Bridge: make some headers submissions free (#4102)

supersedes https://github.com/paritytech/parity-bridges-common/pull/2873 Draft because of couple of TODOs: - [x] fix remaining TODOs; - [x] double check that all changes from https://github.com/paritytech/parity-bridges-common/pull/2873 are correctly ported; - [x] create a separate PR (on top of that one or a follow up?) for https://github.com/paritytech/polkadot-sdk/tree/sv-try-new-bridge-fees; - [x] fix compilation issues (haven't checked, but there should be many). --------- Co-authored-by: Adrian Catangiu <adrian@parity.io>
2026-07-17 05:25:42 +00:00 · 2024-04-25 08:26:16 +03:00
parent 4f3d43a0c4
commit a633e954f3
54 changed files with 3731 additions and 616 deletions
@@ -15,20 +15,24 @@
 // along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.

 use crate::{
-	weights::WeightInfo, BridgedBlockNumber, BridgedHeader, Config, CurrentAuthoritySet, Error,
-	Pallet,
+	weights::WeightInfo, BestFinalized, BridgedBlockNumber, BridgedHeader, Config,
+	CurrentAuthoritySet, Error, FreeHeadersRemaining, Pallet,
 };
 use bp_header_chain::{
 	justification::GrandpaJustification, max_expected_submit_finality_proof_arguments_size,
 	ChainWithGrandpa, GrandpaConsensusLogReader,
 };
-use bp_runtime::{BlockNumberOf, OwnedBridgeModule};
+use bp_runtime::{BlockNumberOf, Chain, OwnedBridgeModule};
 use codec::Encode;
-use frame_support::{dispatch::CallableCallFor, traits::IsSubType, weights::Weight};
+use frame_support::{
+	dispatch::CallableCallFor,
+	traits::{Get, IsSubType},
+	weights::Weight,
+};
 use sp_consensus_grandpa::SetId;
 use sp_runtime::{
-	traits::{Header, Zero},
-	transaction_validity::{InvalidTransaction, TransactionValidity, ValidTransaction},
+	traits::{CheckedSub, Header, Zero},
+	transaction_validity::{InvalidTransaction, TransactionValidityError},
 	RuntimeDebug, SaturatedConversion,
 };

@@ -40,6 +44,11 @@ pub struct SubmitFinalityProofInfo<N> {
 	/// An identifier of the validators set that has signed the submitted justification.
 	/// It might be `None` if deprecated version of the `submit_finality_proof` is used.
 	pub current_set_id: Option<SetId>,
+	/// If `true`, then the call proves new **mandatory** header.
+	pub is_mandatory: bool,
+	/// If `true`, then the call must be free (assuming that everything else is valid) to
+	/// be treated as valid.
+	pub is_free_execution_expected: bool,
 	/// Extra weight that we assume is included in the call.
 	///
 	/// We have some assumptions about headers and justifications of the bridged chain.
@@ -54,6 +63,16 @@ pub struct SubmitFinalityProofInfo<N> {
 	pub extra_size: u32,
 }

+/// Verified `SubmitFinalityProofInfo<N>`.
+#[derive(Copy, Clone, PartialEq, RuntimeDebug)]
+pub struct VerifiedSubmitFinalityProofInfo<N> {
+	/// Base call information.
+	pub base: SubmitFinalityProofInfo<N>,
+	/// A difference between bundled bridged header and best bridged header known to us
+	/// before the call.
+	pub improved_by: N,
+}
+
 impl<N> SubmitFinalityProofInfo<N> {
 	/// Returns `true` if call size/weight is below our estimations for regular calls.
 	pub fn fits_limits(&self) -> bool {
@@ -67,14 +86,86 @@ pub struct SubmitFinalityProofHelper<T: Config<I>, I: 'static> {
 }

