Adds syntax for marking calls feeless

Cargo.toml

@@ -382,6 +382,7 @@ members = [
 	"substrate/frame/transaction-payment/asset-tx-payment",
 	"substrate/frame/transaction-payment/rpc",
 	"substrate/frame/transaction-payment/rpc/runtime-api",
+	"substrate/frame/transaction-payment/skip-feeless-payment",
 	"substrate/frame/transaction-storage",
 	"substrate/frame/treasury",
 	"substrate/frame/try-runtime",

prdoc/pr_1926.prdoc

@@ -0,0 +1,30 @@
+title: Adds syntax for marking calls feeless
+
+doc:
+  - audience: Core Dev
+    description: |
+      1. Adds an attribute `#[pallet::feeless_if]` that can be optionally attached to a `pallet::call`.
+      2. Adds a signed extension SkipCheckIfFeeless<T: SignedExtension> that wraps a transaction 
+      payment processor to potentially skip payment fees for such calls.
+      Note that both the attribute and the signed extension are needed to make the call feeless.
+
+migrations:
+  db: []
+
+  runtime: []
+
+crates:
+  - name: "frame-support-procedural"
+    semver: minor
+  - name: "pallet-skip-feeless-payment"
+    semver: major
+  - pallet-example-kitchensink
+    semver: patch
+  - kitchensink-runtime
+    semver: major
+  - node-testing
+    semver: patch
+  - node-cli
+    semver: patch
+
+host_functions: []

substrate/bin/node/cli/Cargo.toml

@@ -96,6 +96,7 @@ pallet-assets = { path = "../../../frame/assets" }
 pallet-asset-conversion-tx-payment = { path = "../../../frame/transaction-payment/asset-conversion-tx-payment" }
 pallet-asset-tx-payment = { path = "../../../frame/transaction-payment/asset-tx-payment" }
 pallet-im-online = { path = "../../../frame/im-online", default-features = false}
+pallet-skip-feeless-payment = { path = "../../../frame/transaction-payment/skip-feeless-payment", default-features = false}
 
 # node-specific dependencies
 kitchensink-runtime = { path = "../runtime" }
@@ -168,6 +169,7 @@ runtime-benchmarks = [
 	"pallet-assets/runtime-benchmarks",
 	"pallet-balances/runtime-benchmarks",
 	"pallet-im-online/runtime-benchmarks",
+	"pallet-skip-feeless-payment/runtime-benchmarks",
 	"pallet-timestamp/runtime-benchmarks",
 	"sc-client-db/runtime-benchmarks",
 	"sc-service/runtime-benchmarks",
@@ -183,6 +185,7 @@ try-runtime = [
 	"pallet-assets/try-runtime",
 	"pallet-balances/try-runtime",
 	"pallet-im-online/try-runtime",
+	"pallet-skip-feeless-payment/try-runtime",
 	"pallet-timestamp/try-runtime",
 	"sp-runtime/try-runtime",
 	"substrate-cli-test-utils/try-runtime",

