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Drop ED Requirement for Transaction Payment with Exchangeable Asset (p…
…olkadot-fellows#310) Drop the Existential Deposit (ED) requirement for the asset amount exchangeable for the fee asset (DOT/KSM) during transaction payments. 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. Example: https://assethub-polkadot.subscan.io/extrinsic/6204018-9?event=6204018-79 Given the importance of this scenario for CEX, I propose a solution that does not entail breaking changes to the pallets' API and open PR to the runtimes without waiting for new polkadot-sdk version. Additionally, I have opened a draft PR with these types in their respective pallets in FRAME, where they have been tested against existing tests for types implementing the same contract. PR - paritytech/polkadot-sdk#4455 Target implementation with breaking changes: paritytech/polkadot-sdk#4488 --------- Co-authored-by: joe petrowski <[email protected]>
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system-parachains/asset-hubs/asset-hub-kusama/src/impls.rs
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| // Copyright (C) Parity Technologies (UK) Ltd. | ||
| // SPDX-License-Identifier: Apache-2.0 | ||
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|
||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
|
|
||
| use crate::*; | ||
|
|
||
| // TODO: move implementations to the polkadot-sdk. | ||
| pub mod tx_payment { | ||
| use super::*; | ||
| use core::marker::PhantomData; | ||
| use frame_support::{ | ||
| ensure, | ||
| pallet_prelude::{InvalidTransaction, TransactionValidityError}, | ||
| traits::{ | ||
| fungibles::{Balanced as FungiblesBalanced, Inspect as FungiblesInspect}, | ||
| tokens::{Fortitude, Precision, Preservation}, | ||
| Defensive, OnUnbalanced, SameOrOther, | ||
| }, | ||
| }; | ||
| use pallet_asset_conversion::{Pallet as AssetConversion, SwapCredit}; | ||
| use pallet_asset_conversion_tx_payment::OnChargeAssetTransaction; | ||
| use pallet_transaction_payment::OnChargeTransaction; | ||
| use sp_core::Get; | ||
| use sp_runtime::{ | ||
| traits::{DispatchInfoOf, PostDispatchInfoOf, Zero}, | ||
| Saturating, | ||
| }; | ||
|
|
||
| /// Implements [`OnChargeTransaction`] for [`pallet_transaction_payment`], where the asset class | ||
| /// used to pay the fee is defined with the `A` type parameter (eg. KSM location) and accessed | ||
| /// via the type implementing the [`frame_support::traits::fungibles`] trait. | ||
| /// | ||
| /// This implementation with the `fungibles` trait is necessary to set up | ||
| /// [`pallet_asset_conversion_tx_payment`] with the [`SwapCreditAdapter`] type. For both types, | ||
| /// the credit types they handle must be the same, therefore they must be credits of | ||
| /// `fungibles`. | ||
| pub struct FungiblesAdapter<F, A, OU>(PhantomData<(F, A, OU)>); | ||
|
|
||
| impl<T, F, A, OU> OnChargeTransaction<T> for FungiblesAdapter<F, A, OU> | ||
| where | ||
| T: pallet_transaction_payment::Config, | ||
| F: fungibles::Balanced<T::AccountId>, | ||
| A: Get<F::AssetId>, | ||
| OU: OnUnbalanced<fungibles::Credit<T::AccountId, F>>, | ||
| { | ||
| type LiquidityInfo = Option<fungibles::Credit<T::AccountId, F>>; | ||
| type Balance = F::Balance; | ||
|
|
||
| fn withdraw_fee( | ||
| who: &<T>::AccountId, | ||
| _call: &<T>::RuntimeCall, | ||
| _dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>, | ||
| fee: Self::Balance, | ||
| _tip: Self::Balance, | ||
| ) -> Result<Self::LiquidityInfo, TransactionValidityError> { | ||
| if fee.is_zero() { | ||
| return Ok(None) | ||
| } | ||
|
|
||
| match F::withdraw( | ||
| A::get(), | ||
| who, | ||
| fee, | ||
| Precision::Exact, | ||
| Preservation::Preserve, | ||
| Fortitude::Polite, | ||
| ) { | ||
| Ok(imbalance) => Ok(Some(imbalance)), | ||
| Err(_) => Err(InvalidTransaction::Payment.into()), | ||
| } | ||
| } | ||
|
|
||
| fn correct_and_deposit_fee( | ||
| who: &<T>::AccountId, | ||
| _dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>, | ||
| _post_info: &PostDispatchInfoOf<<T>::RuntimeCall>, | ||
| corrected_fee: Self::Balance, | ||
| _tip: Self::Balance, | ||
| already_withdrawn: Self::LiquidityInfo, | ||