 impl<T: Config<I>, I: 'static> SubmitFinalityProofHelper<T, I> {
+	/// Returns `true` if we may fit more free headers into the current block. If `false` is
+	/// returned, the call will be paid even if `is_free_execution_expected` has been set
+	/// to `true`.
+	pub fn has_free_header_slots() -> bool {
+		// `unwrap_or(u32::MAX)` means that if `FreeHeadersRemaining` is `None`, we may accept
+		// this header for free. That is a small cheat - it is `None` if executed outside of
+		// transaction (e.g. during block initialization). Normal relayer would never submit
+		// such calls, but if he did, that is not our problem. During normal transactions,
+		// the `FreeHeadersRemaining` is always `Some(_)`.
+		let free_headers_remaining = FreeHeadersRemaining::<T, I>::get().unwrap_or(u32::MAX);
+		free_headers_remaining > 0
+	}
+
+	/// Check that the: (1) GRANDPA head provided by the `SubmitFinalityProof` is better than the
+	/// best one we know (2) if `current_set_id` matches the current authority set id, if specified
+	/// and (3) whether transaction MAY be free for the submitter if `is_free_execution_expected`
+	/// is `true`.
+	///
+	/// Returns number of headers between the current best finalized header, known to the pallet
+	/// and the bundled header.
+	pub fn check_obsolete_from_extension(
+		call_info: &SubmitFinalityProofInfo<BlockNumberOf<T::BridgedChain>>,
+	) -> Result<BlockNumberOf<T::BridgedChain>, Error<T, I>> {
+		// do basic checks first
+		let improved_by = Self::check_obsolete(call_info.block_number, call_info.current_set_id)?;
+
+		// if submitter has NOT specified that it wants free execution, then we are done
+		if !call_info.is_free_execution_expected {
+			return Ok(improved_by);
+		}
+
+		// else - if we can not accept more free headers, "reject" the transaction
+		if !Self::has_free_header_slots() {
+			log::trace!(
+				target: crate::LOG_TARGET,
+				"Cannot accept free {:?} header {:?}. No more free slots remaining",
+				T::BridgedChain::ID,
+				call_info.block_number,
+			);
+
+			return Err(Error::<T, I>::FreeHeadersLimitExceded);
+		}
+
+		// ensure that the `improved_by` is larger than the configured free interval
+		if !call_info.is_mandatory {
+			if let Some(free_headers_interval) = T::FreeHeadersInterval::get() {
+				if improved_by < free_headers_interval.into() {
+					log::trace!(
+						target: crate::LOG_TARGET,
+						"Cannot accept free {:?} header {:?}. Too small difference \
+						between submitted headers: {:?} vs {}",
+						T::BridgedChain::ID,
+						call_info.block_number,
+						improved_by,
+						free_headers_interval,
+					);
+
+					return Err(Error::<T, I>::BelowFreeHeaderInterval);
+				}
+			}
+		}
+
+		// we do not check whether the header matches free submission criteria here - it is the
+		// relayer responsibility to check that
+
+		Ok(improved_by)
+	}
+
 	/// Check that the GRANDPA head provided by the `SubmitFinalityProof` is better than the best
 	/// one we know. Additionally, checks if `current_set_id` matches the current authority set
-	/// id, if specified.
+	/// id, if specified. This method is called by the call code and the transaction extension,
+	/// so it does not check the free execution.
+	///
+	/// Returns number of headers between the current best finalized header, known to the pallet
+	/// and the bundled header.
 	pub fn check_obsolete(
 		finality_target: BlockNumberOf<T::BridgedChain>,
 		current_set_id: Option<SetId>,
-	) -> Result<(), Error<T, I>> {
-		let best_finalized = crate::BestFinalized::<T, I>::get().ok_or_else(|| {
+	) -> Result<BlockNumberOf<T::BridgedChain>, Error<T, I>> {
+		let best_finalized = BestFinalized::<T, I>::get().ok_or_else(|| {
 			log::trace!(
 				target: crate::LOG_TARGET,
 				"Cannot finalize header {:?} because pallet is not yet initialized",
@@ -83,16 +174,19 @@ impl<T: Config<I>, I: 'static> SubmitFinalityProofHelper<T, I> {
 			<Error<T, I>>::NotInitialized
 		})?;

-		if best_finalized.number() >= finality_target {
-			log::trace!(
-				target: crate::LOG_TARGET,
-				"Cannot finalize obsolete header: bundled {:?}, best {:?}",
-				finality_target,
-				best_finalized,
-			);
+		let improved_by = match finality_target.checked_sub(&best_finalized.number()) {
+			Some(improved_by) if improved_by > Zero::zero() => improved_by,
+			_ => {
+				log::trace!(
+					target: crate::LOG_TARGET,
+					"Cannot finalize obsolete header: bundled {:?}, best {:?}",
+					finality_target,
+					best_finalized,
+				);

-			return Err(Error::<T, I>::OldHeader)
-		}
+				return Err(Error::<T, I>::OldHeader)
+			},
+		};

 		if let Some(current_set_id) = current_set_id {
 			let actual_set_id = <CurrentAuthoritySet<T, I>>::get().set_id;
@@ -108,12 +202,12 @@ impl<T: Config<I>, I: 'static> SubmitFinalityProofHelper<T, I> {
 			}
 		}

-		Ok(())
+		Ok(improved_by)
 	}

 	/// Check if the `SubmitFinalityProof` was successfully executed.
 	pub fn was_successful(finality_target: BlockNumberOf<T::BridgedChain>) -> bool {
-		match crate::BestFinalized::<T, I>::get() {
+		match BestFinalized::<T, I>::get() {
 			Some(best_finalized) => best_finalized.number() == finality_target,
 			None => false,
 		}
@@ -135,17 +229,20 @@ pub trait CallSubType<T: Config<I, RuntimeCall = Self>, I: 'static>:
 				finality_target,
 				justification,
 				None,
+				false,
 			))
 		} else if let Some(crate::Call::<T, I>::submit_finality_proof_ex {
 			finality_target,
 			justification,
 			current_set_id,
+			is_free_execution_expected,
 		}) = self.is_sub_type()
 		{
 			return Some(submit_finality_proof_info_from_args::<T, I>(
 				finality_target,
 				justification,
 				Some(*current_set_id),
+				*is_free_execution_expected,
 			))
 		}

@@ -155,26 +252,36 @@ pub trait CallSubType<T: Config<I, RuntimeCall = Self>, I: 'static>:
 	/// Validate Grandpa headers in order to avoid "mining" transactions that provide outdated
 	/// bridged chain headers. Without this validation, even honest relayers may lose their funds
 	/// if there are multiple relays running and submitting the same information.
-	fn check_obsolete_submit_finality_proof(&self) -> TransactionValidity
+	///
+	/// Returns `Ok(None)` if the call is not the `submit_finality_proof` call of our pallet.
+	/// Returns `Ok(Some(_))` if the call is the `submit_finality_proof` call of our pallet and
+	/// we believe the call brings header that improves the pallet state.
+	/// Returns `Err(_)` if the call is the `submit_finality_proof` call of our pallet and we
+	/// believe that the call will fail.
+	fn check_obsolete_submit_finality_proof(
+		&self,
+	) -> Result<
+		Option<VerifiedSubmitFinalityProofInfo<BridgedBlockNumber<T, I>>>,
+		TransactionValidityError,
+	>
 	where
 		Self: Sized,
 	{
-		let finality_target = match self.submit_finality_proof_info() {
+		let call_info = match self.submit_finality_proof_info() {
 			Some(finality_proof) => finality_proof,
-			_ => return Ok(ValidTransaction::default()),
+			_ => return Ok(None),
 		};

 		if Pallet::<T, I>::ensure_not_halted().is_err() {
-			return InvalidTransaction::Call.into()
+			return Err(InvalidTransaction::Call.into())
 		}