substrate/bin/node/cli/src/service.rs

@@ -91,21 +91,24 @@ pub fn create_extrinsic(
 		.map(|c| c / 2)
 		.unwrap_or(2) as u64;
 	let tip = 0;
-	let extra: kitchensink_runtime::SignedExtra = (
-		frame_system::CheckNonZeroSender::<kitchensink_runtime::Runtime>::new(),
-		frame_system::CheckSpecVersion::<kitchensink_runtime::Runtime>::new(),
-		frame_system::CheckTxVersion::<kitchensink_runtime::Runtime>::new(),
-		frame_system::CheckGenesis::<kitchensink_runtime::Runtime>::new(),
-		frame_system::CheckEra::<kitchensink_runtime::Runtime>::from(generic::Era::mortal(
-			period,
-			best_block.saturated_into(),
-		)),
-		frame_system::CheckNonce::<kitchensink_runtime::Runtime>::from(nonce),
-		frame_system::CheckWeight::<kitchensink_runtime::Runtime>::new(),
-		pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<kitchensink_runtime::Runtime>::from(
-			tip, None,
-		),
-	);
+	let extra: kitchensink_runtime::SignedExtra =
+		(
+			frame_system::CheckNonZeroSender::<kitchensink_runtime::Runtime>::new(),
+			frame_system::CheckSpecVersion::<kitchensink_runtime::Runtime>::new(),
+			frame_system::CheckTxVersion::<kitchensink_runtime::Runtime>::new(),
+			frame_system::CheckGenesis::<kitchensink_runtime::Runtime>::new(),
+			frame_system::CheckEra::<kitchensink_runtime::Runtime>::from(generic::Era::mortal(
+				period,
+				best_block.saturated_into(),
+			)),
+			frame_system::CheckNonce::<kitchensink_runtime::Runtime>::from(nonce),
+			frame_system::CheckWeight::<kitchensink_runtime::Runtime>::new(),
+			pallet_skip_feeless_payment::SkipCheckIfFeeless::from(
+				pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<
+					kitchensink_runtime::Runtime,
+				>::from(tip, None),
+			),
+		);
 
 	let raw_payload = kitchensink_runtime::SignedPayload::from_raw(
 		function.clone(),
@@ -879,8 +882,9 @@ mod tests {
 				let check_era = frame_system::CheckEra::from(Era::Immortal);
 				let check_nonce = frame_system::CheckNonce::from(index);
 				let check_weight = frame_system::CheckWeight::new();
-				let tx_payment =
-					pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::from(0, None);
+				let tx_payment = pallet_skip_feeless_payment::SkipCheckIfFeeless::from(
+					pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::from(0, None),
+				);
 				let extra = (
 					check_non_zero_sender,
 					check_spec_version,

substrate/bin/node/runtime/Cargo.toml

@@ -129,6 +129,7 @@ pallet-transaction-payment = { path = "../../../frame/transaction-payment", defa
 pallet-transaction-payment-rpc-runtime-api = { path = "../../../frame/transaction-payment/rpc/runtime-api", default-features = false}
 pallet-asset-conversion-tx-payment = { path = "../../../frame/transaction-payment/asset-conversion-tx-payment", default-features = false}
 pallet-asset-tx-payment = { path = "../../../frame/transaction-payment/asset-tx-payment", default-features = false}
+pallet-skip-feeless-payment = { path = "../../../frame/transaction-payment/skip-feeless-payment", default-features = false}
 pallet-transaction-storage = { path = "../../../frame/transaction-storage", default-features = false}
 pallet-uniques = { path = "../../../frame/uniques", default-features = false}
 pallet-vesting = { path = "../../../frame/vesting", default-features = false}
@@ -212,6 +213,7 @@ std = [
 	"pallet-scheduler/std",
 	"pallet-session-benchmarking?/std",
 	"pallet-session/std",
+	"pallet-skip-feeless-payment/std",
 	"pallet-society/std",
 	"pallet-staking-runtime-api/std",
 	"pallet-staking/std",
@@ -308,6 +310,7 @@ runtime-benchmarks = [
 	"pallet-salary/runtime-benchmarks",
 	"pallet-scheduler/runtime-benchmarks",
 	"pallet-session-benchmarking/runtime-benchmarks",
+	"pallet-skip-feeless-payment/runtime-benchmarks",
 	"pallet-society/runtime-benchmarks",
 	"pallet-staking/runtime-benchmarks",
 	"pallet-state-trie-migration/runtime-benchmarks",
@@ -381,6 +384,7 @@ try-runtime = [
 	"pallet-salary/try-runtime",
 	"pallet-scheduler/try-runtime",
 	"pallet-session/try-runtime",
+	"pallet-skip-feeless-payment/try-runtime",
 	"pallet-society/try-runtime",
 	"pallet-staking/try-runtime",
 	"pallet-state-trie-migration/try-runtime",