| ) -> Result<(), TransactionValidityError> { | ||
| let Some(paid) = already_withdrawn else { | ||
| return Ok(()); | ||
| }; | ||
| // Make sure the credit is in desired asset id. | ||
| ensure!(paid.asset() == A::get(), InvalidTransaction::Payment); | ||
| // Calculate how much refund we should return. | ||
| let refund_amount = paid.peek().saturating_sub(corrected_fee); | ||
| // Refund to the the account that paid the fees if it was not removed by the | ||
| // dispatched function. If fails for any reason (eg. ED requirement is not met) no | ||
| // refund given. | ||
| let refund_debt = | ||
| if F::total_balance(A::get(), who).is_zero() || refund_amount.is_zero() { | ||
| fungibles::Debt::<T::AccountId, F>::zero(A::get()) | ||
| } else { | ||
| F::deposit(A::get(), who, refund_amount, Precision::BestEffort) | ||
| .unwrap_or_else(|_| fungibles::Debt::<T::AccountId, F>::zero(A::get())) | ||
| }; | ||
| // Merge the imbalance caused by paying the fees and refunding parts of it again. | ||
| let adjusted_paid: fungibles::Credit<T::AccountId, F> = | ||
| match paid.offset(refund_debt).defensive_proof("credits should be identical") { | ||
| Ok(SameOrOther::Same(credit)) => credit, | ||
| // Paid amount is fully refunded. | ||
| Ok(SameOrOther::None) => fungibles::Credit::<T::AccountId, F>::zero(A::get()), | ||
| // Should never fail as at this point the asset id is always valid and the | ||
| // refund amount is not greater than paid amount. | ||
| _ => return Err(InvalidTransaction::Payment.into()), | ||
| }; | ||
| // No separation for simplicity. | ||
| // In our case the fees and the tips are deposited to the same pot. | ||
| // We cannot call [`OnUnbalanced::on_unbalanceds`] since fungibles credit does not | ||
| // implement `Imbalanced` trait. | ||
| OU::on_unbalanced(adjusted_paid); | ||
| Ok(()) | ||
| } | ||
| } | ||
|
|
||
| type LiquidityInfoOf<T> = | ||
| <<T as pallet_transaction_payment::Config>::OnChargeTransaction as OnChargeTransaction< | ||
| T, | ||
| >>::LiquidityInfo; | ||
|
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||
| /// Implements [`OnChargeAssetTransaction`] for [`pallet_asset_conversion_tx_payment`], where | ||
| /// the asset class used to pay the fee is defined with the `A` type parameter (eg. DOT | ||
| /// location) and accessed via the type implementing the [`frame_support::traits::fungibles`] | ||
| /// trait. | ||
| pub struct SwapCreditAdapter<A, S>(PhantomData<(A, S)>); | ||
| impl<A, S, T> OnChargeAssetTransaction<T> for SwapCreditAdapter<A, S> | ||
| where | ||
| A: Get<S::AssetKind>, | ||
| S: SwapCredit< | ||
| T::AccountId, | ||
| Balance = T::Balance, | ||
| AssetKind = T::AssetKind, | ||
| Credit = fungibles::Credit<T::AccountId, T::Assets>, | ||
| >, | ||
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| T: pallet_asset_conversion_tx_payment::Config, | ||
| T::Fungibles: | ||
| fungibles::Inspect<T::AccountId, Balance = T::Balance, AssetId = T::AssetKind>, | ||
| T::OnChargeTransaction: | ||
| OnChargeTransaction<T, Balance = T::Balance, LiquidityInfo = Option<S::Credit>>, | ||
| { | ||
| type AssetId = T::AssetKind; | ||
| type Balance = T::Balance; | ||
| type LiquidityInfo = T::Balance; | ||
|
|
||
| fn withdraw_fee( | ||
| who: &<T>::AccountId, | ||
| _call: &<T>::RuntimeCall, | ||
| _dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>, | ||
| asset_id: Self::AssetId, | ||
| fee: Self::Balance, | ||
| _tip: Self::Balance, | ||
| ) -> Result<(LiquidityInfoOf<T>, Self::LiquidityInfo, T::Balance), TransactionValidityError> | ||
| { | ||
| let asset_fee = AssetConversion::<T>::quote_price_tokens_for_exact_tokens( | ||
| asset_id.clone(), | ||
| A::get(), | ||
| fee, | ||
| true, | ||
| ) | ||
| .ok_or(InvalidTransaction::Payment)?; | ||
|
|
||
| let asset_fee_credit = T::Assets::withdraw( | ||
| asset_id.clone(), | ||
| who, | ||
| asset_fee, | ||
| Precision::Exact, | ||
| Preservation::Preserve, | ||
| Fortitude::Polite, | ||
| ) | ||
| .map_err(|_| TransactionValidityError::from(InvalidTransaction::Payment))?; | ||
|
|
||
| let (fee_credit, change) = match S::swap_tokens_for_exact_tokens( | ||
| vec![asset_id, A::get()], | ||
| asset_fee_credit, | ||
| fee, | ||