-		match SubmitFinalityProofHelper::<T, I>::check_obsolete(
-			finality_target.block_number,
-			finality_target.current_set_id,
-		) {
-			Ok(_) => Ok(ValidTransaction::default()),
-			Err(Error::<T, I>::OldHeader) => InvalidTransaction::Stale.into(),
-			Err(_) => InvalidTransaction::Call.into(),
+		let result = SubmitFinalityProofHelper::<T, I>::check_obsolete_from_extension(&call_info);
+		match result {
+			Ok(improved_by) =>
+				Ok(Some(VerifiedSubmitFinalityProofInfo { base: call_info, improved_by })),
+			Err(Error::<T, I>::OldHeader) => Err(InvalidTransaction::Stale.into()),
+			Err(_) => Err(InvalidTransaction::Call.into()),
 		}
 	}
 }
@@ -189,6 +296,7 @@ pub(crate) fn submit_finality_proof_info_from_args<T: Config<I>, I: 'static>(
 	finality_target: &BridgedHeader<T, I>,
 	justification: &GrandpaJustification<BridgedHeader<T, I>>,
 	current_set_id: Option<SetId>,
+	is_free_execution_expected: bool,
 ) -> SubmitFinalityProofInfo<BridgedBlockNumber<T, I>> {
 	let block_number = *finality_target.number();

@@ -230,16 +338,26 @@ pub(crate) fn submit_finality_proof_info_from_args<T: Config<I>, I: 'static>(
 	);
 	let extra_size = actual_call_size.saturating_sub(max_expected_call_size);

-	SubmitFinalityProofInfo { block_number, current_set_id, extra_weight, extra_size }
+	SubmitFinalityProofInfo {
+		block_number,
+		current_set_id,
+		is_mandatory: is_mandatory_finality_target,
+		is_free_execution_expected,
+		extra_weight,
+		extra_size,
+	}
 }

 #[cfg(test)]
 mod tests {
 	use crate::{
 		call_ext::CallSubType,
-		mock::{run_test, test_header, RuntimeCall, TestBridgedChain, TestNumber, TestRuntime},
-		BestFinalized, Config, CurrentAuthoritySet, PalletOperatingMode, StoredAuthoritySet,
-		SubmitFinalityProofInfo, WeightInfo,
+		mock::{
+			run_test, test_header, FreeHeadersInterval, RuntimeCall, TestBridgedChain, TestNumber,
+			TestRuntime,
+		},
+		BestFinalized, Config, CurrentAuthoritySet, FreeHeadersRemaining, PalletOperatingMode,
+		StoredAuthoritySet, SubmitFinalityProofInfo, WeightInfo,
 	};
 	use bp_header_chain::ChainWithGrandpa;
 	use bp_runtime::{BasicOperatingMode, HeaderId};
@@ -247,6 +365,7 @@ mod tests {
 		make_default_justification, make_justification_for_header, JustificationGeneratorParams,
 		TEST_GRANDPA_SET_ID,
 	};
+	use codec::Encode;
 	use frame_support::weights::Weight;
 	use sp_runtime::{testing::DigestItem, traits::Header as _, SaturatedConversion};

@@ -256,6 +375,7 @@ mod tests {
 			justification: make_default_justification(&test_header(num)),
 			// not initialized => zero
 			current_set_id: 0,
+			is_free_execution_expected: false,
 		};
 		RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
 			bridge_grandpa_call,
@@ -311,6 +431,121 @@ mod tests {
 		});
 	}