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

@@ -569,6 +569,10 @@ impl pallet_asset_conversion_tx_payment::Config for Runtime {
 		pallet_asset_conversion_tx_payment::AssetConversionAdapter<Balances, AssetConversion>;
 }
 
+impl pallet_skip_feeless_payment::Config for Runtime {
+	type RuntimeEvent = RuntimeEvent;
+}
+
 parameter_types! {
 	pub const MinimumPeriod: Moment = SLOT_DURATION / 2;
 }
@@ -1394,7 +1398,11 @@ where
 			frame_system::CheckEra::<Runtime>::from(era),
 			frame_system::CheckNonce::<Runtime>::from(nonce),
 			frame_system::CheckWeight::<Runtime>::new(),
-			pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<Runtime>::from(tip, None),
+			pallet_skip_feeless_payment::SkipCheckIfFeeless::from(
+				pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::<Runtime>::from(
+					tip, None,
+				),
+			),
 		);
 		let raw_payload = SignedPayload::new(call, extra)
 			.map_err(|e| {
@@ -2134,6 +2142,7 @@ construct_runtime!(
 		Statement: pallet_statement,
 		Broker: pallet_broker,
 		Mixnet: pallet_mixnet,
+		SkipFeelessPayment: pallet_skip_feeless_payment,
 	}
 );
 
@@ -2160,7 +2169,10 @@ pub type SignedExtra = (
 	frame_system::CheckEra<Runtime>,
 	frame_system::CheckNonce<Runtime>,
 	frame_system::CheckWeight<Runtime>,
-	pallet_asset_conversion_tx_payment::ChargeAssetTxPayment<Runtime>,
+	pallet_skip_feeless_payment::SkipCheckIfFeeless<
+		Runtime,
+		pallet_asset_conversion_tx_payment::ChargeAssetTxPayment<Runtime>,
+	>,
 );
 
 /// Unchecked extrinsic type as expected by this runtime.

substrate/bin/node/testing/Cargo.toml

@@ -26,6 +26,7 @@ pallet-asset-conversion = { path = "../../../frame/asset-conversion" }
 pallet-assets = { path = "../../../frame/assets" }
 pallet-asset-conversion-tx-payment = { path = "../../../frame/transaction-payment/asset-conversion-tx-payment" }
 pallet-asset-tx-payment = { path = "../../../frame/transaction-payment/asset-tx-payment" }
+pallet-skip-feeless-payment = { path = "../../../frame/transaction-payment/skip-feeless-payment" }
 sc-block-builder = { path = "../../../client/block-builder" }
 sc-client-api = { path = "../../../client/api" }
 sc-client-db = { path = "../../../client/db", features = ["rocksdb"]}

substrate/bin/node/testing/src/keyring.rs

@@ -78,7 +78,9 @@ pub fn signed_extra(nonce: Nonce, extra_fee: Balance) -> SignedExtra {
 		frame_system::CheckEra::from(Era::mortal(256, 0)),
 		frame_system::CheckNonce::from(nonce),
 		frame_system::CheckWeight::new(),
-		pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::from(extra_fee, None),
+		pallet_skip_feeless_payment::SkipCheckIfFeeless::from(
+			pallet_asset_conversion_tx_payment::ChargeAssetTxPayment::from(extra_fee, None),
+		),
 	)
 }
 

substrate/frame/examples/kitchensink/src/lib.rs

@@ -206,6 +206,10 @@ pub mod pallet {
 	impl<T: Config> Pallet<T> {
 		#[pallet::call_index(0)]
 		#[pallet::weight(T::WeightInfo::set_foo_benchmark())]
+		/// Marks this call as feeless if `new_foo` is zero.
+		#[pallet::feeless_if(|_origin: &OriginFor<T>, new_foo: &u32, _other_compact: &u128| -> bool {
+			*new_foo == 0
+		})]
 		pub fn set_foo(
 			_: OriginFor<T>,
 			new_foo: u32,

