inclusion_inherent.rs

// Copyright 2020 Parity Technologies (UK) Ltd.
// This file is part of Polkadot.

// Polkadot is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Polkadot is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

//! Provides glue code over the scheduler and inclusion modules, and accepting
//! one inherent per block that can include new para candidates and bitfields.
//!
//! Unlike other modules in this crate, it does not need to be initialized by the initializer,
//! as it has no initialization logic and its finalization logic depends only on the details of
//! this module.

use sp_std::prelude::*;
use primitives::v1::{
	BackedCandidate, SignedAvailabilityBitfields, INCLUSION_INHERENT_IDENTIFIER, Header,
};
use frame_support::{
	decl_error, decl_module, decl_storage, ensure,
	dispatch::DispatchResultWithPostInfo,
	weights::{DispatchClass, Weight},
	traits::Get,
};
use frame_system::ensure_none;
use crate::{
	inclusion,
	scheduler::{self, FreedReason},
	ump,
};
use inherents::{InherentIdentifier, InherentData, MakeFatalError, ProvideInherent};

// In the future, we should benchmark these consts; these are all untested assumptions for now.
const BACKED_CANDIDATE_WEIGHT: Weight = 100_000;
const INCLUSION_INHERENT_CLAIMED_WEIGHT: Weight = 1_000_000_000;
// we assume that 75% of an inclusion inherent's weight is used processing backed candidates
const MINIMAL_INCLUSION_INHERENT_WEIGHT: Weight = INCLUSION_INHERENT_CLAIMED_WEIGHT / 4;

pub trait Config: inclusion::Config + scheduler::Config {}

decl_storage! {
	trait Store for Module<T: Config> as ParaInclusionInherent {
		/// Whether the inclusion inherent was included within this block.
		///
		/// The `Option<()>` is effectively a bool, but it never hits storage in the `None` variant
		/// due to the guarantees of FRAME's storage APIs.
		///
		/// If this is `None` at the end of the block, we panic and render the block invalid.
		Included: Option<()>;
	}
}

decl_error! {
	pub enum Error for Module<T: Config> {
		/// Inclusion inherent called more than once per block.
		TooManyInclusionInherents,
		/// The hash of the submitted parent header doesn't correspond to the saved block hash of
		/// the parent.
		InvalidParentHeader,
	}
}

decl_module! {
	/// The inclusion inherent module.
	pub struct Module<T: Config> for enum Call where origin: <T as frame_system::Config>::Origin {
		type Error = Error<T>;

		fn on_initialize() -> Weight {
			T::DbWeight::get().reads_writes(1, 1) // in on_finalize.
		}

		fn on_finalize() {
			if Included::take().is_none() {
				panic!("Bitfields and heads must be included every block");
			}
		}

		/// Include backed candidates and bitfields.
		#[weight = (
			MINIMAL_INCLUSION_INHERENT_WEIGHT + backed_candidates.len() as Weight * BACKED_CANDIDATE_WEIGHT,
			DispatchClass::Mandatory,
		)]
		pub fn inclusion(
			origin,
			signed_bitfields: SignedAvailabilityBitfields,
			backed_candidates: Vec<BackedCandidate<T::Hash>>,
			parent_header: Header,
		) -> DispatchResultWithPostInfo {
			ensure_none(origin)?;
			ensure!(!<Included>::exists(), Error::<T>::TooManyInclusionInherents);

			// Check that the submitted parent header indeed corresponds to the previous block hash.
			let parent_hash = <frame_system::Module<T>>::parent_hash();
			ensure!(
				parent_header.hash().as_ref() == parent_hash.as_ref(),
				Error::<T>::InvalidParentHeader,
			);

			// Process new availability bitfields, yielding any availability cores whose
			// work has now concluded.
			let freed_concluded = <inclusion::Module<T>>::process_bitfields(
				signed_bitfields,
				<scheduler::Module<T>>::core_para,
			)?;

			// Handle timeouts for any availability core work.
			let availability_pred = <scheduler::Module<T>>::availability_timeout_predicate();
			let freed_timeout = if let Some(pred) = availability_pred {
				<inclusion::Module<T>>::collect_pending(pred)
			} else {
				Vec::new()
			};

			// Schedule paras again, given freed cores, and reasons for freeing.
			let freed = freed_concluded.into_iter().map(|c| (c, FreedReason::Concluded))
				.chain(freed_timeout.into_iter().map(|c| (c, FreedReason::TimedOut)));

			<scheduler::Module<T>>::schedule(freed);

			let backed_candidates = limit_backed_candidates::<T>(backed_candidates);
			let backed_candidates_len = backed_candidates.len() as Weight;

