pallet_staking_async_ah_client/

lib.rs

// This file is part of Substrate.

// Copyright (C) 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.

//! The client for AssetHub, intended to be used in the relay chain.
//!
//! The counter-part for this pallet is `pallet-staking-async-rc-client` on AssetHub.
//!
//! This documentation is divided into the following sections:
//!
//! 1. Incoming messages: the messages that we receive from the relay chian.
//! 2. Outgoing messages: the messaged that we sent to the relay chain.
//! 3. Local interfaces: the interfaces that we expose to other pallets in the runtime.
//!
//! ## Incoming Messages
//!
//! All incoming messages are handled via [`Call`]. They are all gated to be dispatched only by
//! [`Config::AssetHubOrigin`]. The only one is:
//!
//! * [`Call::validator_set`]: A new validator set for a planning session index.
//!
//! ## Outgoing Messages
//!
//! All outgoing messages are handled by a single trait
//! [`pallet_staking_async_rc_client::SendToAssetHub`]. They match the incoming messages of the
//! `rc-client` pallet.
//!
//! ## Local Interfaces:
//!
//! Living on the relay chain, this pallet must:
//!
//! * Implement [`pallet_session::SessionManager`] (and historical variant thereof) to _give_
//!   information to the session pallet.
//! * Implements [`SessionInterface`] to _receive_ information from the session pallet
//! * Implement [`sp_staking::offence::OnOffenceHandler`].
//! * Implement reward related APIs ([`frame_support::traits::RewardsReporter`]).
//!
//! ## Future Plans
//!
//! * Governance functions to force set validators.

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

pub use pallet::*;

#[cfg(test)]
pub mod mock;

extern crate alloc;
use alloc::vec::Vec;
use frame_support::{
	pallet_prelude::*,
	traits::{Defensive, DefensiveSaturating, RewardsReporter},
};
pub use pallet_staking_async_rc_client::SendToAssetHub;
use pallet_staking_async_rc_client::{self as rc_client};
use sp_runtime::SaturatedConversion;
use sp_staking::{
	offence::{OffenceDetails, OffenceSeverity},
	SessionIndex,
};

/// The balance type seen from this pallet's PoV.
pub type BalanceOf<T> = <T as Config>::CurrencyBalance;

/// Type alias for offence details
pub type OffenceDetailsOf<T> = OffenceDetails<
	<T as frame_system::Config>::AccountId,
	(
		<T as frame_system::Config>::AccountId,
		sp_staking::Exposure<<T as frame_system::Config>::AccountId, BalanceOf<T>>,
	),
>;

const LOG_TARGET: &str = "runtime::staking-async::ah-client";

// syntactic sugar for logging.
#[macro_export]
macro_rules! log {
	($level:tt, $patter:expr $(, $values:expr)* $(,)?) => {
		log::$level!(
			target: $crate::LOG_TARGET,
			concat!("[{:?}] ⬇️ ", $patter), <frame_system::Pallet<T>>::block_number() $(, $values)*
		)
	};
}

/// Interface to talk to the local session pallet.
pub trait SessionInterface {
	/// The validator id type of the session pallet
	type ValidatorId: Clone;

	fn validators() -> Vec<Self::ValidatorId>;

	/// prune up to the given session index.
	fn prune_up_to(index: SessionIndex);

	/// Report an offence.
	///
	/// This is used to disable validators directly on the RC, until the next validator set.
	fn report_offence(offender: Self::ValidatorId, severity: OffenceSeverity);
}

impl<T: Config + pallet_session::Config + pallet_session::historical::Config> SessionInterface
	for T
{
	type ValidatorId = <T as pallet_session::Config>::ValidatorId;

	fn validators() -> Vec<Self::ValidatorId> {
		pallet_session::Pallet::<T>::validators()
	}

	fn prune_up_to(index: SessionIndex) {
		pallet_session::historical::Pallet::<T>::prune_up_to(index)
	}
	fn report_offence(offender: Self::ValidatorId, severity: OffenceSeverity) {
		pallet_session::Pallet::<T>::report_offence(offender, severity)
	}
}

/// Represents the operating mode of the pallet.
#[derive(
	Default,
	DecodeWithMemTracking,
	Encode,
	Decode,
	MaxEncodedLen,
	TypeInfo,
	Clone,
	PartialEq,
	Eq,
	RuntimeDebug,
	serde::Serialize,
	serde::Deserialize,
)]
pub enum OperatingMode {
	/// Fully delegated mode.
	///
	/// In this mode, the pallet performs no core logic and forwards all relevant operations
	/// to the fallback implementation defined in the pallet's `Config::Fallback`.
	///
	/// This mode is useful when staking is in synchronous mode and waiting for the signal to
	/// transition to asynchronous mode.
	#[default]
	Passive,

	/// Buffered mode for deferred execution.
	///
	/// In this mode, offences are accepted and buffered for later transmission to AssetHub.
	/// However, session change reports are dropped.
	///
	/// This mode is useful when the counterpart pallet `pallet-staking-async-rc-client` on
	/// AssetHub is not yet ready to process incoming messages.
	Buffered,

