (excluding size of storage value itself) of storage proof, built at
 /// Polkadot-like chain. This mostly depends on number of entries in the storage trie.

bridges/relays/bin-substrate/Cargo.toml

@@ -10,6 +10,7 @@ anyhow = "1.0"
 async-std = "1.9.0"
 async-trait = "0.1"
 codec = { package = "parity-scale-codec", version = "3.1.5" }
+env_logger = "0.10"
 futures = "0.3.29"
 hex = "0.4"
 log = "0.4.20"

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

@@ -23,8 +23,9 @@ use bp_header_chain::{
 		verify_and_optimize_justification, GrandpaEquivocationsFinder, GrandpaJustification,
 		JustificationVerificationContext,
 	},
-	AuthoritySet, ConsensusLogReader, FinalityProof, FindEquivocations, GrandpaConsensusLogReader,
-	HeaderFinalityInfo, HeaderGrandpaInfo, StoredHeaderGrandpaInfo,
+	max_expected_submit_finality_proof_arguments_size, AuthoritySet, ConsensusLogReader,
+	FinalityProof, FindEquivocations, GrandpaConsensusLogReader, HeaderFinalityInfo,
+	HeaderGrandpaInfo, StoredHeaderGrandpaInfo,
 };
 use bp_runtime::{BasicOperatingMode, HeaderIdProvider, OperatingMode};
 use codec::{Decode, Encode};
@@ -35,9 +36,22 @@ use relay_substrate_client::{
 };
 use sp_consensus_grandpa::{AuthorityList as GrandpaAuthoritiesSet, GRANDPA_ENGINE_ID};
 use sp_core::{storage::StorageKey, Bytes};
-use sp_runtime::{scale_info::TypeInfo, traits::Header, ConsensusEngineId};
+use sp_runtime::{scale_info::TypeInfo, traits::Header, ConsensusEngineId, SaturatedConversion};
 use std::{fmt::Debug, marker::PhantomData};
 
+/// Result of checking maximal expected call size.
+pub enum MaxExpectedCallSizeCheck {
+	/// Size is ok and call will be refunded.
+	Ok,
+	/// The call size exceeds the maximal expected and relayer will only get partial refund.
+	Exceeds {
+		/// Actual call size.
+		call_size: u32,
+		/// Maximal expected call size.
+		max_call_size: u32,
+	},
+}
+
 /// Finality engine, used by the Substrate chain.
 #[async_trait]
 pub trait Engine<C: Chain>: Send {
@@ -111,6 +125,14 @@ pub trait Engine<C: Chain>: Send {
 		proof: &mut Self::FinalityProof,
 	) -> Result<(), SubstrateError>;
 
+	/// Checks whether the given `header` and its finality `proof` fit the maximal expected
+	/// call size limit. If result is `MaxExpectedCallSizeCheck::Exceeds { .. }`, this
+	/// submission won't be fully refunded and relayer will spend its own funds on that.
+	fn check_max_expected_call_size(
+		header: &C::Header,
+		proof: &Self::FinalityProof,
+	) -> MaxExpectedCallSizeCheck;
+
 	/// Prepare initialization data for the finality bridge pallet.
 	async fn prepare_initialization_data(
 		client: Client<C>,
@@ -219,6 +241,24 @@ impl<C: ChainWithGrandpa> Engine<C> for Grandpa<C> {
 		})
 	}
 
+	fn check_max_expected_call_size(
+		header: &C::Header,
+		proof: &Self::FinalityProof,
+	) -> MaxExpectedCallSizeCheck {
+		let is_mandatory = Self::ConsensusLogReader::schedules_authorities_change(header.digest());
+		let call_size: u32 =
+			header.encoded_size().saturating_add(proof.encoded_size()).saturated_into();
+		let max_call_size = max_expected_submit_finality_proof_arguments_size::<C>(
+			is_mandatory,
+			proof.commit.precommits.len().saturated_into(),
+		);
+		if call_size > max_call_size {
+			MaxExpectedCallSizeCheck::Exceeds { call_size, max_call_size }
+		} else {
+			MaxExpectedCallSizeCheck::Ok
+		}
+	}
+
 	/// Prepare initialization data for the GRANDPA verifier pallet.
 	async fn prepare_initialization_data(
 		source_client: Client<C>,

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

@@ -16,14 +16,16 @@
 
 //! On-demand Substrate -> Substrate header finality relay.
 