			// Process backed candidates according to scheduled cores.
			let parent_storage_root = parent_header.state_root;
			let occupied = <inclusion::Module<T>>::process_candidates(
				parent_storage_root,
				backed_candidates,
				<scheduler::Module<T>>::scheduled(),
				<scheduler::Module<T>>::group_validators,
			)?;

			// Note which of the scheduled cores were actually occupied by a backed candidate.
			<scheduler::Module<T>>::occupied(&occupied);

			// Give some time slice to dispatch pending upward messages.
			<ump::Module<T>>::process_pending_upward_messages();

			// And track that we've finished processing the inherent for this block.
			Included::set(Some(()));

			Ok(Some(
				MINIMAL_INCLUSION_INHERENT_WEIGHT +
				(backed_candidates_len * BACKED_CANDIDATE_WEIGHT)
			).into())
		}
	}
}

/// Limit the number of backed candidates processed in order to stay within block weight limits.
///
/// Use a configured assumption about the weight required to process a backed candidate and the
/// current block weight as of the execution of this function to ensure that we don't overload
/// the block with candidate processing.
///
/// If the backed candidates exceed the available block weight remaining, then skips all of them.
/// This is somewhat less desirable than attempting to fit some of them, but is more fair in the
/// even that we can't trust the provisioner to provide a fair / random ordering of candidates.
fn limit_backed_candidates<T: Config>(
	backed_candidates: Vec<BackedCandidate<T::Hash>>,
) -> Vec<BackedCandidate<T::Hash>> {
	// the weight of the inclusion inherent is already included in the current block weight,
	// so our operation is simple: if the block is currently overloaded, make this intrinsic smaller
	if frame_system::Module::<T>::block_weight().total() > <T as frame_system::Config>::BlockWeights::get().max_block {
		Vec::new()
	} else {
		backed_candidates
	}
}

impl<T: Config> ProvideInherent for Module<T> {
	type Call = Call<T>;
	type Error = MakeFatalError<()>;
	const INHERENT_IDENTIFIER: InherentIdentifier = INCLUSION_INHERENT_IDENTIFIER;

	fn create_inherent(data: &InherentData) -> Option<Self::Call> {
		data.get_data(&Self::INHERENT_IDENTIFIER)
			.expect("inclusion inherent data failed to decode")
			.map(
				|(signed_bitfields, backed_candidates, parent_header): (
					SignedAvailabilityBitfields,
					Vec<BackedCandidate<T::Hash>>,
					Header,
				)| {
					// Sanity check: session changes can invalidate an inherent, and we _really_ don't want that to happen.
					// See github.com/paritytech/polkadot/issues/1327
					let (signed_bitfields, backed_candidates) = match Self::inclusion(
						frame_system::RawOrigin::None.into(),
						signed_bitfields.clone(),
						backed_candidates.clone(),
						parent_header.clone(),
					) {
						Ok(_) => (signed_bitfields, backed_candidates),
						Err(err) => {
							frame_support::debug::warn!(
								target: "runtime_inclusion_inherent",
								"dropping signed_bitfields and backed_candidates because they produced \
								an invalid inclusion inherent: {:?}",
								err,
							);
							(Vec::new().into(), Vec::new())
						}
					};
					Call::inclusion(signed_bitfields, backed_candidates, parent_header)
				}
			)
	}
}

#[cfg(test)]
mod tests {
	use super::*;

	use crate::mock::{
		new_test_ext, System, GenesisConfig as MockGenesisConfig, Test
	};

	mod limit_backed_candidates {
		use super::*;

		#[test]
		fn does_not_truncate_on_empty_block() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let backed_candidates = vec![BackedCandidate::default()];
				System::set_block_consumed_resources(0, 0);
				assert_eq!(limit_backed_candidates::<Test>(backed_candidates).len(), 1);
			});
		}

		#[test]
		fn does_not_truncate_on_exactly_full_block() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let backed_candidates = vec![BackedCandidate::default()];
				let max_block_weight = <Test as frame_system::Config>::BlockWeights::get().max_block;
				// if the consumed resources are precisely equal to the max block weight, we do not truncate.
				System::set_block_consumed_resources(max_block_weight, 0);
				assert_eq!(limit_backed_candidates::<Test>(backed_candidates).len(), 1);
			});
		}

		#[test]
		fn truncates_on_over_full_block() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let backed_candidates = vec![BackedCandidate::default()];
				let max_block_weight = <Test as frame_system::Config>::BlockWeights::get().max_block;
				// if the consumed resources are precisely equal to the max block weight, we do not truncate.
				System::set_block_consumed_resources(max_block_weight + 1, 0);
				assert_eq!(limit_backed_candidates::<Test>(backed_candidates).len(), 0);
			});
		}