+	#[test]
+	fn extension_rejects_new_header_if_free_execution_is_requested_and_free_submissions_are_not_accepted(
+	) {
+		run_test(|| {
+			let bridge_grandpa_call = crate::Call::<TestRuntime, ()>::submit_finality_proof_ex {
+				finality_target: Box::new(test_header(10 + FreeHeadersInterval::get() as u64)),
+				justification: make_default_justification(&test_header(
+					10 + FreeHeadersInterval::get() as u64,
+				)),
+				current_set_id: 0,
+				is_free_execution_expected: true,
+			};
+			sync_to_header_10();
+
+			// when we can accept free headers => Ok
+			FreeHeadersRemaining::<TestRuntime, ()>::put(2);
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_ok());
+
+			// when we can NOT accept free headers => Err
+			FreeHeadersRemaining::<TestRuntime, ()>::put(0);
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_err());
+
+			// when called outside of transaction => Ok
+			FreeHeadersRemaining::<TestRuntime, ()>::kill();
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call,
+			),)
+			.is_ok());
+		})
+	}
+
+	#[test]
+	fn extension_rejects_new_header_if_free_execution_is_requested_and_improved_by_is_below_expected(
+	) {
+		run_test(|| {
+			let bridge_grandpa_call = crate::Call::<TestRuntime, ()>::submit_finality_proof_ex {
+				finality_target: Box::new(test_header(100)),
+				justification: make_default_justification(&test_header(100)),
+				current_set_id: 0,
+				is_free_execution_expected: true,
+			};
+			sync_to_header_10();
+
+			// when `improved_by` is less than the free interval
+			BestFinalized::<TestRuntime, ()>::put(HeaderId(
+				100 - FreeHeadersInterval::get() as u64 + 1,
+				sp_core::H256::default(),
+			));
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_err());
+
+			// when `improved_by` is equal to the free interval
+			BestFinalized::<TestRuntime, ()>::put(HeaderId(
+				100 - FreeHeadersInterval::get() as u64,
+				sp_core::H256::default(),
+			));
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_ok());
+
+			// when `improved_by` is larger than the free interval
+			BestFinalized::<TestRuntime, ()>::put(HeaderId(
+				100 - FreeHeadersInterval::get() as u64 - 1,
+				sp_core::H256::default(),
+			));
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_ok());
+
+			// when `improved_by` is less than the free interval BUT it is a mandatory header
+			let mut mandatory_header = test_header(100);
+			let consensus_log = sp_consensus_grandpa::ConsensusLog::<TestNumber>::ScheduledChange(
+				sp_consensus_grandpa::ScheduledChange {
+					next_authorities: bp_test_utils::authority_list(),
+					delay: 0,
+				},
+			);
+			mandatory_header.digest = sp_runtime::Digest {
+				logs: vec![DigestItem::Consensus(
+					sp_consensus_grandpa::GRANDPA_ENGINE_ID,
+					consensus_log.encode(),
+				)],
+			};
+			let justification = make_justification_for_header(JustificationGeneratorParams {
+				header: mandatory_header.clone(),
+				set_id: 1,
+				..Default::default()
+			});
+			let bridge_grandpa_call = crate::Call::<TestRuntime, ()>::submit_finality_proof_ex {
+				finality_target: Box::new(mandatory_header),
+				justification,
+				current_set_id: 0,
+				is_free_execution_expected: true,
+			};
+			BestFinalized::<TestRuntime, ()>::put(HeaderId(
+				100 - FreeHeadersInterval::get() as u64 + 1,
+				sp_core::H256::default(),
+			));
+			assert!(RuntimeCall::check_obsolete_submit_finality_proof(&RuntimeCall::Grandpa(
+				bridge_grandpa_call.clone(),
+			),)
+			.is_ok());
+		})
+	}
+
 	#[test]
 	fn extension_accepts_new_header() {
 		run_test(|| {
@@ -336,6 +571,8 @@ mod tests {
 				current_set_id: None,
 				extra_weight: Weight::zero(),
 				extra_size: 0,
+				is_mandatory: false,
+				is_free_execution_expected: false,
 			})
 		);

@@ -345,6 +582,7 @@ mod tests {
 				finality_target: Box::new(test_header(42)),
 				justification: make_default_justification(&test_header(42)),
 				current_set_id: 777,
+				is_free_execution_expected: false,
 			});
 		assert_eq!(
 			deprecated_call.submit_finality_proof_info(),
@@ -353,6 +591,8 @@ mod tests {
 				current_set_id: Some(777),
 				extra_weight: Weight::zero(),
 				extra_size: 0,
+				is_mandatory: false,
+				is_free_execution_expected: false,
 			})
 		);
 	}
@@ -370,6 +610,7 @@ mod tests {
 			finality_target: Box::new(small_finality_target),
 			justification: small_justification,
 			current_set_id: TEST_GRANDPA_SET_ID,
+			is_free_execution_expected: false,
 		});
 		assert_eq!(small_call.submit_finality_proof_info().unwrap().extra_size, 0);

@@ -387,6 +628,7 @@ mod tests {
 			finality_target: Box::new(large_finality_target),
 			justification: large_justification,
 			current_set_id: TEST_GRANDPA_SET_ID,
+			is_free_execution_expected: false,
 		});
 		assert_ne!(large_call.submit_finality_proof_info().unwrap().extra_size, 0);
 	}
@@ -406,6 +648,7 @@ mod tests {
 			finality_target: Box::new(finality_target.clone()),
 			justification,
 			current_set_id: TEST_GRANDPA_SET_ID,
+			is_free_execution_expected: false,
 		});
 		assert_eq!(call.submit_finality_proof_info().unwrap().extra_weight, Weight::zero());

@@ -420,7 +663,52 @@ mod tests {
 			finality_target: Box::new(finality_target),
 			justification,
 			current_set_id: TEST_GRANDPA_SET_ID,
+			is_free_execution_expected: false,
 		});
 		assert_eq!(call.submit_finality_proof_info().unwrap().extra_weight, call_weight);
 	}
+
+	#[test]
+	fn check_obsolete_submit_finality_proof_returns_correct_improved_by() {
+		run_test(|| {
+			fn make_call(number: u64) -> RuntimeCall {
+				RuntimeCall::Grandpa(crate::Call::<TestRuntime, ()>::submit_finality_proof_ex {
+					finality_target: Box::new(test_header(number)),
+					justification: make_default_justification(&test_header(number)),
+					current_set_id: 0,
+					is_free_execution_expected: false,
+				})
+			}
+
+			sync_to_header_10();
+
+			// when the difference between headers is 1
+			assert_eq!(
+				RuntimeCall::check_obsolete_submit_finality_proof(&make_call(11))
+					.unwrap()
+					.unwrap()
+					.improved_by,
+				1,
+			);
+
+			// when the difference between headers is 2
+			assert_eq!(
+				RuntimeCall::check_obsolete_submit_finality_proof(&make_call(12))
+					.unwrap()
+					.unwrap()
+					.improved_by,
+				2,
+			);
+		})
+	}
+
+	#[test]
+	fn check_obsolete_submit_finality_proof_ignores_other_calls() {
+		run_test(|| {
+			let call =
+				RuntimeCall::System(frame_system::Call::<TestRuntime>::remark { remark: vec![42] });
+
+			assert_eq!(RuntimeCall::check_obsolete_submit_finality_proof(&call), Ok(None));
+		})
+	}
 }
@@ -44,6 +44,7 @@ use bp_header_chain::{
 };
 use bp_runtime::{BlockNumberOf, HashOf, HasherOf, HeaderId, HeaderOf, OwnedBridgeModule};
 use frame_support::{dispatch::PostDispatchInfo, ensure, DefaultNoBound};
+use sp_consensus_grandpa::SetId;
 use sp_runtime::{
 	traits::{Header as HeaderT, Zero},
 	SaturatedConversion,
@@ -57,6 +58,7 @@ mod storage_types;