	/// Fully active mode.
	///
	/// The pallet performs all core logic directly and handles messages immediately.
	///
	/// This mode is useful when staking is ready to execute in asynchronous mode and the
	/// counterpart pallet `pallet-staking-async-rc-client` is ready to accept messages.
	Active,
}

impl OperatingMode {
	fn can_accept_validator_set(&self) -> bool {
		matches!(self, OperatingMode::Active)
	}
}

/// See `pallet_staking::DefaultExposureOf`. This type is the same, except it is duplicated here so
/// that an rc-runtime can use it after `pallet-staking` is fully removed as a dependency.
pub struct DefaultExposureOf<T>(core::marker::PhantomData<T>);

impl<T: Config>
	sp_runtime::traits::Convert<
		T::AccountId,
		Option<sp_staking::Exposure<T::AccountId, BalanceOf<T>>>,
	> for DefaultExposureOf<T>
{
	fn convert(
		validator: T::AccountId,
	) -> Option<sp_staking::Exposure<T::AccountId, BalanceOf<T>>> {
		T::SessionInterface::validators()
			.contains(&validator)
			.then_some(Default::default())
	}
}

#[frame_support::pallet]
pub mod pallet {
	use crate::*;
	use alloc::vec;
	use frame_support::traits::{Hooks, UnixTime};
	use frame_system::pallet_prelude::*;
	use pallet_session::{historical, SessionManager};
	use pallet_staking_async_rc_client::SessionReport;
	use sp_runtime::{Perbill, Saturating};
	use sp_staking::{
		offence::{OffenceSeverity, OnOffenceHandler},
		SessionIndex,
	};

	const STORAGE_VERSION: StorageVersion = StorageVersion::new(1);

	#[pallet::config]
	pub trait Config: frame_system::Config {
		/// The balance type of the runtime's currency interface.
		type CurrencyBalance: sp_runtime::traits::AtLeast32BitUnsigned
			+ codec::FullCodec
			+ DecodeWithMemTracking
			+ codec::HasCompact<Type: DecodeWithMemTracking>
			+ Copy
			+ MaybeSerializeDeserialize
			+ core::fmt::Debug
			+ Default
			+ From<u64>
			+ TypeInfo
			+ Send
			+ Sync
			+ MaxEncodedLen;

		/// An origin type that ensures an incoming message is from asset hub.
		type AssetHubOrigin: EnsureOrigin<Self::RuntimeOrigin>;

		/// The origin that can control this pallet's operations.
		type AdminOrigin: EnsureOrigin<Self::RuntimeOrigin>;

		/// Our communication interface to AssetHub.
		type SendToAssetHub: SendToAssetHub<AccountId = Self::AccountId>;

		/// A safety measure that asserts an incoming validator set must be at least this large.
		type MinimumValidatorSetSize: Get<u32>;

		/// A safety measure that asserts when iterating over validator points (to be sent to AH),
		/// we don't iterate too many times.
		///
		/// Validator may change session to session, and if session reports are not sent, validator
		/// points that we store may well grow beyond the size of the validator set. Yet, a too
		/// large of an upper bound may also exceed the maximum size of a single DMP message.
		/// Consult the test `message_queue_sizes` for more information.
		///
		/// Note that in case a single session report is larger than a single DMP message, it might
		/// still be sent over if we use
		/// [`pallet_staking_async_rc_client::XCMSender::split_then_send`]. This will make the size
		/// of each individual message smaller, yet, it will still try and push them all to the
		/// queue at the same time.
		type MaximumValidatorsWithPoints: Get<u32>;

		/// A type that gives us a reliable unix timestamp.
		type UnixTime: UnixTime;

		/// Number of points to award a validator per block authored.
		type PointsPerBlock: Get<u32>;

		/// Maximum number of offences to batch in a single message to AssetHub. Actual sending
		/// happens `on_initialize`. Offences get infinite "retries", and are never dropped.
		///
		/// A sensible value should be such that sending this batch is small enough to not exhaust
		/// the DMP queue. The size of a single offence is documented in `message_queue_sizes` test
		/// (74 bytes).
		type MaxOffenceBatchSize: Get<u32>;

		/// Interface to talk to the local Session pallet.
		type SessionInterface: SessionInterface<ValidatorId = Self::AccountId>;

		/// A fallback implementation to delegate logic to when the pallet is in
		/// [`OperatingMode::Passive`].
		///
		/// This type must implement the `historical::SessionManager` and `OnOffenceHandler`
		/// interface and is expected to behave as a stand-in for this pallet’s core logic when
		/// delegation is active.
		type Fallback: pallet_session::SessionManager<Self::AccountId>
			+ OnOffenceHandler<
				Self::AccountId,
				(Self::AccountId, sp_staking::Exposure<Self::AccountId, BalanceOf<Self>>),
				Weight,
			> + frame_support::traits::RewardsReporter<Self::AccountId>
			+ pallet_authorship::EventHandler<Self::AccountId, BlockNumberFor<Self>>;

