// This file is part of Substrate.

// Copyright (C) 2017-2022 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.

//! An opt-in utility module for reporting equivocations.
//!
//! This module defines an offence type for GRANDPA equivocations
//! and some utility traits to wire together:
//! - a key ownership proof system (e.g. to prove that a given authority was
//! part of a session);
//! - a system for reporting offences;
//! - a system for signing and submitting transactions;
//! - a way to get the current block author;
//!
//! These can be used in an offchain context in order to submit equivocation
//! reporting extrinsics (from the client that's running the GRANDPA protocol).
//! And in a runtime context, so that the GRANDPA module can validate the
//! equivocation proofs in the extrinsic and report the offences.
//!
//! IMPORTANT:
//! When using this module for enabling equivocation reporting it is required
//! that the `ValidateUnsigned` for the GRANDPA pallet is used in the runtime
//! definition.

use sp_std::prelude::*;

use codec::{self as codec, Decode, Encode};
use frame_support::traits::{Get, KeyOwnerProofSystem};
use sp_finality_grandpa::{EquivocationProof, RoundNumber, SetId};
use sp_runtime::{
	transaction_validity::{
		InvalidTransaction, TransactionPriority, TransactionSource, TransactionValidity,
		TransactionValidityError, ValidTransaction,
	},
	DispatchResult, Perbill,
};
use sp_staking::{
	offence::{Kind, Offence, OffenceError, ReportOffence},
	SessionIndex,
};

use super::{Call, Config, Pallet, LOG_TARGET};

/// A trait with utility methods for handling equivocation reports in GRANDPA.
/// The offence type is generic, and the trait provides , reporting an offence
/// triggered by a valid equivocation report, and also for creating and
/// submitting equivocation report extrinsics (useful only in offchain context).
pub trait HandleEquivocation<T: Config> {
	/// The offence type used for reporting offences on valid equivocation reports.
	type Offence: GrandpaOffence<T::KeyOwnerIdentification>;

	/// The longevity, in blocks, that the equivocation report is valid for. When using the staking
	/// pallet this should be equal to the bonding duration (in blocks, not eras).
	type ReportLongevity: Get<u64>;

	/// Report an offence proved by the given reporters.
	fn report_offence(
		reporters: Vec<T::AccountId>,
		offence: Self::Offence,
	) -> Result<(), OffenceError>;

	/// Returns true if all of the offenders at the given time slot have already been reported.
	fn is_known_offence(
		offenders: &[T::KeyOwnerIdentification],
		time_slot: &<Self::Offence as Offence<T::KeyOwnerIdentification>>::TimeSlot,
	) -> bool;

	/// Create and dispatch an equivocation report extrinsic.
	fn submit_unsigned_equivocation_report(
		equivocation_proof: EquivocationProof<T::Hash, T::BlockNumber>,
		key_owner_proof: T::KeyOwnerProof,
	) -> DispatchResult;

	/// Fetch the current block author id, if defined.
	fn block_author() -> Option<T::AccountId>;
}

impl<T: Config> HandleEquivocation<T> for () {
	type Offence = GrandpaEquivocationOffence<T::KeyOwnerIdentification>;
	type ReportLongevity = ();

	fn report_offence(
		_reporters: Vec<T::AccountId>,
		_offence: GrandpaEquivocationOffence<T::KeyOwnerIdentification>,
	) -> Result<(), OffenceError> {
		Ok(())
	}

	fn is_known_offence(
		_offenders: &[T::KeyOwnerIdentification],
		_time_slot: &GrandpaTimeSlot,
	) -> bool {
		true
	}

	fn submit_unsigned_equivocation_report(
		_equivocation_proof: EquivocationProof<T::Hash, T::BlockNumber>,
		_key_owner_proof: T::KeyOwnerProof,
	) -> DispatchResult {
		Ok(())
	}

	fn block_author() -> Option<T::AccountId> {
		None
	}
}

/// Generic equivocation handler. This type implements `HandleEquivocation`
/// using existing subsystems that are part of frame (type bounds described
/// below) and will dispatch to them directly, it's only purpose is to wire all
/// subsystems together.
pub struct EquivocationHandler<I, R, L, O = GrandpaEquivocationOffence<I>> {
	_phantom: sp_std::marker::PhantomData<(I, R, L, O)>,
}