-use crate::finality::SubmitFinalityProofCallBuilder;
+use crate::{
+	finality::SubmitFinalityProofCallBuilder, finality_base::engine::MaxExpectedCallSizeCheck,
+};
 
 use async_std::sync::{Arc, Mutex};
 use async_trait::async_trait;
 use bp_header_chain::ConsensusLogReader;
 use bp_runtime::HeaderIdProvider;
 use futures::{select, FutureExt};
-use num_traits::{One, Zero};
+use num_traits::{One, Saturating, Zero};
 use sp_runtime::traits::Header;
 
 use finality_relay::{FinalitySyncParams, TargetClient as FinalityTargetClient};
@@ -133,29 +135,61 @@ impl<P: SubstrateFinalitySyncPipeline> OnDemandRelay<P::SourceChain, P::TargetCh
 		&self,
 		required_header: BlockNumberOf<P::SourceChain>,
 	) -> Result<(HeaderIdOf<P::SourceChain>, Vec<CallOf<P::TargetChain>>), SubstrateError> {
-		// first find proper header (either `required_header`) or its descendant
-		let finality_source = SubstrateFinalitySource::<P>::new(self.source_client.clone(), None);
-		let (header, mut proof) = finality_source.prove_block_finality(required_header).await?;
-		let header_id = header.id();
+		const MAX_ITERATIONS: u32 = 4;
+		let mut iterations = 0;
+		let mut current_required_header = required_header;
+		loop {
+			// first find proper header (either `current_required_header`) or its descendant
+			let finality_source =
+				SubstrateFinalitySource::<P>::new(self.source_client.clone(), None);
+			let (header, mut proof) =
+				finality_source.prove_block_finality(current_required_header).await?;
+			let header_id = header.id();
+
+			// optimize justification before including it into the call
+			P::FinalityEngine::optimize_proof(&self.target_client, &header, &mut proof).await?;
+
+			// now we have the header and its proof, but we want to minimize our losses, so let's
+			// check if we'll get the full refund for submitting this header
+			let check_result = P::FinalityEngine::check_max_expected_call_size(&header, &proof);
+			if let MaxExpectedCallSizeCheck::Exceeds { call_size, max_call_size } = check_result {
+				iterations += 1;
+				current_required_header = header_id.number().saturating_add(One::one());
+				if iterations < MAX_ITERATIONS {
+					log::debug!(
+						target: "bridge",
+						"[{}] Requested to prove {} head {:?}. Selected to prove {} head {:?}. But it is too large: {} vs {}. \
+						Going to select next header",
+						self.relay_task_name,
+						P::SourceChain::NAME,
+						required_header,
+						P::SourceChain::NAME,
+						header_id,
+						call_size,
+						max_call_size,
+					);
 
-		// optimize justification before including it into the call
-		P::FinalityEngine::optimize_proof(&self.target_client, &header, &mut proof).await?;
+					continue;
+				}
+			}
 
-		log::debug!(
-			target: "bridge",
-			"[{}] Requested to prove {} head {:?}. Selected to prove {} head {:?}",
-			self.relay_task_name,
-			P::SourceChain::NAME,
-			required_header,
-			P::SourceChain::NAME,
-			header_id,
-		);
+			log::debug!(
+				target: "bridge",
+				"[{}] Requested to prove {} head {:?}. Selected to prove {} head {:?} (after {} iterations)",
+				self.relay_task_name,
+				P::SourceChain::NAME,
+				required_header,
+				P::SourceChain::NAME,
+				header_id,
+				iterations,
+			);
 
-		// and then craft the submit-proof call
-		let call =
-			P::SubmitFinalityProofCallBuilder::build_submit_finality_proof_call(header, proof);
+			// and then craft the submit-proof call
+			let call =
+				P::SubmitFinalityProofCallBuilder::build_submit_finality_proof_call(header, proof);
 
-		Ok((header_id, vec![call]))
+			return Ok((header_id, vec![call]));
+		}
 	}
 }