		/// Maximum number of times we try to send a session report to AssetHub, after which, if
		/// sending still fails, we drop it.
		type MaxSessionReportRetries: Get<u32>;
	}

	#[pallet::pallet]
	#[pallet::storage_version(STORAGE_VERSION)]
	pub struct Pallet<T>(_);

	/// The queued validator sets for a given planning session index.
	///
	/// This is received via a call from AssetHub.
	#[pallet::storage]
	#[pallet::unbounded]
	pub type ValidatorSet<T: Config> = StorageValue<_, (u32, Vec<T::AccountId>), OptionQuery>;

	/// An incomplete validator set report.
	#[pallet::storage]
	#[pallet::unbounded]
	pub type IncompleteValidatorSetReport<T: Config> =
		StorageValue<_, rc_client::ValidatorSetReport<T::AccountId>, OptionQuery>;

	/// All of the points of the validators.
	///
	/// This is populated during a session, and is flushed and sent over via [`SendToAssetHub`]
	/// at each session end.
	#[pallet::storage]
	pub type ValidatorPoints<T: Config> =
		StorageMap<_, Twox64Concat, T::AccountId, u32, ValueQuery>;

	/// Indicates the current operating mode of the pallet.
	///
	/// This value determines how the pallet behaves in response to incoming and outgoing messages,
	/// particularly whether it should execute logic directly, defer it, or delegate it entirely.
	#[pallet::storage]
	pub type Mode<T: Config> = StorageValue<_, OperatingMode, ValueQuery>;

	/// A storage value that is set when a `new_session` gives a new validator set to the session
	/// pallet, and is cleared on the next call.
	///
	/// The inner u32 is the id of the said activated validator set. While not relevant here, good
	/// to know this is the planning era index of staking-async on AH.
	///
	/// Once cleared, we know a validator set has been activated, and therefore we can send a
	/// timestamp to AH.
	#[pallet::storage]
	pub type NextSessionChangesValidators<T: Config> = StorageValue<_, u32, OptionQuery>;

	/// The session index at which the latest elected validator set was applied.
	///
	/// This is used to determine if an offence, given a session index, is in the current active era
	/// or not.
	#[pallet::storage]
	pub type ValidatorSetAppliedAt<T: Config> = StorageValue<_, SessionIndex, OptionQuery>;

	/// A session report that is outgoing, and should be sent.
	///
	/// This will be attempted to be sent, possibly on every `on_initialize` call, until it is sent,
	/// or the second value reaches zero, at which point we drop it.
	#[pallet::storage]
	#[pallet::unbounded]
	pub type OutgoingSessionReport<T: Config> =
		StorageValue<_, (SessionReport<T::AccountId>, u32), OptionQuery>;

	/// Wrapper struct for storing offences, and getting them back page by page.
	///
	/// It has only two interfaces:
	///
	/// * [`OffenceSendQueue::append`], to add a single offence.
	/// * [`OffenceSendQueue::get_and_maybe_delete`] which retrieves the last page. Depending on the
	///   closure, it may also delete that page. The returned value is indeed
	///   [`Config::MaxOffenceBatchSize`] or less items.
	///
	/// Internally, it manages `OffenceSendQueueOffences` and `OffenceSendQueueCursor`, both of
	/// which should NEVER be used manually.
	pub struct OffenceSendQueue<T: Config>(core::marker::PhantomData<T>);

	/// A single buffered offence in [`OffenceSendQueue`].
	pub type QueuedOffenceOf<T> =
		(SessionIndex, rc_client::Offence<<T as frame_system::Config>::AccountId>);
	/// A page of buffered offences in [`OffenceSendQueue`].
	pub type QueuedOffencePageOf<T> =
		BoundedVec<QueuedOffenceOf<T>, <T as Config>::MaxOffenceBatchSize>;

	impl<T: Config> OffenceSendQueue<T> {
		/// Add a single offence to the queue.
		pub fn append(o: QueuedOffenceOf<T>) {
			let mut index = OffenceSendQueueCursor::<T>::get();
			match OffenceSendQueueOffences::<T>::try_mutate(index, |b| b.try_push(o.clone())) {
				Ok(_) => {
					// `index` had empty slot -- all good.
				},
				Err(_) => {
					debug_assert!(
						!OffenceSendQueueOffences::<T>::contains_key(index + 1),
						"next page should be empty"
					);
					index += 1;
					OffenceSendQueueOffences::<T>::insert(
						index,
						BoundedVec::<_, _>::try_from(vec![o]).defensive_unwrap_or_default(),
					);
					OffenceSendQueueCursor::<T>::mutate(|i| *i += 1);
				},
			}
		}

