limit maximal "free" parachain head size

bin/runtime-common/src/mock.rs

@@ -406,6 +406,7 @@ impl Chain for BridgedUnderlyingParachain {
 
 impl Parachain for BridgedUnderlyingParachain {
 	const PARACHAIN_ID: u32 = 42;
+	const MAX_HEADER_SIZE: u32 = 1_024;
 }
 
 /// The other, bridged chain, used in tests.

modules/parachains/src/lib.rs

@@ -505,6 +505,7 @@ pub mod pallet {
 				}
 
 				// convert from parachain head into stored parachain head data
+				let parachain_head_size = parachain_head.0.len();
 				let parachain_head_data =
 					match T::ParaStoredHeaderDataBuilder::try_build(parachain, &parachain_head) {
 						Some(parachain_head_data) => parachain_head_data,
@@ -521,15 +522,17 @@ pub mod pallet {
 
 				let update_result: Result<_, ()> =
 					ParasInfo::<T, I>::try_mutate(parachain, |stored_best_head| {
-						let is_free = match stored_best_head {
-							Some(ref best_head)
-								if at_relay_block.0.saturating_sub(
-									best_head.best_head_hash.at_relay_block_number,
-								) >= free_headers_interval =>
-								true,
-							Some(_) => false,
-							None => true,
-						};
+						let is_free = parachain_head_size <
+							T::ParaStoredHeaderDataBuilder::max_free_head_size() as usize &&
+							match stored_best_head {
+								Some(ref best_head)
+									if at_relay_block.0.saturating_sub(
+										best_head.best_head_hash.at_relay_block_number,
+									) >= free_headers_interval =>
+									true,
+								Some(_) => false,
+								None => true,
+							};
 						let artifacts = Pallet::<T, I>::update_parachain_head(
 							parachain,
 							stored_best_head.take(),
@@ -582,7 +585,8 @@ pub mod pallet {
 				&& SubmitFinalityProofHelper::<T, T::BridgesGrandpaPalletInstance>::can_import_anything_for_free();
 			let pays_fee = if is_free {
 				log::trace!(target: LOG_TARGET, "Parachain heads update transaction is free");
-				pallet_bridge_grandpa::on_free_header_imported::<T, T::BridgesGrandpaPalletInstance>();
+				pallet_bridge_grandpa::on_free_header_imported::<T, T::BridgesGrandpaPalletInstance>(
+				);
 				Pays::No
 			} else {
 				log::trace!(target: LOG_TARGET, "Parachain heads update transaction is paid");
@@ -818,7 +822,7 @@ pub fn initialize_for_benchmarks<T: Config<I>, I: 'static, PC: Parachain<Hash =
 pub(crate) mod tests {
 	use super::*;
 	use crate::mock::{
-		run_test, test_relay_header, BigParachainHeader, FreeHeadersInterval,
+		run_test, test_relay_header, BigParachain, BigParachainHeader, FreeHeadersInterval,
 		RegularParachainHasher, RegularParachainHeader, RelayBlockHeader,
 		RuntimeEvent as TestEvent, RuntimeOrigin, TestRuntime, UNTRACKED_PARACHAIN_ID,
 	};
@@ -1809,6 +1813,21 @@ pub(crate) mod tests {
 				proof,
 			);
 			assert_eq!(result.unwrap().pays_fee, Pays::No);
+			// then we submit new BIG head, proved after `FreeHeadersInterval` => Pays::Yes
+			// then we submit new head, proved after `FreeHeadersInterval` => Pays::No
+			let mut large_head = head_data(2, 100);
+			large_head.0.extend(&[42u8; BigParachain::MAX_HEADER_SIZE as _]);
+			let (state_root, proof, parachains) =
+				prepare_parachain_heads_proof::<RegularParachainHeader>(vec![(2, large_head)]);
+			let relay_block_number = relay_block_number + FreeHeadersInterval::get();
+			proceed(relay_block_number, state_root);
+			let result = Pallet::<TestRuntime>::submit_parachain_heads(
+				RuntimeOrigin::signed(1),
+				(relay_block_number, test_relay_header(relay_block_number, state_root).hash()),
+				parachains,
+				proof,
+			);
+			assert_eq!(result.unwrap().pays_fee, Pays::Yes);
 		})
 	}
 }

modules/parachains/src/mock.rs

@@ -70,6 +70,7 @@ impl Chain for Parachain1 {
 
 impl Parachain for Parachain1 {
 	const PARACHAIN_ID: u32 = 1;
+	const MAX_HEADER_SIZE: u32 = 1_024;
 }
 
 pub struct Parachain2;
@@ -96,6 +97,7 @@ impl Chain for Parachain2 {
 
 impl Parachain for Parachain2 {
 	const PARACHAIN_ID: u32 = 2;
+	const MAX_HEADER_SIZE: u32 = 1_024;
 }
 
 pub struct Parachain3;
@@ -122,6 +124,7 @@ impl Chain for Parachain3 {
 
 impl Parachain for Parachain3 {
 	const PARACHAIN_ID: u32 = 3;
+	const MAX_HEADER_SIZE: u32 = 1_024;
 }
 
 // this parachain is using u128 as block number and stored head data size exceeds limit
@@ -149,6 +152,7 @@ impl Chain for BigParachain {
 
 impl Parachain for BigParachain {
 	const PARACHAIN_ID: u32 = 4;
+	const MAX_HEADER_SIZE: u32 = 2_048;
 }
 
 construct_runtime! {

primitives/chain-bridge-hub-cumulus/src/lib.rs

@@ -39,6 +39,9 @@ use frame_support::{
 use frame_system::limits;
 use sp_std::time::Duration;
 
