Allow PostDispatchInfo to disable fees (#6749)

* initial mock

* add test

* remove unneeded clone

* Update frame/support/src/weights.rs

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

* fix compile

* Update frame/support/src/weights.rs

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/sudo/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Alexander Theißen <alex.theissen@me.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/contracts/src/gas.rs

@@ -194,6 +194,7 @@ impl<T: Trait> GasMeter<T> {
 	{
 		let post_info = PostDispatchInfo {
 			actual_weight: Some(self.gas_spent()),
+			pays_fee: Default::default(),
 		};
 
 		result

substrate/frame/contracts/src/tests.rs

@@ -291,6 +291,7 @@ fn returns_base_call_cost() {
 			Ok(
 				PostDispatchInfo {
 					actual_weight: Some(67500000),
+					pays_fee: Default::default(),
 				}
 			)
 		);

substrate/frame/sudo/src/lib.rs

@@ -93,7 +93,11 @@ use sp_runtime::{DispatchResult, traits::StaticLookup};
 use frame_support::{
 	Parameter, decl_module, decl_event, decl_storage, decl_error, ensure,
 };
-use frame_support::{weights::{Weight, GetDispatchInfo}, traits::UnfilteredDispatchable};
+use frame_support::{
+	weights::{Weight, GetDispatchInfo, Pays},
+	traits::UnfilteredDispatchable,
+	dispatch::DispatchResultWithPostInfo,
+};
 use frame_system::ensure_signed;
 
 #[cfg(test)]
@@ -127,13 +131,15 @@ decl_module! {
 		/// - Weight of derivative `call` execution + 10,000.
 		/// # </weight>
 		#[weight = (call.get_dispatch_info().weight + 10_000, call.get_dispatch_info().class)]
-		fn sudo(origin, call: Box<<T as Trait>::Call>) {
+		fn sudo(origin, call: Box<<T as Trait>::Call>) -> DispatchResultWithPostInfo {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let sender = ensure_signed(origin)?;
 			ensure!(sender == Self::key(), Error::<T>::RequireSudo);
 
 			let res = call.dispatch_bypass_filter(frame_system::RawOrigin::Root.into());
 			Self::deposit_event(RawEvent::Sudid(res.map(|_| ()).map_err(|e| e.error)));
+			// Sudo user does not pay a fee.
+			Ok(Pays::No.into())
 		}
 
 		/// Authenticates the sudo key and dispatches a function call with `Root` origin.
@@ -147,13 +153,15 @@ decl_module! {
 		/// - The weight of this call is defined by the caller.
 		/// # </weight>
 		#[weight = (*_weight, call.get_dispatch_info().class)]
-		fn sudo_unchecked_weight(origin, call: Box<<T as Trait>::Call>, _weight: Weight) {
+		fn sudo_unchecked_weight(origin, call: Box<<T as Trait>::Call>, _weight: Weight) -> DispatchResultWithPostInfo {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let sender = ensure_signed(origin)?;
 			ensure!(sender == Self::key(), Error::<T>::RequireSudo);
 
 			let res = call.dispatch_bypass_filter(frame_system::RawOrigin::Root.into());
 			Self::deposit_event(RawEvent::Sudid(res.map(|_| ()).map_err(|e| e.error)));
+			// Sudo user does not pay a fee.
+			Ok(Pays::No.into())
 		}
 
 		/// Authenticates the current sudo key and sets the given AccountId (`new`) as the new sudo key.
@@ -166,7 +174,7 @@ decl_module! {
 		/// - One DB change.
 		/// # </weight>
 		#[weight = 0]
-		fn set_key(origin, new: <T::Lookup as StaticLookup>::Source) {
+		fn set_key(origin, new: <T::Lookup as StaticLookup>::Source) -> DispatchResultWithPostInfo {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let sender = ensure_signed(origin)?;
 			ensure!(sender == Self::key(), Error::<T>::RequireSudo);
@@ -174,6 +182,8 @@ decl_module! {
 
 			Self::deposit_event(RawEvent::KeyChanged(Self::key()));
 			<Key<T>>::put(new);
+			// Sudo user does not pay a fee.
+			Ok(Pays::No.into())
 		}
 
