Add `ConstFeeMultiplier` to the transaction payment pallet (#12222)

* fix: FeeMultiplierUpdate

* fix: cargo fmt

* fix: rustdoc

* Revert "fix: rustdoc"

This reverts commit 96b6ad80a4cd4d856cf5a830889858c4dd4c385b.

* Revert "fix: cargo fmt"

This reverts commit 13016527bdbc53d9484642d6b52430550c0efc55.

* Revert "fix: FeeMultiplierUpdate"

This reverts commit 2cbddd0b85e0293d0eeda859807ddf70cee29067.

* feat: add ConstFeeMultiplier

* fix: use cConstFeeMultiplier in the template node

substrate/bin/node-template/runtime/src/lib.rs

@@ -14,7 +14,9 @@ use sp_consensus_aura::sr25519::AuthorityId as AuraId;
 use sp_core::{crypto::KeyTypeId, OpaqueMetadata};
 use sp_runtime::{
 	create_runtime_str, generic, impl_opaque_keys,
-	traits::{AccountIdLookup, BlakeTwo256, Block as BlockT, IdentifyAccount, NumberFor, Verify},
+	traits::{
+		AccountIdLookup, BlakeTwo256, Block as BlockT, IdentifyAccount, NumberFor, One, Verify,
+	},
 	transaction_validity::{TransactionSource, TransactionValidity},
 	ApplyExtrinsicResult, MultiSignature,
 };
@@ -38,7 +40,7 @@ pub use frame_support::{
 pub use frame_system::Call as SystemCall;
 pub use pallet_balances::Call as BalancesCall;
 pub use pallet_timestamp::Call as TimestampCall;
-use pallet_transaction_payment::CurrencyAdapter;
+use pallet_transaction_payment::{ConstFeeMultiplier, CurrencyAdapter, Multiplier};
 #[cfg(any(feature = "std", test))]
 pub use sp_runtime::BuildStorage;
 pub use sp_runtime::{Perbill, Permill};
@@ -251,13 +253,17 @@ impl pallet_balances::Config for Runtime {
 	type WeightInfo = pallet_balances::weights::SubstrateWeight<Runtime>;
 }
 
+parameter_types! {
+	pub FeeMultiplier: Multiplier = Multiplier::one();
+}
+
 impl pallet_transaction_payment::Config for Runtime {
 	type Event = Event;
 	type OnChargeTransaction = CurrencyAdapter<Balances, ()>;
 	type OperationalFeeMultiplier = ConstU8<5>;
 	type WeightToFee = IdentityFee<Balance>;
 	type LengthToFee = IdentityFee<Balance>;
-	type FeeMultiplierUpdate = ();
+	type FeeMultiplierUpdate = ConstFeeMultiplier<FeeMultiplier>;
 }
 
 impl pallet_sudo::Config for Runtime {

substrate/frame/transaction-payment/src/lib.rs

@@ -226,6 +226,30 @@ where
 	}
 }
 
+/// A struct to make the fee multiplier a constant
+pub struct ConstFeeMultiplier<M: Get<Multiplier>>(sp_std::marker::PhantomData<M>);
+
+impl<M: Get<Multiplier>> MultiplierUpdate for ConstFeeMultiplier<M> {
+	fn min() -> Multiplier {
+		M::get()
+	}
+	fn target() -> Perquintill {
+		Default::default()
+	}
+	fn variability() -> Multiplier {
+		Default::default()
+	}
+}
+
+impl<M> Convert<Multiplier, Multiplier> for ConstFeeMultiplier<M>
+where
+	M: Get<Multiplier>,
+{
+	fn convert(_previous: Multiplier) -> Multiplier {
+		Self::min()
+	}
+}
+
 /// Storage releases of the pallet.
 #[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug, TypeInfo, MaxEncodedLen)]
 enum Releases {
@@ -371,7 +395,8 @@ pub mod pallet {
 			// add 1 percent;
 			let addition = target / 100;
 			if addition == Weight::zero() {
-				// this is most likely because in a test setup we set everything to ().
+				// this is most likely because in a test setup we set everything to ()
+				// or to `ConstFeeMultiplier`.
 				return
 			}