Unsigned Validation best practices (#5563)

* Configurable Unsigned Priority.

* Use the new builder.

* Fix tests.

* Fix benches.

* Remove unused import.

* Rename for_pallet

substrate/frame/staking/src/lib.rs

@@ -292,7 +292,7 @@ use sp_runtime::{
 	},
 	transaction_validity::{
 		TransactionValidityError, TransactionValidity, ValidTransaction, InvalidTransaction,
-		TransactionSource,
+		TransactionSource, TransactionPriority,
 	},
 };
 use sp_staking::{
@@ -782,6 +782,12 @@ pub trait Trait: frame_system::Trait {
 	/// For each validator only the `$MaxNominatorRewardedPerValidator` biggest stakers can claim
 	/// their reward. This used to limit the i/o cost for the nominator payout.
 	type MaxNominatorRewardedPerValidator: Get<u32>;
+
+	/// A configuration for base priority of unsigned transactions.
+	///
+	/// This is exposed so that it can be tuned for particular runtime, when
+	/// multiple pallets send unsigned transactions.
+	type UnsignedPriority: Get<TransactionPriority>;
 }
 
 /// Mode of era-forcing.
@@ -3224,24 +3230,24 @@ impl<T: Trait> frame_support::unsigned::ValidateUnsigned for Module<T> {
 				era,
 			);
 
-			Ok(ValidTransaction {
+			ValidTransaction::with_tag_prefix("StakingOffchain")
 				// The higher the score[0], the better a solution is.
-				priority: score[0].saturated_into(),
-				// no requires.
-				requires: vec![],
+				.priority(T::UnsignedPriority::get().saturating_add(score[0].saturated_into()))
 				// Defensive only. A single solution can exist in the pool per era. Each validator
 				// will run OCW at most once per era, hence there should never exist more than one
 				// transaction anyhow.
-				provides: vec![("StakingOffchain", era).encode()],
+				.and_provides(era)
 				// Note: this can be more accurate in the future. We do something like
 				// `era_end_block - current_block` but that is not needed now as we eagerly run
 				// offchain workers now and the above should be same as `T::ElectionLookahead`
 				// without the need to query more storage in the validation phase. If we randomize
 				// offchain worker, then we might re-consider this.
-				longevity: TryInto::<u64>::try_into(T::ElectionLookahead::get()).unwrap_or(DEFAULT_LONGEVITY),
+				.longevity(TryInto::<u64>::try_into(
+						T::ElectionLookahead::get()).unwrap_or(DEFAULT_LONGEVITY)
+				)
 				// We don't propagate this. This can never the validated at a remote node.
-				propagate: false,
-			})
+				.propagate(false)
+				.build()
 		} else {
 			InvalidTransaction::Call.into()
 		}