		// Get the last page of offences, and delete it if `op` returns `Ok(())`.
		pub fn get_and_maybe_delete(op: impl FnOnce(QueuedOffencePageOf<T>) -> Result<(), ()>) {
			let index = OffenceSendQueueCursor::<T>::get();
			let page = OffenceSendQueueOffences::<T>::get(index);
			let res = op(page);
			match res {
				Ok(_) => {
					OffenceSendQueueOffences::<T>::remove(index);
					OffenceSendQueueCursor::<T>::mutate(|i| *i = i.saturating_sub(1))
				},
				Err(_) => {
					// nada
				},
			}
		}

		#[cfg(feature = "std")]
		pub fn pages() -> u32 {
			let last_page = if Self::last_page_empty() { 0 } else { 1 };
			OffenceSendQueueCursor::<T>::get().saturating_add(last_page)
		}

		#[cfg(feature = "std")]
		pub fn count() -> u32 {
			let last_index = OffenceSendQueueCursor::<T>::get();
			let last_page = OffenceSendQueueOffences::<T>::get(last_index);
			let last_page_count = last_page.len() as u32;
			last_index.saturating_mul(T::MaxOffenceBatchSize::get()) + last_page_count
		}

		#[cfg(feature = "std")]
		fn last_page_empty() -> bool {
			OffenceSendQueueOffences::<T>::get(OffenceSendQueueCursor::<T>::get()).is_empty()
		}
	}

	/// Internal storage item of [`OffenceSendQueue`]. Should not be used manually.
	#[pallet::storage]
	#[pallet::unbounded]
	pub(crate) type OffenceSendQueueOffences<T: Config> =
		StorageMap<_, Twox64Concat, u32, QueuedOffencePageOf<T>, ValueQuery>;
	/// Internal storage item of [`OffenceSendQueue`]. Should not be used manually.
	#[pallet::storage]
	pub(crate) type OffenceSendQueueCursor<T: Config> = StorageValue<_, u32, ValueQuery>;

	#[pallet::genesis_config]
	#[derive(frame_support::DefaultNoBound, frame_support::DebugNoBound)]
	pub struct GenesisConfig<T: Config> {
		/// The initial operating mode of the pallet.
		pub operating_mode: OperatingMode,
		pub _marker: core::marker::PhantomData<T>,
	}

	#[pallet::genesis_build]
	impl<T: Config> BuildGenesisConfig for GenesisConfig<T> {
		fn build(&self) {
			// Set the initial operating mode of the pallet.
			Mode::<T>::put(self.operating_mode.clone());
		}
	}

	#[pallet::error]
	pub enum Error<T> {
		/// Could not process incoming message because incoming messages are blocked.
		Blocked,
	}

	#[pallet::event]
	#[pallet::generate_deposit(fn deposit_event)]
	pub enum Event<T: Config> {
		/// A new validator set has been received.
		ValidatorSetReceived {
			id: u32,
			new_validator_set_count: u32,
			prune_up_to: Option<SessionIndex>,
			leftover: bool,
		},
		/// We could not merge, and therefore dropped a buffered message.
		///
		/// Note that this event is more resembling an error, but we use an event because in this
		/// pallet we need to mutate storage upon some failures.
		CouldNotMergeAndDropped,
		/// The validator set received is way too small, as per
		/// [`Config::MinimumValidatorSetSize`].
		SetTooSmallAndDropped,
		/// Something occurred that should never happen under normal operation. Logged as an event
		/// for fail-safe observability.
		Unexpected(UnexpectedKind),
	}

	/// Represents unexpected or invariant-breaking conditions encountered during execution.
	///
	/// These variants are emitted as [`Event::Unexpected`] and indicate a defensive check has
	/// failed. While these should never occur under normal operation, they are useful for
	/// diagnosing issues in production or test environments.
	#[derive(Clone, Encode, Decode, DecodeWithMemTracking, PartialEq, TypeInfo, RuntimeDebug)]
	pub enum UnexpectedKind {
		/// A validator set was received while the pallet is in [`OperatingMode::Passive`].
		ReceivedValidatorSetWhilePassive,

		/// An unexpected transition was applied between operating modes.
		///
		/// Expected transitions are linear and forward-only: `Passive` → `Buffered` → `Active`.
		UnexpectedModeTransition,

		/// A session report failed to be sent.
		///
		/// We will store, and retry it for a number of more block.
		SessionReportSendFailed,

		/// A session report failed enough times that we should drop it.
		///
		/// We will retain the validator points, and send them over in the next session we receive
		/// from pallet-session.
		SessionReportDropped,

		/// An offence report failed to be sent.
		///
		/// It will be retried again in the next block. We never drop them.
		OffenceSendFailed,

		/// Some validator points didn't make it to be included in the session report. Should
		/// never happen, and means:
		///
		/// * a too low of a value is assigned to [`Config::MaximumValidatorsWithPoints`]
		/// * Those who are calling into our `RewardsReporter` likely have a bad view of the
		///   validator set, and are spamming us.
		ValidatorPointDropped,
	}

