check session index first as cheap failfast

Suggested-by: Serban Iorga <[email protected]>

substrate/frame/beefy/src/equivocation.rs

@@ -269,6 +269,14 @@ where
 		let session_index = key_owner_proofs[0].session();
 		let validator_set_count = key_owner_proofs[0].validator_count();
 
+		// Check that the session id for the membership proof is within the
+		// bounds of the set id reported in the equivocation.
+		let set_id_session_index = crate::SetIdSession::<T>::get(set_id)
+			.ok_or(Error::<T>::InvalidEquivocationProofSession)?;
+		if session_index != set_id_session_index {
+			return Err(Error::<T>::InvalidEquivocationProofSession.into())
+		}
+
 		// Validate the key ownership proof extracting the ids of the offenders.
 		let offenders = offenders
 			.into_iter()
@@ -332,14 +340,6 @@ where
 			},
 		}
 
-		// Check that the session id for the membership proof is within the
-		// bounds of the set id reported in the equivocation.
-		let set_id_session_index = crate::SetIdSession::<T>::get(set_id)
-			.ok_or(Error::<T>::InvalidEquivocationProofSession)?;
-		if session_index != set_id_session_index {
-			return Err(Error::<T>::InvalidEquivocationProofSession.into())
-		}
-
 		let offence = EquivocationOffence {
 			time_slot: TimeSlot { set_id, round },
 			session_index,