Tx Payment: drop ED requirements for tx payments with exchangeable as…

…set (paritytech#4488)

Drop the Existential Deposit (ED) requirement for the asset amount
exchangeable for the fee asset (eg. DOT/KSM) during transaction
payments.

This achieved by using `SwapCredit` implementation of swap, which works
with imbalances and does not require a temporary balance account within
the transaction payment.

### Problem
Currently, every swap during transaction payment, processed with asset
`A` for native asset, must be for an amount greater than the ED of a
native asset if the user lacks a native asset account. Since fees are
typically smaller, the current implementation necessitates additional
swaps to meet the ED during `pre_dispatch`, with refunds for excess ED
swap occurring during `post_dispatch`. Further details can be found
[here](https://github.com/paritytech/polkadot-sdk/blob/115c2477eb287df55107cd95594100ba395ed239/substrate/frame/transaction-payment/asset-conversion-tx-payment/src/payment.rs#L115).

This setup presents an issue where a user is unable to transfer their
entire balance and close the account. Instead, the user must transfer
slightly less of asset `A` to ensure there is enough not only for the
fee payment but also some extra to meet the ED requirement for their
native account during `pre_dispatch`. In some cases during
`post_dispatch`, the user will have the excess ED swapped back to asset
`A`, while in other cases, it may not be sufficient to meet the ED
requirement for asset `A`, leading it being left in the user's
'temporary' native asset account.

---------

Co-authored-by: Oliver Tale-Yazdi <[email protected]>

cumulus/parachains/runtimes/assets/asset-hub-rococo/src/lib.rs

@@ -71,7 +71,7 @@ use frame_system::{
 	limits::{BlockLength, BlockWeights},
 	EnsureRoot, EnsureSigned, EnsureSignedBy,
 };
-use pallet_asset_conversion_tx_payment::AssetConversionAdapter;
+use pallet_asset_conversion_tx_payment::SwapAssetAdapter;
 use pallet_nfts::PalletFeatures;
 use parachains_common::{
 	impls::DealWithFees,
@@ -798,11 +798,19 @@ impl pallet_collator_selection::Config for Runtime {
 	type WeightInfo = weights::pallet_collator_selection::WeightInfo<Runtime>;
 }
 
+parameter_types! {
+	pub StakingPot: AccountId = CollatorSelection::account_id();
+}
+
 impl pallet_asset_conversion_tx_payment::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
-	type Fungibles = LocalAndForeignAssets;
-	type OnChargeAssetTransaction =
-		AssetConversionAdapter<Balances, AssetConversion, TokenLocationV3>;
+	type AssetId = xcm::v3::Location;
+	type OnChargeAssetTransaction = SwapAssetAdapter<
+		TokenLocationV3,
+		NativeAndAssets,
+		AssetConversion,
+		ResolveAssetTo<StakingPot, NativeAndAssets>,
+	>;
 }
 
 parameter_types! {

cumulus/parachains/runtimes/assets/asset-hub-westend/src/lib.rs

@@ -55,7 +55,7 @@ use frame_system::{
 	limits::{BlockLength, BlockWeights},
 	EnsureRoot, EnsureSigned, EnsureSignedBy,
 };
-use pallet_asset_conversion_tx_payment::AssetConversionAdapter;
+use pallet_asset_conversion_tx_payment::SwapAssetAdapter;
 use pallet_nfts::{DestroyWitness, PalletFeatures};
 use pallet_xcm::EnsureXcm;
 use parachains_common::{
@@ -787,11 +787,19 @@ impl pallet_collator_selection::Config for Runtime {
 	type WeightInfo = weights::pallet_collator_selection::WeightInfo<Runtime>;
 }
 