	#[pallet::call]
	impl<T: Config> Pallet<T> {
		#[pallet::call_index(0)]
		#[pallet::weight(
			// Reads:
			// - OperatingMode
			// - IncompleteValidatorSetReport
			// Writes:
			// - IncompleteValidatorSetReport or ValidatorSet
			// ignoring `T::SessionInterface::prune_up_to`
			T::DbWeight::get().reads_writes(2, 1)
		)]
		pub fn validator_set(
			origin: OriginFor<T>,
			report: rc_client::ValidatorSetReport<T::AccountId>,
		) -> DispatchResult {
			// Ensure the origin is one of Root or whatever is representing AssetHub.
			log!(debug, "Received new validator set report {}", report);
			T::AssetHubOrigin::ensure_origin_or_root(origin)?;

			// Check the operating mode.
			let mode = Mode::<T>::get();
			ensure!(mode.can_accept_validator_set(), Error::<T>::Blocked);

			let maybe_merged_report = match IncompleteValidatorSetReport::<T>::take() {
				Some(old) => old.merge(report.clone()),
				None => Ok(report),
			};

			if maybe_merged_report.is_err() {
				Self::deposit_event(Event::CouldNotMergeAndDropped);
				debug_assert!(
					IncompleteValidatorSetReport::<T>::get().is_none(),
					"we have ::take() it above, we don't want to keep the old data"
				);
				return Ok(());
			}

			let report = maybe_merged_report.expect("checked above; qed");

			if report.leftover {
				// buffer it, and nothing further to do.
				Self::deposit_event(Event::ValidatorSetReceived {
					id: report.id,
					new_validator_set_count: report.new_validator_set.len() as u32,
					prune_up_to: report.prune_up_to,
					leftover: report.leftover,
				});
				IncompleteValidatorSetReport::<T>::put(report);
			} else {
				// message is complete, process it.
				let rc_client::ValidatorSetReport {
					id,
					leftover,
					mut new_validator_set,
					prune_up_to,
				} = report;

				// ensure the validator set, deduplicated, is not too big.
				new_validator_set.sort();
				new_validator_set.dedup();

				if (new_validator_set.len() as u32) < T::MinimumValidatorSetSize::get() {
					Self::deposit_event(Event::SetTooSmallAndDropped);
					debug_assert!(
						IncompleteValidatorSetReport::<T>::get().is_none(),
						"we have ::take() it above, we don't want to keep the old data"
					);
					return Ok(());
				}

				Self::deposit_event(Event::ValidatorSetReceived {
					id,
					new_validator_set_count: new_validator_set.len() as u32,
					prune_up_to,
					leftover,
				});

				// Save the validator set.
				ValidatorSet::<T>::put((id, new_validator_set));
				if let Some(index) = prune_up_to {
					T::SessionInterface::prune_up_to(index);
				}
			}

			Ok(())
		}

		/// Allows governance to force set the operating mode of the pallet.
		#[pallet::call_index(1)]
		#[pallet::weight(T::DbWeight::get().writes(1))]
		pub fn set_mode(origin: OriginFor<T>, mode: OperatingMode) -> DispatchResult {
			T::AdminOrigin::ensure_origin(origin)?;
			Self::do_set_mode(mode);
			Ok(())
		}

		/// manually do what this pallet was meant to do at the end of the migration.
		#[pallet::call_index(2)]
		#[pallet::weight(T::DbWeight::get().writes(1))]
		pub fn force_on_migration_end(origin: OriginFor<T>) -> DispatchResult {
			T::AdminOrigin::ensure_origin(origin)?;
			Self::on_migration_end();
			Ok(())
		}
	}

	#[pallet::hooks]
	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {
		fn on_initialize(_n: BlockNumberFor<T>) -> Weight {
			let mut weight = Weight::zero();

			let mode = Mode::<T>::get();
			weight = weight.saturating_add(T::DbWeight::get().reads(1));
			if mode != OperatingMode::Active {
				return weight;
			}

			// if we have any pending session reports, send it.
			weight.saturating_accrue(T::DbWeight::get().reads(1));
			if let Some((session_report, retries_left)) = OutgoingSessionReport::<T>::take() {
				match T::SendToAssetHub::relay_session_report(session_report.clone()) {
					Ok(()) => {
						// report was sent, all good, it is already deleted.
					},
					Err(()) => {
						log!(error, "Failed to send session report to assethub");
						Self::deposit_event(Event::<T>::Unexpected(
							UnexpectedKind::SessionReportSendFailed,
						));
						if let Some(new_retries_left) = retries_left.checked_sub(One::one()) {
							OutgoingSessionReport::<T>::put((session_report, new_retries_left))
						} else {
							// recreate the validator points, so they will be sent in the next
							// report.
							session_report.validator_points.into_iter().for_each(|(v, p)| {
								ValidatorPoints::<T>::mutate(v, |existing_points| {
									*existing_points = existing_points.defensive_saturating_add(p)
								});
							});

							Self::deposit_event(Event::<T>::Unexpected(
								UnexpectedKind::SessionReportDropped,
							));
						}
					},
				}
			}