 		/// Authenticates the sudo key and dispatches a function call with `Signed` origin from
@@ -188,7 +198,10 @@ decl_module! {
 		/// - Weight of derivative `call` execution + 10,000.
 		/// # </weight>
 		#[weight = (call.get_dispatch_info().weight + 10_000, call.get_dispatch_info().class)]
-		fn sudo_as(origin, who: <T::Lookup as StaticLookup>::Source, call: Box<<T as Trait>::Call>) {
+		fn sudo_as(origin,
+			who: <T::Lookup as StaticLookup>::Source,
+			call: Box<<T as Trait>::Call>
+		) -> DispatchResultWithPostInfo {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let sender = ensure_signed(origin)?;
 			ensure!(sender == Self::key(), Error::<T>::RequireSudo);
@@ -204,6 +217,8 @@ decl_module! {
 			};
 
 			Self::deposit_event(RawEvent::SudoAsDone(res));
+			// Sudo user does not pay a fee.
+			Ok(Pays::No.into())
 		}
 	}
 }

substrate/frame/support/src/weights.rs

@@ -263,10 +263,13 @@ pub trait GetDispatchInfo {
 }
 
 /// Weight information that is only available post dispatch.
+/// NOTE: This can only be used to reduce the weight or fee, not increase it.
 #[derive(Clone, Copy, Eq, PartialEq, Default, RuntimeDebug, Encode, Decode)]
 pub struct PostDispatchInfo {
 	/// Actual weight consumed by a call or `None` which stands for the worst case static weight.
 	pub actual_weight: Option<Weight>,
+	/// Whether this transaction should pay fees when all is said and done.
+	pub pays_fee: Pays,
 }
 
 impl PostDispatchInfo {
@@ -283,6 +286,20 @@ impl PostDispatchInfo {
 			info.weight
 		}
 	}
+
+	/// Determine if user should actually pay fees at the end of the dispatch.
+	pub fn pays_fee(&self, info: &DispatchInfo) -> Pays {
+		// If they originally were not paying fees, or the post dispatch info
+		// says they should not pay fees, then they don't pay fees.
+		// This is because the pre dispatch information must contain the
+		// worst case for weight and fees paid.
+		if info.pays_fee == Pays::No || self.pays_fee == Pays::No {
+			Pays::No
+		} else {
+			// Otherwise they pay.
+			Pays::Yes
+		}
+	}
 }
 
 /// Extract the actual weight from a dispatch result if any or fall back to the default weight.
@@ -293,10 +310,30 @@ pub fn extract_actual_weight(result: &DispatchResultWithPostInfo, info: &Dispatc
 	}.calc_actual_weight(info)
 }
 
+impl From<(Option<Weight>, Pays)> for PostDispatchInfo {
+	fn from(post_weight_info: (Option<Weight>, Pays)) -> Self {
+		let (actual_weight, pays_fee) = post_weight_info;
+		Self {
+			actual_weight,
+			pays_fee,
+		}
+	}
+}
+
+impl From<Pays> for PostDispatchInfo {
+	fn from(pays_fee: Pays) -> Self {
+		Self {
+			actual_weight: None,
+			pays_fee,
+		}
+	}
+}
+
 impl From<Option<Weight>> for PostDispatchInfo {
 	fn from(actual_weight: Option<Weight>) -> Self {
 		Self {
 			actual_weight,
+			pays_fee: Default::default(),
 		}
 	}
 }
@@ -305,6 +342,7 @@ impl From<()> for PostDispatchInfo {
 	fn from(_: ()) -> Self {
 		Self {
 			actual_weight: None,
+			pays_fee: Default::default(),
 		}
 	}
 }
@@ -315,6 +353,11 @@ impl sp_runtime::traits::Printable for PostDispatchInfo {
 		match self.actual_weight {
 			Some(weight) => weight.print(),
 			None => "max-weight".print(),
+		};
+		"pays_fee=".print();
+		match self.pays_fee {
+			Pays::Yes => "Yes".print(),
+			Pays::No => "No".print(),
 		}
 	}
 }
@@ -338,7 +381,10 @@ impl<T> WithPostDispatchInfo for T where
 {
 	fn with_weight(self, actual_weight: Weight) -> DispatchErrorWithPostInfo {
 		DispatchErrorWithPostInfo {
-			post_info: PostDispatchInfo { actual_weight: Some(actual_weight) },
+			post_info: PostDispatchInfo {
+				actual_weight: Some(actual_weight),
+				pays_fee: Default::default(),
+			},
 			error: self.into(),
 		}
 	}