+parameter_types! {
+	pub StakingPot: AccountId = CollatorSelection::account_id();
+}
+
 impl pallet_asset_conversion_tx_payment::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
-	type Fungibles = LocalAndForeignAssets;
-	type OnChargeAssetTransaction =
-		AssetConversionAdapter<Balances, AssetConversion, WestendLocationV3>;
+	type AssetId = xcm::v3::Location;
+	type OnChargeAssetTransaction = SwapAssetAdapter<
+		WestendLocationV3,
+		NativeAndAssets,
+		AssetConversion,
+		ResolveAssetTo<StakingPot, NativeAndAssets>,
+	>;
 }
 
 parameter_types! {

prdoc/pr_4488.prdoc

@@ -0,0 +1,25 @@
+title: "Tx Payment: drop ED requirements for tx payments with exchangeable asset"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+        Drop the Existential Deposit requirement for the asset amount exchangeable for the fee asset
+        (eg. DOT/KSM) during transaction payments.
+
+        This achieved by using `SwapCredit` implementation of asset conversion, which works with
+        imbalances and does not require a temporary balance account within the transaction payment.
+
+        This is a breaking change for the `pallet-asset-conversion-tx-payment` pallet, use examples 
+        from PR for the migration.
+
+crates:
+  - name: pallet-asset-conversion-tx-payment
+    bump: major
+  - name: pallet-transaction-payment
+    bump: patch
+  - name: pallet-asset-conversion
+    bump: patch
+  - name: asset-hub-rococo-runtime
+    bump: patch
+  - name: asset-hub-westend-runtime
+    bump: patch

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

@@ -69,6 +69,7 @@ use frame_system::{
 pub use node_primitives::{AccountId, Signature};
 use node_primitives::{AccountIndex, Balance, BlockNumber, Hash, Moment, Nonce};
 use pallet_asset_conversion::{AccountIdConverter, Ascending, Chain, WithFirstAsset};
+use pallet_asset_conversion_tx_payment::SwapAssetAdapter;
 use pallet_broker::{CoreAssignment, CoreIndex, CoretimeInterface, PartsOf57600};
 use pallet_election_provider_multi_phase::{GeometricDepositBase, SolutionAccuracyOf};
 use pallet_identity::legacy::IdentityInfo;
@@ -123,7 +124,7 @@ pub use sp_runtime::BuildStorage;
 pub mod impls;
 #[cfg(not(feature = "runtime-benchmarks"))]
 use impls::AllianceIdentityVerifier;
-use impls::{AllianceProposalProvider, Author, CreditToBlockAuthor};
+use impls::{AllianceProposalProvider, Author};
 
 /// Constant values used within the runtime.
 pub mod constants;
@@ -574,22 +575,14 @@ impl pallet_transaction_payment::Config for Runtime {
 	>;
 }
 
-impl pallet_asset_tx_payment::Config for Runtime {
-	type RuntimeEvent = RuntimeEvent;
-	type Fungibles = Assets;
-	type OnChargeAssetTransaction = pallet_asset_tx_payment::FungiblesAdapter<
-		pallet_assets::BalanceToAssetBalance<Balances, Runtime, ConvertInto, Instance1>,
-		CreditToBlockAuthor,
-	>;
-}
-
 impl pallet_asset_conversion_tx_payment::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
-	type Fungibles = Assets;
-	type OnChargeAssetTransaction = pallet_asset_conversion_tx_payment::AssetConversionAdapter<
-		Balances,
-		AssetConversion,
+	type AssetId = NativeOrWithId<u32>;
+	type OnChargeAssetTransaction = SwapAssetAdapter<
 		Native,
+		NativeAndAssets,
+		AssetConversion,
+		ResolveAssetTo<TreasuryAccount, NativeAndAssets>,
 	>;
 }
 
@@ -1705,12 +1698,15 @@ parameter_types! {
 	pub const Native: NativeOrWithId<u32> = NativeOrWithId::Native;
 }
 