			// then, take a page from our send queue, and if present, send it.
			weight.saturating_accrue(T::DbWeight::get().reads(2));
			OffenceSendQueue::<T>::get_and_maybe_delete(|page| {
				if page.is_empty() {
					return Ok(())
				}
				// send the page if not empty. If sending returns `Ok`, we delete this page.
				T::SendToAssetHub::relay_new_offence_paged(page.into_inner()).inspect_err(|_| {
					Self::deposit_event(Event::Unexpected(UnexpectedKind::OffenceSendFailed));
				})
			});

			weight
		}

		fn integrity_test() {
			assert!(T::MaxOffenceBatchSize::get() > 0, "Offence Batch size must be at least 1");
		}
	}

	impl<T: Config>
		historical::SessionManager<T::AccountId, sp_staking::Exposure<T::AccountId, BalanceOf<T>>>
		for Pallet<T>
	{
		fn new_session(
			new_index: sp_staking::SessionIndex,
		) -> Option<
			Vec<(
				<T as frame_system::Config>::AccountId,
				sp_staking::Exposure<T::AccountId, BalanceOf<T>>,
			)>,
		> {
			<Self as pallet_session::SessionManager<_>>::new_session(new_index)
				.map(|v| v.into_iter().map(|v| (v, sp_staking::Exposure::default())).collect())
		}

		fn new_session_genesis(
			new_index: SessionIndex,
		) -> Option<Vec<(T::AccountId, sp_staking::Exposure<T::AccountId, BalanceOf<T>>)>> {
			if Mode::<T>::get() == OperatingMode::Passive {
				T::Fallback::new_session_genesis(new_index).map(|validators| {
					validators.into_iter().map(|v| (v, sp_staking::Exposure::default())).collect()
				})
			} else {
				None
			}
		}

		fn start_session(start_index: SessionIndex) {
			<Self as pallet_session::SessionManager<_>>::start_session(start_index)
		}

		fn end_session(end_index: SessionIndex) {
			<Self as pallet_session::SessionManager<_>>::end_session(end_index)
		}
	}

	impl<T: Config> pallet_session::SessionManager<T::AccountId> for Pallet<T> {
		fn new_session(session_index: u32) -> Option<Vec<T::AccountId>> {
			match Mode::<T>::get() {
				OperatingMode::Passive => T::Fallback::new_session(session_index),
				// In `Buffered` mode, we drop the session report and do nothing.
				OperatingMode::Buffered => None,
				OperatingMode::Active => Self::do_new_session(),
			}
		}

		fn start_session(session_index: u32) {
			if Mode::<T>::get() == OperatingMode::Passive {
				T::Fallback::start_session(session_index)
			}
		}

		fn new_session_genesis(new_index: SessionIndex) -> Option<Vec<T::AccountId>> {
			if Mode::<T>::get() == OperatingMode::Passive {
				T::Fallback::new_session_genesis(new_index)
			} else {
				None
			}
		}

		fn end_session(session_index: u32) {
			match Mode::<T>::get() {
				OperatingMode::Passive => T::Fallback::end_session(session_index),
				// In `Buffered` mode, we drop the session report and do nothing.
				OperatingMode::Buffered => (),
				OperatingMode::Active => Self::do_end_session(session_index),
			}
		}
	}

	impl<T: Config>
		OnOffenceHandler<
			T::AccountId,
			(T::AccountId, sp_staking::Exposure<T::AccountId, BalanceOf<T>>),
			Weight,
		> for Pallet<T>
	{
		fn on_offence(
			offenders: &[OffenceDetails<
				T::AccountId,
				(T::AccountId, sp_staking::Exposure<T::AccountId, BalanceOf<T>>),
			>],
			slash_fraction: &[Perbill],
			slash_session: SessionIndex,
		) -> Weight {
			match Mode::<T>::get() {
				OperatingMode::Passive => {
					// delegate to the fallback implementation.
					T::Fallback::on_offence(offenders, slash_fraction, slash_session)
				},
				OperatingMode::Buffered =>
					Self::on_offence_buffered(offenders, slash_fraction, slash_session),
				OperatingMode::Active =>
					Self::on_offence_active(offenders, slash_fraction, slash_session),
			}
		}
	}

	impl<T: Config> RewardsReporter<T::AccountId> for Pallet<T> {
		fn reward_by_ids(rewards: impl IntoIterator<Item = (T::AccountId, u32)>) {
			match Mode::<T>::get() {
				OperatingMode::Passive => T::Fallback::reward_by_ids(rewards),
				OperatingMode::Buffered | OperatingMode::Active => Self::do_reward_by_ids(rewards),
			}
		}
	}

	impl<T: Config> pallet_authorship::EventHandler<T::AccountId, BlockNumberFor<T>> for Pallet<T> {
		fn note_author(author: T::AccountId) {
			match Mode::<T>::get() {
				OperatingMode::Passive => T::Fallback::note_author(author),
				OperatingMode::Buffered | OperatingMode::Active => Self::do_note_author(author),
			}
		}
	}

	impl<T: Config> Pallet<T> {
		/// Hook to be called when the AssetHub migration begins.
		///
		/// This transitions the pallet into [`OperatingMode::Buffered`], meaning it will act as the
		/// primary staking module on the relay chain but will buffer outgoing messages instead of
		/// sending them to AssetHub.
		///
		/// While in this mode, the pallet stops delegating to the fallback implementation and
		/// temporarily accumulates events for later processing.
		pub fn on_migration_start() {
			debug_assert!(
				Mode::<T>::get() == OperatingMode::Passive,
				"we should only be called when in passive mode"
			);
			Self::do_set_mode(OperatingMode::Buffered);
		}