substrate/frame/system/src/extensions/check_weight.rs

@@ -575,7 +575,10 @@ mod tests {
 		new_test_ext().execute_with(|| {
 			// This is half of the max block weight
 			let info = DispatchInfo { weight: 512, ..Default::default() };
-			let post_info = PostDispatchInfo { actual_weight: Some(128), };
+			let post_info = PostDispatchInfo {
+				actual_weight: Some(128),
+				pays_fee: Default::default(),
+			};
 			let len = 0_usize;
 
 			// We allow 75% for normal transaction, so we put 25% - extrinsic base weight
@@ -601,7 +604,10 @@ mod tests {
 	fn signed_ext_check_weight_actual_weight_higher_than_max_is_capped() {
 		new_test_ext().execute_with(|| {
 			let info = DispatchInfo { weight: 512, ..Default::default() };
-			let post_info = PostDispatchInfo { actual_weight: Some(700), };
+			let post_info = PostDispatchInfo {
+				actual_weight: Some(700),
+				pays_fee: Default::default(),
+			};
 			let len = 0_usize;
 
 			BlockWeight::mutate(|current_weight| {

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

@@ -371,7 +371,7 @@ impl<T: Trait> Module<T> where
 	) -> BalanceOf<T> where
 		T::Call: Dispatchable<Info=DispatchInfo,PostInfo=PostDispatchInfo>,
 	{
-		Self::compute_fee_raw(len, post_info.calc_actual_weight(info), tip, info.pays_fee)
+		Self::compute_fee_raw(len, post_info.calc_actual_weight(info), tip, post_info.pays_fee(info))
 	}
 
 	fn compute_fee_raw(
@@ -762,11 +762,24 @@ mod tests {
 	}
 
 	fn post_info_from_weight(w: Weight) -> PostDispatchInfo {
-		PostDispatchInfo { actual_weight: Some(w), }
+		PostDispatchInfo {
+			actual_weight: Some(w),
+			pays_fee: Default::default(),
+		}
+	}
+
+	fn post_info_from_pays(p: Pays) -> PostDispatchInfo {
+		PostDispatchInfo {
+			actual_weight: None,
+			pays_fee: p,
+		}
 	}
 
 	fn default_post_info() -> PostDispatchInfo {
-		PostDispatchInfo { actual_weight: None, }
+		PostDispatchInfo {
+			actual_weight: None,
+			pays_fee: Default::default(),
+		}
 	}
 
 	#[test]
@@ -1211,4 +1224,38 @@ mod tests {
 			assert_eq!(refund_based_fee, actual_fee);
 		});
 	}
+
+	#[test]
+	fn post_info_can_change_pays_fee() {
+		ExtBuilder::default()
+			.balance_factor(10)
+			.base_weight(7)
+			.build()
+			.execute_with(||
+		{
+			let info = info_from_weight(100);
+			let post_info = post_info_from_pays(Pays::No);
+			let prev_balance = Balances::free_balance(2);
+			let len = 10;
+			let tip = 5;
+
+			NextFeeMultiplier::put(Multiplier::saturating_from_rational(5, 4));
+
+			let pre = ChargeTransactionPayment::<Runtime>::from(tip)
+				.pre_dispatch(&2, CALL, &info, len)
+				.unwrap();
+
+			ChargeTransactionPayment::<Runtime>
+				::post_dispatch(pre, &info, &post_info, len, &Ok(()))
+				.unwrap();
+
+			let refund_based_fee = prev_balance - Balances::free_balance(2);
+			let actual_fee = Module::<Runtime>
+				::compute_actual_fee(len as u32, &info, &post_info, tip);
+
+			// Only 5 tip is paid
+			assert_eq!(actual_fee, 5);
+			assert_eq!(refund_based_fee, actual_fee);
+		});
+	}
 }