 /// Module, containing weights for this pallet.
 pub mod weights;
+pub mod weights_ext;

 #[cfg(feature = "runtime-benchmarks")]
 pub mod benchmarking;
@@ -65,6 +67,7 @@ pub mod benchmarking;
 pub use call_ext::*;
 pub use pallet::*;
 pub use weights::WeightInfo;
+pub use weights_ext::WeightInfoExt;

 /// The target that will be used when publishing logs related to this pallet.
 pub const LOG_TARGET: &str = "runtime::bridge-grandpa";
@@ -101,17 +104,31 @@ pub mod pallet {
 		/// The chain we are bridging to here.
 		type BridgedChain: ChainWithGrandpa;

-		/// Maximal number of "free" mandatory header transactions per block.
+		/// Maximal number of "free" header transactions per block.
 		///
 		/// To be able to track the bridged chain, the pallet requires all headers that are
 		/// changing GRANDPA authorities set at the bridged chain (we call them mandatory).
-		/// So it is a common good deed to submit mandatory headers to the pallet. However, if the
-		/// bridged chain gets compromised, its validators may generate as many mandatory headers
-		/// as they want. And they may fill the whole block (at this chain) for free. This constants
-		/// limits number of calls that we may refund in a single block. All calls above this
-		/// limit are accepted, but are not refunded.
+		/// So it is a common good deed to submit mandatory headers to the pallet.
+		///
+		/// The pallet may be configured (see `[Self::FreeHeadersInterval]`) to import some
+		/// non-mandatory headers for free as well. It also may be treated as a common good
+		/// deed, because it may help to reduce bridge fees - this cost may be deducted from
+		/// bridge fees, paid by message senders.
+		///
+		/// However, if the bridged chain gets compromised, its validators may generate as many
+		/// "free" headers as they want. And they may fill the whole block (at this chain) for
+		/// free. This constants limits number of calls that we may refund in a single block.
+		/// All calls above this limit are accepted, but are not refunded.
 		#[pallet::constant]
-		type MaxFreeMandatoryHeadersPerBlock: Get<u32>;
+		type MaxFreeHeadersPerBlock: Get<u32>;
+
+		/// The distance between bridged chain headers, that may be submitted for free. The
+		/// first free header is header number zero, the next one is header number
+		/// `FreeHeadersInterval::get()` or any of its descendant if that header has not
+		/// been submitted. In other words, interval between free headers should be at least
+		/// `FreeHeadersInterval`.
+		#[pallet::constant]
+		type FreeHeadersInterval: Get<Option<u32>>;

 		/// Maximal number of finalized headers to keep in the storage.
 		///
@@ -124,7 +141,7 @@ pub mod pallet {
 		type HeadersToKeep: Get<u32>;

 		/// Weights gathered through benchmarking.
-		type WeightInfo: WeightInfo;
+		type WeightInfo: WeightInfoExt;
 	}

 	#[pallet::pallet]
@@ -133,12 +150,12 @@ pub mod pallet {
 	#[pallet::hooks]
 	impl<T: Config<I>, I: 'static> Hooks<BlockNumberFor<T>> for Pallet<T, I> {
 		fn on_initialize(_n: BlockNumberFor<T>) -> Weight {
-			FreeMandatoryHeadersRemaining::<T, I>::put(T::MaxFreeMandatoryHeadersPerBlock::get());
+			FreeHeadersRemaining::<T, I>::put(T::MaxFreeHeadersPerBlock::get());
 			Weight::zero()
 		}

 		fn on_finalize(_n: BlockNumberFor<T>) {
-			FreeMandatoryHeadersRemaining::<T, I>::kill();
+			FreeHeadersRemaining::<T, I>::kill();
 		}
 	}

@@ -155,7 +172,7 @@ pub mod pallet {
 		/// `submit_finality_proof_ex` instead. Semantically, this call is an equivalent of the
 		/// `submit_finality_proof_ex` call without current authority set id check.
 		#[pallet::call_index(0)]
-		#[pallet::weight(<T::WeightInfo as WeightInfo>::submit_finality_proof(
+		#[pallet::weight(T::WeightInfo::submit_finality_proof_weight(
 			justification.commit.precommits.len().saturated_into(),
 			justification.votes_ancestries.len().saturated_into(),
 		))]
@@ -175,6 +192,8 @@ pub mod pallet {
 				// the `submit_finality_proof_ex` also reads this value, but it is done from the
 				// cache, so we don't treat it as an additional db access
 				<CurrentAuthoritySet<T, I>>::get().set_id,
+				// cannot enforce free execution using this call
+				false,
 			)
 		}