impl<I, R, L, O> Default for EquivocationHandler<I, R, L, O> {
	fn default() -> Self {
		Self { _phantom: Default::default() }
	}
}

impl<T, R, L, O> HandleEquivocation<T> for EquivocationHandler<T::KeyOwnerIdentification, R, L, O>
where
	// We use the authorship pallet to fetch the current block author and use
	// `offchain::SendTransactionTypes` for unsigned extrinsic creation and
	// submission.
	T: Config + pallet_authorship::Config + frame_system::offchain::SendTransactionTypes<Call<T>>,
	// A system for reporting offences after valid equivocation reports are
	// processed.
	R: ReportOffence<T::AccountId, T::KeyOwnerIdentification, O>,
	// The longevity (in blocks) that the equivocation report is valid for. When using the staking
	// pallet this should be the bonding duration.
	L: Get<u64>,
	// The offence type that should be used when reporting.
	O: GrandpaOffence<T::KeyOwnerIdentification>,
{
	type Offence = O;
	type ReportLongevity = L;

	fn report_offence(reporters: Vec<T::AccountId>, offence: O) -> Result<(), OffenceError> {
		R::report_offence(reporters, offence)
	}

	fn is_known_offence(offenders: &[T::KeyOwnerIdentification], time_slot: &O::TimeSlot) -> bool {
		R::is_known_offence(offenders, time_slot)
	}

	fn submit_unsigned_equivocation_report(
		equivocation_proof: EquivocationProof<T::Hash, T::BlockNumber>,
		key_owner_proof: T::KeyOwnerProof,
	) -> DispatchResult {
		use frame_system::offchain::SubmitTransaction;

		let call = Call::report_equivocation_unsigned {
			equivocation_proof: Box::new(equivocation_proof),
			key_owner_proof,
		};

		match SubmitTransaction::<T, Call<T>>::submit_unsigned_transaction(call.into()) {
			Ok(()) => log::info!(target: LOG_TARGET, "Submitted GRANDPA equivocation report.",),
			Err(e) =>
				log::error!(target: LOG_TARGET, "Error submitting equivocation report: {:?}", e,),
		}

		Ok(())
	}

	fn block_author() -> Option<T::AccountId> {
		<pallet_authorship::Pallet<T>>::author()
	}
}

/// A round number and set id which point on the time of an offence.
#[derive(Copy, Clone, PartialOrd, Ord, Eq, PartialEq, Encode, Decode)]
pub struct GrandpaTimeSlot {
	// The order of these matters for `derive(Ord)`.
	/// Grandpa Set ID.
	pub set_id: SetId,
	/// Round number.
	pub round: RoundNumber,
}

/// Methods for the `ValidateUnsigned` implementation:
/// It restricts calls to `report_equivocation_unsigned` to local calls (i.e. extrinsics generated
/// on this node) or that already in a block. This guarantees that only block authors can include
/// unsigned equivocation reports.
impl<T: Config> Pallet<T> {
	pub fn validate_unsigned(source: TransactionSource, call: &Call<T>) -> TransactionValidity {
		if let Call::report_equivocation_unsigned { equivocation_proof, key_owner_proof } = call {
			// discard equivocation report not coming from the local node
			match source {
				TransactionSource::Local | TransactionSource::InBlock => { /* allowed */ },
				_ => {
					log::warn!(
						target: LOG_TARGET,
						"rejecting unsigned report equivocation transaction because it is not local/in-block."
					);

					return InvalidTransaction::Call.into()
				},
			}

			// check report staleness
			is_known_offence::<T>(equivocation_proof, key_owner_proof)?;

			let longevity =
				<T::HandleEquivocation as HandleEquivocation<T>>::ReportLongevity::get();