		/// Hook to be called when the AssetHub migration is complete.
		///
		/// This transitions the pallet into [`OperatingMode::Active`], meaning the counterpart
		/// pallet on AssetHub is ready to accept incoming messages, and this pallet can resume
		/// sending them.
		///
		/// In this mode, the pallet becomes fully active and processes all staking-related events
		/// directly.
		pub fn on_migration_end() {
			debug_assert!(
				Mode::<T>::get() == OperatingMode::Buffered,
				"we should only be called when in buffered mode"
			);
			Self::do_set_mode(OperatingMode::Active);

			// Buffered offences will be processed gradually by on_initialize
			// using MaxOffenceBatchSize to prevent block overload.
		}

		fn do_set_mode(new_mode: OperatingMode) {
			let old_mode = Mode::<T>::get();
			let unexpected = match new_mode {
				// `Passive` is the initial state, and not expected to be set by the user.
				OperatingMode::Passive => true,
				OperatingMode::Buffered => old_mode != OperatingMode::Passive,
				OperatingMode::Active => old_mode != OperatingMode::Buffered,
			};

			// this is a defensive check, and should never happen under normal operation.
			if unexpected {
				log!(warn, "Unexpected mode transition from {:?} to {:?}", old_mode, new_mode);
				Self::deposit_event(Event::Unexpected(UnexpectedKind::UnexpectedModeTransition));
			}

			// apply new mode anyway.
			Mode::<T>::put(new_mode);
		}

		fn do_new_session() -> Option<Vec<T::AccountId>> {
			ValidatorSet::<T>::take().map(|(id, val_set)| {
				// store the id to be sent back in the next session back to AH
				NextSessionChangesValidators::<T>::put(id);
				val_set
			})
		}

		fn do_end_session(end_index: u32) {
			// take and delete all validator points, limited by `MaximumValidatorsWithPoints`.
			let validator_points = ValidatorPoints::<T>::iter()
				.drain()
				.take(T::MaximumValidatorsWithPoints::get() as usize)
				.collect::<Vec<_>>();

			// If there were more validators than `MaximumValidatorsWithPoints`..
			if ValidatorPoints::<T>::iter().next().is_some() {
				// ..not much more we can do about it other than an event.
				Self::deposit_event(Event::<T>::Unexpected(UnexpectedKind::ValidatorPointDropped))
			}

			let activation_timestamp = NextSessionChangesValidators::<T>::take().map(|id| {
				// keep track of starting session index at which the validator set was applied.
				ValidatorSetAppliedAt::<T>::put(end_index + 1);
				// set the timestamp and the identifier of the validator set.
				(T::UnixTime::now().as_millis().saturated_into::<u64>(), id)
			});

			let session_report = pallet_staking_async_rc_client::SessionReport {
				end_index,
				validator_points,
				activation_timestamp,
				leftover: false,
			};

			// queue the session report to be sent.
			OutgoingSessionReport::<T>::put((session_report, T::MaxSessionReportRetries::get()));
		}

		fn do_reward_by_ids(rewards: impl IntoIterator<Item = (T::AccountId, u32)>) {
			for (validator_id, points) in rewards {
				ValidatorPoints::<T>::mutate(validator_id, |balance| {
					balance.saturating_accrue(points);
				});
			}
		}

		fn do_note_author(author: T::AccountId) {
			ValidatorPoints::<T>::mutate(author, |points| {
				points.saturating_accrue(T::PointsPerBlock::get());
			});
		}

		/// Check if an offence is from the active validator set.
		fn is_ongoing_offence(slash_session: SessionIndex) -> bool {
			ValidatorSetAppliedAt::<T>::get()
				.map(|start_session| slash_session >= start_session)
				.unwrap_or(false)
		}

		/// Handle offences in Buffered mode.
		fn on_offence_buffered(
			offenders: &[OffenceDetailsOf<T>],
			slash_fraction: &[Perbill],
			slash_session: SessionIndex,
		) -> Weight {
			let ongoing_offence = Self::is_ongoing_offence(slash_session);

			offenders.iter().cloned().zip(slash_fraction).for_each(|(offence, fraction)| {
				if ongoing_offence {
					// report the offence to the session pallet.
					T::SessionInterface::report_offence(
						offence.offender.0.clone(),
						OffenceSeverity(*fraction),
					);
				}

				let (offender, _full_identification) = offence.offender;
				let reporters = offence.reporters;