@@ -250,8 +269,14 @@ pub mod pallet {
 		/// - verification is not optimized or invalid;
 		///
 		/// - header contains forced authorities set change or change with non-zero delay.
+		///
+		/// The `is_free_execution_expected` parameter is not really used inside the call. It is
+		/// used by the transaction extension, which should be registered at the runtime level. If
+		/// this parameter is `true`, the transaction will be treated as invalid, if the call won't
+		/// be executed for free. If transaction extension is not used by the runtime, this
+		/// parameter is not used at all.
 		#[pallet::call_index(4)]
-		#[pallet::weight(<T::WeightInfo as WeightInfo>::submit_finality_proof(
+		#[pallet::weight(T::WeightInfo::submit_finality_proof_weight(
 			justification.commit.precommits.len().saturated_into(),
 			justification.votes_ancestries.len().saturated_into(),
 		))]
@@ -260,6 +285,7 @@ pub mod pallet {
 			finality_target: Box<BridgedHeader<T, I>>,
 			justification: GrandpaJustification<BridgedHeader<T, I>>,
 			current_set_id: sp_consensus_grandpa::SetId,
+			_is_free_execution_expected: bool,
 		) -> DispatchResultWithPostInfo {
 			Self::ensure_not_halted().map_err(Error::<T, I>::BridgeModule)?;
 			ensure_signed(origin)?;
@@ -273,7 +299,8 @@ pub mod pallet {

 			// it checks whether the `number` is better than the current best block number
 			// and whether the `current_set_id` matches the best known set id
-			SubmitFinalityProofHelper::<T, I>::check_obsolete(number, Some(current_set_id))?;
+			let improved_by =
+				SubmitFinalityProofHelper::<T, I>::check_obsolete(number, Some(current_set_id))?;

 			let authority_set = <CurrentAuthoritySet<T, I>>::get();
 			let unused_proof_size = authority_set.unused_proof_size();
@@ -283,23 +310,16 @@ pub mod pallet {

 			let maybe_new_authority_set =
 				try_enact_authority_change::<T, I>(&finality_target, set_id)?;
-			let may_refund_call_fee = maybe_new_authority_set.is_some() &&
-				// if we have seen too many mandatory headers in this block, we don't want to refund
-				Self::free_mandatory_headers_remaining() > 0 &&
-				// if arguments out of expected bounds, we don't want to refund
-				submit_finality_proof_info_from_args::<T, I>(&finality_target, &justification, Some(current_set_id))
-					.fits_limits();
+			let may_refund_call_fee = may_refund_call_fee::<T, I>(
+				&finality_target,
+				&justification,
+				current_set_id,
+				improved_by,
+			);
 			if may_refund_call_fee {
-				FreeMandatoryHeadersRemaining::<T, I>::mutate(|count| {
-					*count = count.saturating_sub(1)
-				});
+				on_free_header_imported::<T, I>();
 			}
 			insert_header::<T, I>(*finality_target, hash);
-			log::info!(
-				target: LOG_TARGET,
-				"Successfully imported finalized header with hash {:?}!",
-				hash
-			);

 			// mandatory header is a header that changes authorities set. The pallet can't go
 			// further without importing this header. So every bridge MUST import mandatory headers.
@@ -311,6 +331,13 @@ pub mod pallet {
 			// to pay for the transaction.
 			let pays_fee = if may_refund_call_fee { Pays::No } else { Pays::Yes };

+			log::info!(
+				target: LOG_TARGET,
+				"Successfully imported finalized header with hash {:?}! Free: {}",
+				hash,
+				if may_refund_call_fee { "Yes" } else { "No" },
+			);
+
 			// the proof size component of the call weight assumes that there are
 			// `MaxBridgedAuthorities` in the `CurrentAuthoritySet` (we use `MaxEncodedLen`
 			// estimation). But if their number is lower, then we may "refund" some `proof_size`,
@@ -335,20 +362,18 @@ pub mod pallet {
 		}
 	}

-	/// Number mandatory headers that we may accept in the current block for free (returning
-	/// `Pays::No`).
+	/// Number of free header submissions that we may yet accept in the current block.
 	///
-	/// If the `FreeMandatoryHeadersRemaining` hits zero, all following mandatory headers in the
+	/// If the `FreeHeadersRemaining` hits zero, all following mandatory headers in the
 	/// current block are accepted with fee (`Pays::Yes` is returned).
 	///
-	/// The `FreeMandatoryHeadersRemaining` is an ephemeral value that is set to
-	/// `MaxFreeMandatoryHeadersPerBlock` at each block initialization and is killed on block
+	/// The `FreeHeadersRemaining` is an ephemeral value that is set to
+	/// `MaxFreeHeadersPerBlock` at each block initialization and is killed on block
 	/// finalization. So it never ends up in the storage trie.
 	#[pallet::storage]
 	#[pallet::whitelist_storage]
-	#[pallet::getter(fn free_mandatory_headers_remaining)]
-	pub(super) type FreeMandatoryHeadersRemaining<T: Config<I>, I: 'static = ()> =
-		StorageValue<_, u32, ValueQuery>;
+	pub type FreeHeadersRemaining<T: Config<I>, I: 'static = ()> =
+		StorageValue<_, u32, OptionQuery>;