			ValidTransaction::with_tag_prefix("GrandpaEquivocation")
				// We assign the maximum priority for any equivocation report.
				.priority(TransactionPriority::max_value())
				// Only one equivocation report for the same offender at the same slot.
				.and_provides((
					equivocation_proof.offender().clone(),
					equivocation_proof.set_id(),
					equivocation_proof.round(),
				))
				.longevity(longevity)
				// We don't propagate this. This can never be included on a remote node.
				.propagate(false)
				.build()
		} else {
			InvalidTransaction::Call.into()
		}
	}

	pub fn pre_dispatch(call: &Call<T>) -> Result<(), TransactionValidityError> {
		if let Call::report_equivocation_unsigned { equivocation_proof, key_owner_proof } = call {
			is_known_offence::<T>(equivocation_proof, key_owner_proof)
		} else {
			Err(InvalidTransaction::Call.into())
		}
	}
}

fn is_known_offence<T: Config>(
	equivocation_proof: &EquivocationProof<T::Hash, T::BlockNumber>,
	key_owner_proof: &T::KeyOwnerProof,
) -> Result<(), TransactionValidityError> {
	// check the membership proof to extract the offender's id
	let key = (sp_finality_grandpa::KEY_TYPE, equivocation_proof.offender().clone());

	let offender = T::KeyOwnerProofSystem::check_proof(key, key_owner_proof.clone())
		.ok_or(InvalidTransaction::BadProof)?;

	// check if the offence has already been reported,
	// and if so then we can discard the report.
	let time_slot = <T::HandleEquivocation as HandleEquivocation<T>>::Offence::new_time_slot(
		equivocation_proof.set_id(),
		equivocation_proof.round(),
	);

	let is_known_offence = T::HandleEquivocation::is_known_offence(&[offender], &time_slot);

	if is_known_offence {
		Err(InvalidTransaction::Stale.into())
	} else {
		Ok(())
	}
}

/// A grandpa equivocation offence report.
#[allow(dead_code)]
pub struct GrandpaEquivocationOffence<FullIdentification> {
	/// Time slot at which this incident happened.
	pub time_slot: GrandpaTimeSlot,
	/// The session index in which the incident happened.
	pub session_index: SessionIndex,
	/// The size of the validator set at the time of the offence.
	pub validator_set_count: u32,
	/// The authority which produced this equivocation.
	pub offender: FullIdentification,
}

/// An interface for types that will be used as GRANDPA offences and must also
/// implement the `Offence` trait. This trait provides a constructor that is
/// provided all available data during processing of GRANDPA equivocations.
pub trait GrandpaOffence<FullIdentification>: Offence<FullIdentification> {
	/// Create a new GRANDPA offence using the given equivocation details.
	fn new(
		session_index: SessionIndex,
		validator_set_count: u32,
		offender: FullIdentification,
		set_id: SetId,
		round: RoundNumber,
	) -> Self;

	/// Create a new GRANDPA offence time slot.
	fn new_time_slot(set_id: SetId, round: RoundNumber) -> Self::TimeSlot;
}

impl<FullIdentification: Clone> GrandpaOffence<FullIdentification>
	for GrandpaEquivocationOffence<FullIdentification>
{
	fn new(
		session_index: SessionIndex,
		validator_set_count: u32,
		offender: FullIdentification,
		set_id: SetId,
		round: RoundNumber,
	) -> Self {
		GrandpaEquivocationOffence {
			session_index,
			validator_set_count,
			offender,
			time_slot: GrandpaTimeSlot { set_id, round },
		}
	}

	fn new_time_slot(set_id: SetId, round: RoundNumber) -> Self::TimeSlot {
		GrandpaTimeSlot { set_id, round }
	}
}

impl<FullIdentification: Clone> Offence<FullIdentification>
	for GrandpaEquivocationOffence<FullIdentification>
{
	const ID: Kind = *b"grandpa:equivoca";
	type TimeSlot = GrandpaTimeSlot;

	fn offenders(&self) -> Vec<FullIdentification> {
		vec![self.offender.clone()]
	}

	fn session_index(&self) -> SessionIndex {
		self.session_index
	}

	fn validator_set_count(&self) -> u32 {
		self.validator_set_count
	}

	fn time_slot(&self) -> Self::TimeSlot {
		self.time_slot
	}

	fn slash_fraction(&self, offenders_count: u32) -> Perbill {
		// the formula is min((3k / n)^2, 1)
		let x = Perbill::from_rational(3 * offenders_count, self.validator_set_count);
		// _ ^ 2
		x.square()
	}
}