				// In `Buffered` mode, we buffer the offences for later processing.
				OffenceSendQueue::<T>::append((
					slash_session,
					rc_client::Offence {
						offender: offender.clone(),
						reporters: reporters.into_iter().take(1).collect(),
						slash_fraction: *fraction,
					},
				));
			});

			T::DbWeight::get().reads_writes(1, 1)
		}

		/// Handle offences in Active mode.
		fn on_offence_active(
			offenders: &[OffenceDetailsOf<T>],
			slash_fraction: &[Perbill],
			slash_session: SessionIndex,
		) -> Weight {
			let ongoing_offence = Self::is_ongoing_offence(slash_session);

			offenders.iter().cloned().zip(slash_fraction).for_each(|(offence, fraction)| {
				if ongoing_offence {
					// report the offence to the session pallet.
					T::SessionInterface::report_offence(
						offence.offender.0.clone(),
						OffenceSeverity(*fraction),
					);
				}

				let (offender, _full_identification) = offence.offender;
				let reporters = offence.reporters;

				// prepare an `Offence` instance for the XCM message. Note that we drop
				// the identification.
				let offence = rc_client::Offence {
					offender,
					reporters: reporters.into_iter().take(1).collect(),
					slash_fraction: *fraction,
				};
				OffenceSendQueue::<T>::append((slash_session, offence))
			});

			T::DbWeight::get().reads_writes(2, 2)
		}
	}
}

#[cfg(test)]
mod send_queue_tests {
	use frame_support::hypothetically;
	use sp_runtime::Perbill;

	use super::*;
	use crate::mock::*;

	// (cursor, len_of_pages)
	fn status() -> (u32, Vec<u32>) {
		let mut sorted = OffenceSendQueueOffences::<Test>::iter().collect::<Vec<_>>();
		sorted.sort_by(|x, y| x.0.cmp(&y.0));
		(
			OffenceSendQueueCursor::<Test>::get(),
			sorted.into_iter().map(|(_, v)| v.len() as u32).collect(),
		)
	}

	#[test]
	fn append_and_take() {
		new_test_ext().execute_with(|| {
			let o = (
				42,
				rc_client::Offence {
					offender: 42,
					reporters: vec![],
					slash_fraction: Perbill::from_percent(10),
				},
			);
			let page_size = <Test as Config>::MaxOffenceBatchSize::get();
			assert_eq!(page_size % 2, 0, "page size should be even");

			assert_eq!(status(), (0, vec![]));

			// --- when empty

			assert_eq!(OffenceSendQueue::<Test>::count(), 0);
			assert_eq!(OffenceSendQueue::<Test>::pages(), 0);

			// get and keep
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len(), 0);
					Err(())
				});
				assert_eq!(status(), (0, vec![]));
			});

			// get and delete
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len(), 0);
					Ok(())
				});
				assert_eq!(status(), (0, vec![]));
			});

			// -------- when 1 page half filled
			for _ in 0..page_size / 2 {
				OffenceSendQueue::<Test>::append(o.clone());
			}
			assert_eq!(status(), (0, vec![page_size / 2]));
			assert_eq!(OffenceSendQueue::<Test>::count(), page_size / 2);
			assert_eq!(OffenceSendQueue::<Test>::pages(), 1);

			// get and keep
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len() as u32, page_size / 2);
					Err(())
				});
				assert_eq!(status(), (0, vec![page_size / 2]));
			});

			// get and delete
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len() as u32, page_size / 2);
					Ok(())
				});
				assert_eq!(status(), (0, vec![]));
				assert_eq!(OffenceSendQueue::<Test>::count(), 0);
				assert_eq!(OffenceSendQueue::<Test>::pages(), 0);
			});

			// -------- when 1 page full
			for _ in 0..page_size / 2 {
				OffenceSendQueue::<Test>::append(o.clone());
			}
			assert_eq!(status(), (0, vec![page_size]));
			assert_eq!(OffenceSendQueue::<Test>::count(), page_size);
			assert_eq!(OffenceSendQueue::<Test>::pages(), 1);

			// get and keep
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len() as u32, page_size);
					Err(())
				});
				assert_eq!(status(), (0, vec![page_size]));
			});

			// get and delete
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len() as u32, page_size);
					Ok(())
				});
				assert_eq!(status(), (0, vec![]));
			});

			// -------- when more than 1 page full
			OffenceSendQueue::<Test>::append(o.clone());
			assert_eq!(status(), (1, vec![page_size, 1]));
			assert_eq!(OffenceSendQueue::<Test>::count(), page_size + 1);
			assert_eq!(OffenceSendQueue::<Test>::pages(), 2);

			// get and keep
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len(), 1);
					Err(())
				});
				assert_eq!(status(), (1, vec![page_size, 1]));
			});

			// get and delete
			hypothetically!({
				OffenceSendQueue::<Test>::get_and_maybe_delete(|page| {
					assert_eq!(page.len(), 1);
					Ok(())
				});
				assert_eq!(status(), (0, vec![page_size]));
			});
		})
	}
}