 	/// Hash of the header used to bootstrap the pallet.
 	#[pallet::storage]
@@ -473,6 +498,68 @@ pub mod pallet {
 		/// The `current_set_id` argument of the `submit_finality_proof_ex` doesn't match
 		/// the id of the current set, known to the pallet.
 		InvalidAuthoritySetId,
+		/// The submitter wanted free execution, but we can't fit more free transactions
+		/// to the block.
+		FreeHeadersLimitExceded,
+		/// The submitter wanted free execution, but the difference between best known and
+		/// bundled header numbers is below the `FreeHeadersInterval`.
+		BelowFreeHeaderInterval,
+	}
+
+	/// Called when new free header is imported.
+	pub fn on_free_header_imported<T: Config<I>, I: 'static>() {
+		FreeHeadersRemaining::<T, I>::mutate(|count| {
+			*count = match *count {
+				None => None,
+				// the signed extension expects that `None` means outside of block
+				// execution - i.e. when transaction is validated from the transaction pool,
+				// so use `saturating_sub` and don't go from `Some(0)`->`None`
+				Some(count) => Some(count.saturating_sub(1)),
+			}
+		});
+	}
+
+	/// Return true if we may refund transaction cost to the submitter. In other words,
+	/// this transaction is considered as common good deed w.r.t to pallet configuration.
+	fn may_refund_call_fee<T: Config<I>, I: 'static>(
+		finality_target: &BridgedHeader<T, I>,
+		justification: &GrandpaJustification<BridgedHeader<T, I>>,
+		current_set_id: SetId,
+		improved_by: BridgedBlockNumber<T, I>,
+	) -> bool {
+		// if we have refunded too much at this block => not refunding
+		if FreeHeadersRemaining::<T, I>::get().unwrap_or(0) == 0 {
+			return false;
+		}
+
+		// if size/weight of call is larger than expected => not refunding
+		let call_info = submit_finality_proof_info_from_args::<T, I>(
+			&finality_target,
+			&justification,
+			Some(current_set_id),
+			// this function is called from the transaction body and we do not want
+			// to do MAY-be-free-executed checks here - they had to be done in the
+			// transaction extension before
+			false,
+		);
+		if !call_info.fits_limits() {
+			return false;
+		}
+
+		// if that's a mandatory header => refund
+		if call_info.is_mandatory {
+			return true;
+		}
+
+		// if configuration allows free non-mandatory headers and the header
+		// matches criteria => refund
+		if let Some(free_headers_interval) = T::FreeHeadersInterval::get() {
+			if improved_by >= free_headers_interval.into() {
+				return true;
+			}
+		}
+
+		false
 	}

 	/// Check the given header for a GRANDPA scheduled authority set change. If a change
@@ -692,8 +779,8 @@ pub fn initialize_for_benchmarks<T: Config<I>, I: 'static>(header: BridgedHeader
 mod tests {
 	use super::*;
 	use crate::mock::{
-		run_test, test_header, RuntimeEvent as TestEvent, RuntimeOrigin, System, TestBridgedChain,
-		TestHeader, TestNumber, TestRuntime, MAX_BRIDGED_AUTHORITIES,
+		run_test, test_header, FreeHeadersInterval, RuntimeEvent as TestEvent, RuntimeOrigin,
+		System, TestBridgedChain, TestHeader, TestNumber, TestRuntime, MAX_BRIDGED_AUTHORITIES,
 	};
 	use bp_header_chain::BridgeGrandpaCall;
 	use bp_runtime::BasicOperatingMode;
@@ -747,6 +834,7 @@ mod tests {
 			Box::new(header),
 			justification,
 			TEST_GRANDPA_SET_ID,
+			false,
 		)
 	}

@@ -766,6 +854,7 @@ mod tests {
 			Box::new(header),
 			justification,
 			set_id,
+			false,
 		)
 	}

@@ -794,6 +883,7 @@ mod tests {
 			Box::new(header),
 			justification,
 			set_id,
+			false,
 		)
 	}

@@ -1009,6 +1099,7 @@ mod tests {
 					Box::new(header.clone()),
 					justification.clone(),
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::InvalidJustification
 			);
@@ -1018,6 +1109,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					next_set_id,
+					false,
 				),
 				<Error<TestRuntime>>::InvalidAuthoritySetId
 			);
@@ -1039,6 +1131,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::InvalidJustification
 			);
@@ -1069,6 +1162,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::InvalidAuthoritySet
 			);
@@ -1108,6 +1202,7 @@ mod tests {
 				Box::new(header.clone()),
 				justification.clone(),
 				TEST_GRANDPA_SET_ID,
+				false,
 			);
 			assert_ok!(result);
 			assert_eq!(result.unwrap().pays_fee, frame_support::dispatch::Pays::No);
@@ -1171,6 +1266,7 @@ mod tests {
 				Box::new(header.clone()),
 				justification,
 				TEST_GRANDPA_SET_ID,
+				false,
 			);
 			assert_ok!(result);
 			assert_eq!(result.unwrap().pays_fee, frame_support::dispatch::Pays::Yes);
@@ -1203,6 +1299,7 @@ mod tests {
 				Box::new(header.clone()),
 				justification,
 				TEST_GRANDPA_SET_ID,
+				false,
 			);
 			assert_ok!(result);
 			assert_eq!(result.unwrap().pays_fee, frame_support::dispatch::Pays::Yes);
@@ -1233,6 +1330,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::UnsupportedScheduledChange
 			);
@@ -1259,6 +1357,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::UnsupportedScheduledChange
 			);
@@ -1285,6 +1384,7 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				<Error<TestRuntime>>::TooManyAuthoritiesInSet
 			);
@@ -1350,12 +1450,13 @@ mod tests {
 					Box::new(header),
 					invalid_justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				)
 			};

 			initialize_substrate_bridge();

-			for _ in 0..<TestRuntime as Config>::MaxFreeMandatoryHeadersPerBlock::get() + 1 {
+			for _ in 0..<TestRuntime as Config>::MaxFreeHeadersPerBlock::get() + 1 {
 				assert_err!(submit_invalid_request(), <Error<TestRuntime>>::InvalidJustification);
 			}