+pub type NativeAndAssets =
+	UnionOf<Balances, Assets, NativeFromLeft, NativeOrWithId<u32>, AccountId>;
+
 impl pallet_asset_conversion::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Balance = u128;
 	type HigherPrecisionBalance = sp_core::U256;
 	type AssetKind = NativeOrWithId<u32>;
-	type Assets = UnionOf<Balances, Assets, NativeFromLeft, NativeOrWithId<u32>, AccountId>;
+	type Assets = NativeAndAssets;
 	type PoolId = (Self::AssetKind, Self::AssetKind);
 	type PoolLocator = Chain<
 		WithFirstAsset<
@@ -2259,9 +2255,6 @@ mod runtime {
 	#[runtime::pallet_index(7)]
 	pub type TransactionPayment = pallet_transaction_payment::Pallet<Runtime>;
 
-	#[runtime::pallet_index(8)]
-	pub type AssetTxPayment = pallet_asset_tx_payment::Pallet<Runtime>;
-
 	#[runtime::pallet_index(9)]
 	pub type AssetConversionTxPayment = pallet_asset_conversion_tx_payment::Pallet<Runtime>;
 

substrate/frame/asset-conversion/src/swap.rs

@@ -112,6 +112,37 @@ pub trait SwapCredit<AccountId> {
 	) -> Result<(Self::Credit, Self::Credit), (Self::Credit, DispatchError)>;
 }
 
+/// Trait providing methods to quote swap prices between asset classes.
+///
+/// The quoted price is only guaranteed if no other swaps are made after the price is quoted and
+/// before the target swap (e.g., the swap is made immediately within the same transaction).
+pub trait QuotePrice {
+	/// Measurement units of the asset classes for pricing.
+	type Balance: Balance;
+	/// Type representing the kind of assets for which the price is being quoted.
+	type AssetKind;
+	/// Quotes the amount of `asset1` required to obtain the exact `amount` of `asset2`.
+	///
+	/// If `include_fee` is set to `true`, the price will include the pool's fee.
+	/// If the pool does not exist or the swap cannot be made, `None` is returned.
+	fn quote_price_tokens_for_exact_tokens(
+		asset1: Self::AssetKind,
+		asset2: Self::AssetKind,
+		amount: Self::Balance,
+		include_fee: bool,
+	) -> Option<Self::Balance>;
+	/// Quotes the amount of `asset2` resulting from swapping the exact `amount` of `asset1`.
+	///
+	/// If `include_fee` is set to `true`, the price will include the pool's fee.
+	/// If the pool does not exist or the swap cannot be made, `None` is returned.
+	fn quote_price_exact_tokens_for_tokens(
+		asset1: Self::AssetKind,
+		asset2: Self::AssetKind,
+		amount: Self::Balance,
+		include_fee: bool,
+	) -> Option<Self::Balance>;
+}
+
 impl<T: Config> Swap<T::AccountId> for Pallet<T> {
 	type Balance = T::Balance;
 	type AssetKind = T::AssetKind;
@@ -210,3 +241,24 @@ impl<T: Config> SwapCredit<T::AccountId> for Pallet<T> {
 		.map_err(|_| (Self::Credit::zero(credit_asset), DispatchError::Corruption))?
 	}
 }
+
+impl<T: Config> QuotePrice for Pallet<T> {
+	type Balance = T::Balance;
+	type AssetKind = T::AssetKind;
+	fn quote_price_exact_tokens_for_tokens(
+		asset1: Self::AssetKind,
+		asset2: Self::AssetKind,
+		amount: Self::Balance,
+		include_fee: bool,
+	) -> Option<Self::Balance> {
+		Self::quote_price_exact_tokens_for_tokens(asset1, asset2, amount, include_fee)
+	}
+	fn quote_price_tokens_for_exact_tokens(
+		asset1: Self::AssetKind,
+		asset2: Self::AssetKind,
+		amount: Self::Balance,
+		include_fee: bool,
+	) -> Option<Self::Balance> {
+		Self::quote_price_tokens_for_exact_tokens(asset1, asset2, amount, include_fee)
+	}
+}