+/// Maximal bridge hub header size.
+pub const MAX_BRIDGE_HUB_HEADER_SIZE: u32 = 4_096;
+
 /// Average block interval in Cumulus-based parachains.
 ///
 /// Corresponds to the `MILLISECS_PER_BLOCK` from `parachains_common` crate.

primitives/chain-bridge-hub-kusama/src/lib.rs

@@ -62,6 +62,7 @@ impl Chain for BridgeHubKusama {
 
 impl Parachain for BridgeHubKusama {
 	const PARACHAIN_ID: u32 = BRIDGE_HUB_KUSAMA_PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 = MAX_BRIDGE_HUB_HEADER_SIZE;
 }
 
 impl ChainWithMessages for BridgeHubKusama {

primitives/chain-bridge-hub-polkadot/src/lib.rs

@@ -59,6 +59,7 @@ impl Chain for BridgeHubPolkadot {
 
 impl Parachain for BridgeHubPolkadot {
 	const PARACHAIN_ID: u32 = BRIDGE_HUB_POLKADOT_PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 = MAX_BRIDGE_HUB_HEADER_SIZE;
 }
 
 impl ChainWithMessages for BridgeHubPolkadot {

primitives/chain-bridge-hub-rococo/src/lib.rs

@@ -59,6 +59,7 @@ impl Chain for BridgeHubRococo {
 
 impl Parachain for BridgeHubRococo {
 	const PARACHAIN_ID: u32 = BRIDGE_HUB_ROCOCO_PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 = MAX_BRIDGE_HUB_HEADER_SIZE;
 }
 
 impl ChainWithMessages for BridgeHubRococo {

primitives/chain-bridge-hub-westend/src/lib.rs

@@ -58,6 +58,7 @@ impl Chain for BridgeHubWestend {
 
 impl Parachain for BridgeHubWestend {
 	const PARACHAIN_ID: u32 = BRIDGE_HUB_WESTEND_PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 = MAX_BRIDGE_HUB_HEADER_SIZE;
 }
 
 impl ChainWithMessages for BridgeHubWestend {

primitives/parachains/src/lib.rs

@@ -116,6 +116,10 @@ impl ParaStoredHeaderData {
 
 /// Stored parachain head data builder.
 pub trait ParaStoredHeaderDataBuilder {
+	/// Maximal parachain head size that we may accept for free. All heads above
+	/// this limit are submitted for a regular fee.
+	fn max_free_head_size() -> u32;
+
 	/// Return number of parachains that are supported by this builder.
 	fn supported_parachains() -> u32;
 
@@ -127,6 +131,10 @@ pub trait ParaStoredHeaderDataBuilder {
 pub struct SingleParaStoredHeaderDataBuilder<C: Parachain>(PhantomData<C>);
 
 impl<C: Parachain> ParaStoredHeaderDataBuilder for SingleParaStoredHeaderDataBuilder<C> {
+	fn max_free_head_size() -> u32 {
+		C::MAX_HEADER_SIZE
+	}
+
 	fn supported_parachains() -> u32 {
 		1
 	}
@@ -147,6 +155,17 @@ impl<C: Parachain> ParaStoredHeaderDataBuilder for SingleParaStoredHeaderDataBui
 #[impl_trait_for_tuples::impl_for_tuples(1, 30)]
 #[tuple_types_custom_trait_bound(Parachain)]
 impl ParaStoredHeaderDataBuilder for C {
+	fn max_free_head_size() -> u32 {
+		let mut result = 0_u32;
+		for_tuples!( #(
+			result = sp_std::cmp::max(
+				result,
+				SingleParaStoredHeaderDataBuilder::<C>::max_free_head_size(),
+			);
+		)* );
+		result
+	}
+
 	fn supported_parachains() -> u32 {
 		let mut result = 0;
 		for_tuples!( #(

primitives/runtime/src/chain.rs

@@ -247,6 +247,12 @@ where
 pub trait Parachain: Chain {
 	/// Parachain identifier.
 	const PARACHAIN_ID: u32;
+	/// Maximal size of the parachain header.
+	///
+	/// This isn't a strict limit. The relayer may submit larger headers and the
+	/// pallet will accept the call. The limit is only used to compute whether
+	/// the refund can be made.
+	const MAX_HEADER_SIZE: u32;
 }
 
 impl<T> Parachain for T
@@ -255,6 +261,8 @@ where
 	<T as UnderlyingChainProvider>::Chain: Parachain,
 {
 	const PARACHAIN_ID: u32 = <<T as UnderlyingChainProvider>::Chain as Parachain>::PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 =
+		<<T as UnderlyingChainProvider>::Chain as Parachain>::MAX_HEADER_SIZE;
 }
 
 /// Adapter for `Get<u32>` to access `PARACHAIN_ID` from `trait Parachain`

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

@@ -158,6 +158,7 @@ impl bp_runtime::Chain for BaseBridgeHubRococoAsBridgeHubPolkadot {
 
 impl bp_runtime::Parachain for BaseBridgeHubRococoAsBridgeHubPolkadot {
 	const PARACHAIN_ID: u32 = bp_bridge_hub_rococo::BridgeHubRococo::PARACHAIN_ID;
+	const MAX_HEADER_SIZE: u32 = bp_bridge_hub_rococo::BridgeHubRococo::MAX_HEADER_SIZE;
 }
 
 impl bp_messages::ChainWithMessages for BaseBridgeHubRococoAsBridgeHubPolkadot {

relays/client-substrate/src/test_chain.rs

@@ -97,6 +97,7 @@ impl bp_runtime::Chain for TestParachainBase {
 
 impl bp_runtime::Parachain for TestParachainBase {
 	const PARACHAIN_ID: u32 = 1000;
+	const MAX_HEADER_SIZE: u32 = 1_024;
 }
 
 /// Parachain that may be used in tests.