@@ -1423,6 +1524,64 @@ mod tests {
 		})
 	}

+	#[test]
+	fn may_import_non_mandatory_header_for_free() {
+		run_test(|| {
+			initialize_substrate_bridge();
+
+			// set best finalized to `100`
+			const BEST: u8 = 12;
+			fn reset_best() {
+				BestFinalized::<TestRuntime, ()>::set(Some(HeaderId(
+					BEST as _,
+					Default::default(),
+				)));
+			}
+
+			// non-mandatory header is imported with fee
+			reset_best();
+			let non_free_header_number = BEST + FreeHeadersInterval::get() as u8 - 1;
+			let result = submit_finality_proof(non_free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::Yes);
+
+			// non-mandatory free header is imported without fee
+			reset_best();
+			let free_header_number = BEST + FreeHeadersInterval::get() as u8;
+			let result = submit_finality_proof(free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::No);
+
+			// another non-mandatory free header is imported without fee
+			let free_header_number = BEST + FreeHeadersInterval::get() as u8 * 2;
+			let result = submit_finality_proof(free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::No);
+
+			// now the rate limiter starts charging fees even for free headers
+			let free_header_number = BEST + FreeHeadersInterval::get() as u8 * 3;
+			let result = submit_finality_proof(free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::Yes);
+
+			// check that we can import for free if `improved_by` is larger
+			// than the free interval
+			next_block();
+			reset_best();
+			let free_header_number = FreeHeadersInterval::get() as u8 + 42;
+			let result = submit_finality_proof(free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::No);
+
+			// check that the rate limiter shares the counter between mandatory
+			// and free non-mandatory headers
+			next_block();
+			reset_best();
+			let free_header_number = BEST + FreeHeadersInterval::get() as u8 * 4;
+			let result = submit_finality_proof(free_header_number);
+			assert_eq!(result.unwrap().pays_fee, Pays::No);
+			let result = submit_mandatory_finality_proof(free_header_number + 1, 1);
+			assert_eq!(result.expect("call failed").pays_fee, Pays::No);
+			let result = submit_mandatory_finality_proof(free_header_number + 2, 2);
+			assert_eq!(result.expect("call failed").pays_fee, Pays::Yes);
+		});
+	}
+
 	#[test]
 	fn should_prune_headers_over_headers_to_keep_parameter() {
 		run_test(|| {
@@ -1519,9 +1678,23 @@ mod tests {
 					Box::new(header),
 					justification,
 					TEST_GRANDPA_SET_ID,
+					false,
 				),
 				DispatchError::BadOrigin,
 			);
 		})
 	}
+
+	#[test]
+	fn on_free_header_imported_never_sets_to_none() {
+		run_test(|| {
+			FreeHeadersRemaining::<TestRuntime, ()>::set(Some(2));
+			on_free_header_imported::<TestRuntime, ()>();
+			assert_eq!(FreeHeadersRemaining::<TestRuntime, ()>::get(), Some(1));
+			on_free_header_imported::<TestRuntime, ()>();
+			assert_eq!(FreeHeadersRemaining::<TestRuntime, ()>::get(), Some(0));
+			on_free_header_imported::<TestRuntime, ()>();
+			assert_eq!(FreeHeadersRemaining::<TestRuntime, ()>::get(), Some(0));
+		})
+	}
 }
@@ -48,14 +48,16 @@ impl frame_system::Config for TestRuntime {
 }

 parameter_types! {
-	pub const MaxFreeMandatoryHeadersPerBlock: u32 = 2;
+	pub const MaxFreeHeadersPerBlock: u32 = 2;
+	pub const FreeHeadersInterval: u32 = 32;
 	pub const HeadersToKeep: u32 = 5;
 }

 impl grandpa::Config for TestRuntime {
 	type RuntimeEvent = RuntimeEvent;
 	type BridgedChain = TestBridgedChain;
-	type MaxFreeMandatoryHeadersPerBlock = MaxFreeMandatoryHeadersPerBlock;
+	type MaxFreeHeadersPerBlock = MaxFreeHeadersPerBlock;
+	type FreeHeadersInterval = FreeHeadersInterval;
 	type HeadersToKeep = HeadersToKeep;
 	type WeightInfo = ();
 }
@@ -0,0 +1,58 @@
+// Copyright (C) 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/>.
+
+//! Weight-related utilities.
+
+use crate::weights::{BridgeWeight, WeightInfo};
+
+use frame_support::weights::Weight;
+
+/// Extended weight info.
+pub trait WeightInfoExt: WeightInfo {
+	// Our configuration assumes that the runtime has special signed extensions used to:
+	//
+	// 1) boost priority of `submit_finality_proof` transactions;
+	//
+	// 2) slash relayer if he submits an invalid transaction.
+	//
+	// We read and update storage values of other pallets (`pallet-bridge-relayers` and
+	// balances/assets pallet). So we need to add this weight to the weight of our call.
+	// Hence two following methods.
+
+	/// Extra weight that is added to the `submit_finality_proof` call weight by signed extensions
+	/// that are declared at runtime level.
+	fn submit_finality_proof_overhead_from_runtime() -> Weight;
+
+	// Functions that are directly mapped to extrinsics weights.
+
+	/// Weight of message delivery extrinsic.
+	fn submit_finality_proof_weight(precommits_len: u32, votes_ancestries_len: u32) -> Weight {
+		let base_weight = Self::submit_finality_proof(precommits_len, votes_ancestries_len);
+		base_weight.saturating_add(Self::submit_finality_proof_overhead_from_runtime())
+	}
+}
+
+impl<T: frame_system::Config> WeightInfoExt for BridgeWeight<T> {
+	fn submit_finality_proof_overhead_from_runtime() -> Weight {
+		Weight::zero()
+	}
+}
+
+impl WeightInfoExt for () {
+	fn submit_finality_proof_overhead_from_runtime() -> Weight {
+		Weight::zero()
+	}
+}