		#[test]
		fn all_backed_candidates_get_truncated() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let backed_candidates = vec![BackedCandidate::default(); 10];
				let max_block_weight = <Test as frame_system::Config>::BlockWeights::get().max_block;
				// if the consumed resources are precisely equal to the max block weight, we do not truncate.
				System::set_block_consumed_resources(max_block_weight + 1, 0);
				assert_eq!(limit_backed_candidates::<Test>(backed_candidates).len(), 0);
			});
		}
	}

	mod inclusion_inherent_weight {
		use super::*;

		use crate::mock::{
			new_test_ext, System, GenesisConfig as MockGenesisConfig, Test
		};

		use frame_support::traits::UnfilteredDispatchable;

		fn default_header() -> Header {
			Header {
				parent_hash: Default::default(),
				number: 0,
				state_root: Default::default(),
				extrinsics_root: Default::default(),
				digest: Default::default(),
			}
		}

		/// We expect the weight of the inclusion inherent not to change when no truncation occurs:
		/// its weight is dynamically computed from the size of the backed candidates list, and is
		/// already incorporated into the current block weight when it is selected by the provisioner.
		#[test]
		fn weight_does_not_change_on_happy_path() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let header = default_header();
				System::set_block_number(1);
				System::set_parent_hash(header.hash());

				// number of bitfields doesn't affect the inclusion inherent weight, so we can mock it with an empty one
				let signed_bitfields = Vec::new();
				// backed candidates must not be empty, so we can demonstrate that the weight has not changed
				let backed_candidates = vec![BackedCandidate::default(); 10];

				// the expected weight can always be computed by this formula
				let expected_weight = MINIMAL_INCLUSION_INHERENT_WEIGHT +
					(backed_candidates.len() as Weight * BACKED_CANDIDATE_WEIGHT);

				// we've used half the block weight; there's plenty of margin
				let max_block_weight = <Test as frame_system::Config>::BlockWeights::get().max_block;
				let used_block_weight = max_block_weight / 2;
				System::set_block_consumed_resources(used_block_weight, 0);

				// execute the inclusion inherent
				let post_info = Call::<Test>::inclusion(signed_bitfields, backed_candidates, default_header())
					.dispatch_bypass_filter(None.into()).unwrap_err().post_info;

				// we don't directly check the block's weight post-call. Instead, we check that the
				// call has returned the appropriate post-dispatch weight for refund, and trust
				// Substrate to do the right thing with that information.
				//
				// In this case, the weight system can update the actual weight with the same amount,
				// or return `None` to indicate that the pre-computed weight should not change.
				// Either option is acceptable for our purposes.
				if let Some(actual_weight) = post_info.actual_weight {
					assert_eq!(actual_weight, expected_weight);
				}
			});
		}

		/// We expect the weight of the inclusion inherent to change when truncation occurs: its
		/// weight was initially dynamically computed from the size of the backed candidates list,
		/// but was reduced by truncation.
		#[test]
		fn weight_changes_when_backed_candidates_are_truncated() {
			new_test_ext(MockGenesisConfig::default()).execute_with(|| {
				let header = default_header();
				System::set_block_number(1);
				System::set_parent_hash(header.hash());

				// number of bitfields doesn't affect the inclusion inherent weight, so we can mock it with an empty one
				let signed_bitfields = Vec::new();
				// backed candidates must not be empty, so we can demonstrate that the weight has not changed
				let backed_candidates = vec![BackedCandidate::default(); 10];

				// the expected weight with no blocks is just the minimum weight
				let expected_weight = MINIMAL_INCLUSION_INHERENT_WEIGHT;

				// oops, looks like this mandatory call pushed the block weight over the limit
				let max_block_weight = <Test as frame_system::Config>::BlockWeights::get().max_block;
				let used_block_weight = max_block_weight + 1;
				System::set_block_consumed_resources(used_block_weight, 0);

				// execute the inclusion inherent
				let post_info = Call::<Test>::inclusion(signed_bitfields, backed_candidates, header)
					.dispatch_bypass_filter(None.into()).unwrap();

				// we don't directly check the block's weight post-call. Instead, we check that the
				// call has returned the appropriate post-dispatch weight for refund, and trust
				// Substrate to do the right thing with that information.
				assert_eq!(
					post_info.actual_weight.unwrap(),
					expected_weight,
				);
			});
		}
	}
}