lib.rs

// This file is part of Substrate.

// Copyright (C) 2019-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// 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.

//! # Transaction Payment Module
//!
//! This module provides the basic logic needed to pay the absolute minimum amount needed for a
//! transaction to be included. This includes:
//!   - _weight fee_: A fee proportional to amount of weight a transaction consumes.
//!   - _length fee_: A fee proportional to the encoded length of the transaction.
//!   - _tip_: An optional tip. Tip increases the priority of the transaction, giving it a higher
//!     chance to be included by the transaction queue.
//!
//! Additionally, this module allows one to configure:
//!   - The mapping between one unit of weight to one unit of fee via [`Config::WeightToFee`].
//!   - A means of updating the fee for the next block, via defining a multiplier, based on the
//!     final state of the chain at the end of the previous block. This can be configured via
//!     [`Config::FeeMultiplierUpdate`]
//!   - How the fees are paid via [`Config::OnChargeTransaction`].

#![cfg_attr(not(feature = "std"), no_std)]

use sp_std::prelude::*;
use codec::{Encode, Decode};
use frame_support::{
	decl_storage, decl_module,
	traits::Get,
	weights::{
		Weight, DispatchInfo, PostDispatchInfo, GetDispatchInfo, Pays, WeightToFeePolynomial,
		WeightToFeeCoefficient, DispatchClass,
	},
	dispatch::DispatchResult,
};
use sp_runtime::{
	FixedU128, FixedPointNumber, FixedPointOperand, Perquintill, RuntimeDebug,
	transaction_validity::{
		TransactionPriority, ValidTransaction, TransactionValidityError, TransactionValidity,
	},
	traits::{
		Saturating, SignedExtension, SaturatedConversion, Convert, Dispatchable,
		DispatchInfoOf, PostDispatchInfoOf,
	},
};
use pallet_transaction_payment_rpc_runtime_api::RuntimeDispatchInfo;

mod payment;
pub use payment::*;

/// Fee multiplier.
pub type Multiplier = FixedU128;

type BalanceOf<T> =
	<<T as Config>::OnChargeTransaction as OnChargeTransaction<T>>::Balance;

/// A struct to update the weight multiplier per block. It implements `Convert<Multiplier,
/// Multiplier>`, meaning that it can convert the previous multiplier to the next one. This should
/// be called on `on_finalize` of a block, prior to potentially cleaning the weight data from the
/// system module.
///
/// given:
/// 	s = previous block weight
/// 	s'= ideal block weight
/// 	m = maximum block weight
///		diff = (s - s')/m
///		v = 0.00001
///		t1 = (v * diff)
///		t2 = (v * diff)^2 / 2
///	then:
/// 	next_multiplier = prev_multiplier * (1 + t1 + t2)
///
/// Where `(s', v)` must be given as the `Get` implementation of the `T` generic type. Moreover, `M`
/// must provide the minimum allowed value for the multiplier. Note that a runtime should ensure
/// with tests that the combination of this `M` and `V` is not such that the multiplier can drop to
/// zero and never recover.
///
/// note that `s'` is interpreted as a portion in the _normal transaction_ capacity of the block.
/// For example, given `s' == 0.25` and `AvailableBlockRatio = 0.75`, then the target fullness is
/// _0.25 of the normal capacity_ and _0.1875 of the entire block_.
///
/// This implementation implies the bound:
/// - `v ≤ p / k * (s − s')`
/// - or, solving for `p`: `p >= v * k * (s - s')`
///
/// where `p` is the amount of change over `k` blocks.
///
/// Hence:
/// - in a fully congested chain: `p >= v * k * (1 - s')`.
/// - in an empty chain: `p >= v * k * (-s')`.
///
/// For example, when all blocks are full and there are 28800 blocks per day (default in `substrate-node`)
/// and v == 0.00001, s' == 0.1875, we'd have:
///
/// p >= 0.00001 * 28800 * 0.8125
/// p >= 0.234
///
/// Meaning that fees can change by around ~23% per day, given extreme congestion.
///
/// More info can be found at:
/// <https://w3f-research.readthedocs.io/en/latest/polkadot/Token%20Economics.html>
pub struct TargetedFeeAdjustment<T, S, V, M>(sp_std::marker::PhantomData<(T, S, V, M)>);

/// Something that can convert the current multiplier to the next one.
pub trait MultiplierUpdate: Convert<Multiplier, Multiplier> {
	/// Minimum multiplier
	fn min() -> Multiplier;
	/// Target block saturation level
	fn target() -> Perquintill;
	/// Variability factor
	fn variability() -> Multiplier;
}

impl MultiplierUpdate for () {
	fn min() -> Multiplier {
		Default::default()
	}
	fn target() -> Perquintill {
		Default::default()
	}
	fn variability() -> Multiplier {
		Default::default()
	}
}

impl<T, S, V, M> MultiplierUpdate for TargetedFeeAdjustment<T, S, V, M>
	where T: frame_system::Config, S: Get<Perquintill>, V: Get<Multiplier>, M: Get<Multiplier>,
{
	fn min() -> Multiplier {
		M::get()
	}
	fn target() -> Perquintill {
		S::get()
	}
	fn variability() -> Multiplier {
		V::get()
	}
}

impl<T, S, V, M> Convert<Multiplier, Multiplier> for TargetedFeeAdjustment<T, S, V, M>
	where T: frame_system::Config, S: Get<Perquintill>, V: Get<Multiplier>, M: Get<Multiplier>,
{
	fn convert(previous: Multiplier) -> Multiplier {
		// Defensive only. The multiplier in storage should always be at most positive. Nonetheless
		// we recover here in case of errors, because any value below this would be stale and can
		// never change.
		let min_multiplier = M::get();
		let previous = previous.max(min_multiplier);

		let weights = T::BlockWeights::get();
		// the computed ratio is only among the normal class.
		let normal_max_weight = weights.get(DispatchClass::Normal).max_total
			.unwrap_or_else(|| weights.max_block);
		let current_block_weight = <frame_system::Module<T>>::block_weight();
		let normal_block_weight = *current_block_weight
			.get(DispatchClass::Normal)
			.min(&normal_max_weight);

		let s = S::get();
		let v = V::get();

		let target_weight = (s * normal_max_weight) as u128;
		let block_weight = normal_block_weight as u128;

		// determines if the first_term is positive
		let positive = block_weight >= target_weight;
		let diff_abs = block_weight.max(target_weight) - block_weight.min(target_weight);

		// defensive only, a test case assures that the maximum weight diff can fit in Multiplier
		// without any saturation.
		let diff = Multiplier::saturating_from_rational(diff_abs, normal_max_weight.max(1));
		let diff_squared = diff.saturating_mul(diff);

		let v_squared_2 = v.saturating_mul(v) / Multiplier::saturating_from_integer(2);

		let first_term = v.saturating_mul(diff);
		let second_term = v_squared_2.saturating_mul(diff_squared);

		if positive {
			let excess = first_term.saturating_add(second_term).saturating_mul(previous);
			previous.saturating_add(excess).max(min_multiplier)
		} else {
			// Defensive-only: first_term > second_term. Safe subtraction.
			let negative = first_term.saturating_sub(second_term).saturating_mul(previous);
			previous.saturating_sub(negative).max(min_multiplier)
		}
	}
}