Unsigned Validation best practices (#5563)

* Configurable Unsigned Priority.

* Use the new builder.

* Fix tests.

* Fix benches.

* Remove unused import.

* Rename for_pallet

substrate/frame/im-online/src/lib.rs

@@ -247,6 +247,12 @@ pub trait Trait: frame_system::Trait + pallet_session::historical::Trait {
 			IdentificationTuple<Self>,
 			UnresponsivenessOffence<IdentificationTuple<Self>>,
 		>;
+
+	/// 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>;
 }
 
 decl_event!(
@@ -658,13 +664,14 @@ impl<T: Trait> frame_support::unsigned::ValidateUnsigned for Module<T> {
 				return InvalidTransaction::BadProof.into();
 			}
 
-			Ok(ValidTransaction {
-				priority: TransactionPriority::max_value(),
-				requires: vec![],
-				provides: vec![(current_session, authority_id).encode()],
-				longevity: TryInto::<u64>::try_into(T::SessionDuration::get() / 2.into()).unwrap_or(64_u64),
-				propagate: true,
-			})
+			ValidTransaction::with_tag_prefix("ImOnline")
+				.priority(T::UnsignedPriority::get())
+				.and_provides((current_session, authority_id))
+				.longevity(TryInto::<u64>::try_into(
+					T::SessionDuration::get() / 2.into()
+				).unwrap_or(64_u64))
+				.propagate(true)
+				.build()
 		} else {
 			InvalidTransaction::Call.into()
 		}

substrate/frame/im-online/src/mock.rs

@@ -160,6 +160,10 @@ impl pallet_authorship::Trait for Runtime {
 	type EventHandler = ImOnline;
 }
 
+parameter_types! {
+	pub const UnsignedPriority: u64 = 1 << 20;
+}
+
 impl Trait for Runtime {
 	type AuthorityId = UintAuthorityId;
 	type Event = ();
@@ -167,6 +171,7 @@ impl Trait for Runtime {
 	type SubmitTransaction = SubmitTransaction;
 	type ReportUnresponsiveness = OffenceHandler;
 	type SessionDuration = Period;
+	type UnsignedPriority = UnsignedPriority;
 }
 
 /// Im Online module.