"refund" proof size in GRANDPa pallet (#1863)

* "refund" proof size in GRANDPa pallet

* clippy

* extra_proof_size_bytes_works

* use saturated_into

* fix review comments

bridges/modules/grandpa/src/lib.rs

@@ -44,9 +44,12 @@ use bp_header_chain::{
 };
 use bp_runtime::{BlockNumberOf, Chain, HashOf, HasherOf, HeaderId, HeaderOf, OwnedBridgeModule};
 use finality_grandpa::voter_set::VoterSet;
-use frame_support::{ensure, fail};
+use frame_support::{dispatch::PostDispatchInfo, ensure, fail};
 use sp_finality_grandpa::{ConsensusLog, GRANDPA_ENGINE_ID};
-use sp_runtime::traits::{Header as HeaderT, Zero};
+use sp_runtime::{
+	traits::{Header as HeaderT, Zero},
+	SaturatedConversion,
+};
 use sp_std::{boxed::Box, convert::TryInto};
 
 mod extension;
@@ -152,8 +155,8 @@ pub mod pallet {
 		/// pallet.
 		#[pallet::call_index(0)]
 		#[pallet::weight(T::WeightInfo::submit_finality_proof(
-			justification.commit.precommits.len().try_into().unwrap_or(u32::MAX),
-			justification.votes_ancestries.len().try_into().unwrap_or(u32::MAX),
+			justification.commit.precommits.len().saturated_into(),
+			justification.votes_ancestries.len().saturated_into(),
 		))]
 		pub fn submit_finality_proof(
 			_origin: OriginFor<T>,
@@ -189,6 +192,7 @@ pub mod pallet {
 			ensure!(best_finalized_number < *number, <Error<T, I>>::OldHeader);
 
 			let authority_set = <CurrentAuthoritySet<T, I>>::get();
+			let unused_proof_size = authority_set.unused_proof_size();
 			let set_id = authority_set.set_id;
 			verify_justification::<T, I>(&justification, hash, *number, authority_set.into())?;
 
@@ -210,7 +214,18 @@ pub mod pallet {
 			let is_mandatory_header = is_authorities_change_enacted;
 			let pays_fee = if is_mandatory_header { Pays::No } else { Pays::Yes };
 
-			Ok(pays_fee.into())
+			// 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`,
+			// making proof smaller and leaving block space to other useful transactions
+			let pre_dispatch_weight = T::WeightInfo::submit_finality_proof(
+				justification.commit.precommits.len().saturated_into(),
+				justification.votes_ancestries.len().saturated_into(),
+			);
+			let actual_weight = pre_dispatch_weight
+				.set_proof_size(pre_dispatch_weight.proof_size().saturating_sub(unused_proof_size));
+
+			Ok(PostDispatchInfo { actual_weight: Some(actual_weight), pays_fee })
 		}
 
 		/// Bootstrap the bridge pallet with an initial header and authority set from which to sync.
@@ -819,9 +834,27 @@ mod tests {
 	fn succesfully_imports_header_with_valid_finality() {
 		run_test(|| {
 			initialize_substrate_bridge();
-			let result = submit_finality_proof(1);
+
+			let header_number = 1;
+			let header = test_header(header_number.into());
+			let justification = make_default_justification(&header);
+
+			let pre_dispatch_weight = <TestRuntime as Config>::WeightInfo::submit_finality_proof(
+				justification.commit.precommits.len().try_into().unwrap_or(u32::MAX),
+				justification.votes_ancestries.len().try_into().unwrap_or(u32::MAX),
+			);
+
+			let result = submit_finality_proof(header_number);
 			assert_ok!(result);
 			assert_eq!(result.unwrap().pays_fee, frame_support::dispatch::Pays::Yes);
+			// our test config assumes 2048 max authorities and we are just using couple
+			let pre_dispatch_proof_size = pre_dispatch_weight.proof_size();
+			let actual_proof_size = result.unwrap().actual_weight.unwrap().proof_size();
+			assert!(actual_proof_size > 0);
+			assert!(
+				actual_proof_size < pre_dispatch_proof_size,
+				"Actual proof size {actual_proof_size} must be less than the pre-dispatch {pre_dispatch_proof_size}",
+			);
 
 			let header = test_header(1);
 			assert_eq!(<BestFinalized<TestRuntime>>::get().unwrap().1, header.hash());