substrate/frame/support/procedural/src/construct_runtime/expand/call.rs

@@ -124,6 +124,18 @@ pub fn expand_outer_dispatch(
 			}
 		}
 
+		impl #scrate::dispatch::CheckIfFeeless for RuntimeCall {
+			type Origin = #system_path::pallet_prelude::OriginFor<#runtime>;
+			fn is_feeless(&self, origin: &Self::Origin) -> bool {
+				match self {
+					#(
+						#pallet_attrs
+						#variant_patterns => call.is_feeless(origin),
+					)*
+				}
+			}
+		}
+
 		impl #scrate::traits::GetCallMetadata for RuntimeCall {
 			fn get_call_metadata(&self) -> #scrate::traits::CallMetadata {
 				use #scrate::traits::GetCallName;

substrate/frame/support/procedural/src/lib.rs

@@ -1157,6 +1157,36 @@ pub fn call_index(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()
 }
 
+/// Each dispatchable may be annotated with the `#[pallet::feeless_if($closure)]` attribute,
+/// which explicitly defines the condition for the dispatchable to be feeless.
+///
+/// The arguments for the closure must be the referenced arguments of the dispatchable function.
+///
+/// The closure must return `bool`.
+///
+/// ### Example
+/// ```ignore
+/// #[pallet::feeless_if(|_origin: &OriginFor<T>, something: &u32| -> bool {
+/// 		*something == 0
+/// 	})]
+/// pub fn do_something(origin: OriginFor<T>, something: u32) -> DispatchResult {
+///     ....
+/// }
+/// ```
+///
+/// Please note that this only works for signed dispatchables and requires a signed extension
+/// such as `SkipCheckIfFeeless` as defined in `pallet-skip-feeless-payment` to wrap the existing
+/// payment extension. Else, this is completely ignored and the dispatchable is still charged.
+///
+/// ### Macro expansion
+///
+/// The macro implements the `CheckIfFeeless` trait on the dispatchable and calls the corresponding
+/// closure in the implementation.
+#[proc_macro_attribute]
+pub fn feeless_if(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
 /// Allows you to define some extra constants to be added into constant metadata.
 ///
 /// Item must be defined as:

substrate/frame/support/procedural/src/pallet/expand/call.rs

@@ -241,6 +241,16 @@ pub fn expand_call(def: &mut Def) -> proc_macro2::TokenStream {
 		})
 		.collect::<Vec<_>>();
 
+	let feeless_check = methods.iter().map(|method| &method.feeless_check).collect::<Vec<_>>();
+	let feeless_check_result =
+		feeless_check.iter().zip(args_name.iter()).map(|(feeless_check, arg_name)| {
+			if let Some(feeless_check) = feeless_check {
+				quote::quote!(#feeless_check(origin, #( #arg_name, )*))
+			} else {
+				quote::quote!(false)
+			}
+		});
+
 	quote::quote_spanned!(span =>
 		mod warnings {
 			#(
@@ -347,6 +357,23 @@ pub fn expand_call(def: &mut Def) -> proc_macro2::TokenStream {
 			}
 		}
 
+		impl<#type_impl_gen> #frame_support::dispatch::CheckIfFeeless for #call_ident<#type_use_gen>
+			#where_clause
+		{
+			type Origin = #frame_system::pallet_prelude::OriginFor<T>;
+			#[allow(unused_variables)]
+			fn is_feeless(&self, origin: &Self::Origin) -> bool {
+				match *self {
+					#(
+						Self::#fn_name { #( #args_name_pattern_ref, )* } => {
+							#feeless_check_result
+						},
+					)*
+					Self::__Ignore(_, _) => unreachable!("__Ignore cannot be used"),
+				}
+			}
+		}
+
 		impl<#type_impl_gen> #frame_support::traits::GetCallName for #call_ident<#type_use_gen>
 			#where_clause
 		{