| ) { | ||
| Ok((fee_credit, change)) => (fee_credit, change), | ||
| Err((credit_in, _)) => { | ||
| // The swap should not error since the price quote was successful. | ||
| let _ = T::Assets::resolve(who, credit_in).defensive(); | ||
| return Err(InvalidTransaction::Payment.into()) | ||
| }, | ||
| }; | ||
|
|
||
| // Should be always zero since the exact price was quoted before. | ||
| ensure!(change.peek().is_zero(), InvalidTransaction::Payment); | ||
|
|
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| Ok((Some(fee_credit), fee, asset_fee)) | ||
| } | ||
| fn correct_and_deposit_fee( | ||
| who: &<T>::AccountId, | ||
| dispatch_info: &DispatchInfoOf<<T>::RuntimeCall>, | ||
| post_info: &PostDispatchInfoOf<<T>::RuntimeCall>, | ||
| corrected_fee: Self::Balance, | ||
| tip: Self::Balance, | ||
| fee_paid: LiquidityInfoOf<T>, | ||
| _received_exchanged: Self::LiquidityInfo, | ||
| asset_id: Self::AssetId, | ||
| initial_asset_consumed: T::Balance, | ||
| ) -> Result<T::Balance, TransactionValidityError> { | ||
| let Some(fee_paid) = fee_paid else { | ||
| return Ok(Zero::zero()); | ||
| }; | ||
| // Try to refund if the fee paid is more than the corrected fee and the account was not | ||
| // removed by the dispatched function. | ||
| let (fee, fee_in_asset) = if fee_paid.peek() > corrected_fee && | ||
| !T::Assets::total_balance(asset_id.clone(), who).is_zero() | ||
| { | ||
| let refund_amount = fee_paid.peek().saturating_sub(corrected_fee); | ||
| // Check if the refund amount can be swapped back into the asset used by `who` for | ||
| // fee payment. | ||
| let refund_asset_amount = | ||
| AssetConversion::<T>::quote_price_exact_tokens_for_tokens( | ||
| A::get(), | ||
| asset_id.clone(), | ||
| refund_amount, | ||
| true, | ||
| ) | ||
| // No refund given if it cannot be swapped back. | ||
| .unwrap_or(Zero::zero()); | ||
|
|
||
| // Deposit the refund before the swap to ensure it can be processed. | ||
| let debt = match T::Assets::deposit( | ||
| asset_id.clone(), | ||
| who, | ||
| refund_asset_amount, | ||
| Precision::BestEffort, | ||
| ) { | ||
| Ok(debt) => debt, | ||
| // No refund given since it cannot be deposited. | ||
| Err(_) => fungibles::Debt::<T::AccountId, T::Assets>::zero(asset_id.clone()), | ||
| }; | ||
|
|
||
| if debt.peek().is_zero() { | ||
| // No refund given. | ||
| (fee_paid, initial_asset_consumed) | ||
| } else { | ||
| let (refund, fee_paid) = fee_paid.split(refund_amount); | ||
| match S::swap_exact_tokens_for_tokens( | ||
| vec![A::get(), asset_id], | ||
| refund, | ||
| Some(refund_asset_amount), | ||
| ) { | ||
| Ok(refund_asset) => { | ||
| match refund_asset.offset(debt) { | ||
| Ok(SameOrOther::None) => {}, | ||
| // This arm should never be reached, as the amount of `debt` is | ||
| // expected to be exactly equal to the amount of `refund_asset` | ||
| // credit. | ||
| _ => return Err(InvalidTransaction::Payment.into()), | ||
| }; | ||
| (fee_paid, initial_asset_consumed.saturating_sub(refund_asset_amount)) | ||
| }, | ||
| // The error should not occur since swap was quoted before. | ||
| Err((refund, _)) => { | ||
| match T::Assets::settle(who, debt, Preservation::Expendable) { | ||
| Ok(dust) => | ||
| ensure!(dust.peek().is_zero(), InvalidTransaction::Payment), | ||
| // The error should not occur as the `debt` was just withdrawn | ||
| // above. | ||
| Err(_) => return Err(InvalidTransaction::Payment.into()), | ||
| }; | ||
| let fee_paid = fee_paid.merge(refund).map_err(|_| { | ||
| // The error should never occur since `fee_paid` and `refund` are | ||
| // credits of the same asset. | ||
| TransactionValidityError::from(InvalidTransaction::Payment) | ||
| })?; | ||
| (fee_paid, initial_asset_consumed) | ||
| }, | ||
| } | ||
| } | ||
| } else { | ||
| (fee_paid, initial_asset_consumed) | ||
| }; | ||
|
|
||
| // Refund is already processed. | ||
| let corrected_fee = fee.peek(); | ||
| // Deposit fee. | ||
| T::OnChargeTransaction::correct_and_deposit_fee( | ||
| who, | ||
| dispatch_info, | ||
| post_info, | ||
| corrected_fee, | ||
| tip, | ||
| Some(fee), | ||
| ) | ||
| .map(|_| fee_in_asset) | ||
| } | ||
| } | ||
| } |
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