paras.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/>.

//! The paras module is responsible for storing data on parachains and parathreads.
//!
//! It tracks which paras are parachains, what their current head data is in
//! this fork of the relay chain, what their validation code is, and what their past and upcoming
//! validation code is.
//!
//! A para is not considered live until it is registered and activated in this module. Activation can
//! only occur at session boundaries.

use sp_std::prelude::*;
use sp_std::marker::PhantomData;
use sp_runtime::traits::One;
use primitives::{
	parachain::{Id as ParaId, ValidationCode, HeadData},
};
use frame_support::{
	decl_storage, decl_module, decl_error,
	traits::Get,
	weights::Weight,
};
use codec::{Encode, Decode};
use crate::{configuration, initializer::SessionChangeNotification};

#[cfg(feature = "std")]
use serde::{Serialize, Deserialize};

pub trait Trait: system::Trait + configuration::Trait { }

// the two key times necessary to track for every code replacement.
#[derive(Default, Encode, Decode)]
#[cfg_attr(test, derive(Debug, Clone, PartialEq))]
pub struct ReplacementTimes<N> {
	/// The relay-chain block number that the code upgrade was expected to be activated.
	/// This is when the code change occurs from the para's perspective - after the
	/// first parablock included with a relay-parent with number >= this value.
	expected_at: N,
	/// The relay-chain block number at which the parablock activating the code upgrade was
	/// actually included. This means considered included and available, so this is the time at which
	/// that parablock enters the acceptance period in this fork of the relay-chain.
	activated_at: N,
}

/// Metadata used to track previous parachain validation code that we keep in
/// the state.
#[derive(Default, Encode, Decode)]
#[cfg_attr(test, derive(Debug, Clone, PartialEq))]
pub struct ParaPastCodeMeta<N> {
	/// Block numbers where the code was expected to be replaced and where the code
	/// was actually replaced, respectively. The first is used to do accurate lookups
	/// of historic code in historic contexts, whereas the second is used to do
	/// pruning on an accurate timeframe. These can be used as indices
	/// into the `PastCode` map along with the `ParaId` to fetch the code itself.
	upgrade_times: Vec<ReplacementTimes<N>>,
	/// Tracks the highest pruned code-replacement, if any. This is the `expected_at` value,
	/// not the `activated_at` value.
	last_pruned: Option<N>,
}

#[cfg_attr(test, derive(Debug, PartialEq))]
enum UseCodeAt<N> {
	/// Use the current code.
	Current,
	/// Use the code that was replaced at the given block number.
	/// This is an inclusive endpoint - a parablock in the context of a relay-chain block on this fork
	/// with number N should use the code that is replaced at N.
	ReplacedAt(N),
}

impl<N: Ord + Copy> ParaPastCodeMeta<N> {
	// note a replacement has occurred at a given block number.
	fn note_replacement(&mut self, expected_at: N, activated_at: N) {
		self.upgrade_times.push(ReplacementTimes { expected_at, activated_at })
	}

	// Yields an identifier that should be used for validating a
	// parablock in the context of a particular relay-chain block number.
	//
	// a return value of `None` means that there is no code we are aware of that
	// should be used to validate at the given height.
	#[allow(unused)]
	fn code_at(&self, para_at: N) -> Option<UseCodeAt<N>> {
		// Find out
		// a) if there is a point where code was replaced _after_ execution in this context and
		// b) what the index of that point is.
		let replaced_after_pos = self.upgrade_times.iter().position(|t| t.expected_at >= para_at);

		if let Some(replaced_after_pos) = replaced_after_pos {
			// The earliest stored code replacement needs to be special-cased, since we need to check
			// against the pruning state to see if this replacement represents the correct code, or
			// is simply after a replacement that actually represents the correct code, but has been pruned.
			let was_pruned = replaced_after_pos == 0
				&& self.last_pruned.map_or(false, |t| t >= para_at);

			if was_pruned {
				None
			} else {
				Some(UseCodeAt::ReplacedAt(self.upgrade_times[replaced_after_pos].expected_at))
			}
		} else {
			// No code replacements after this context.
			// This means either that the current code is valid, or `para_at` is so old that
			// we don't know the code necessary anymore. Compare against `last_pruned` to determine.
			self.last_pruned.as_ref().map_or(
				Some(UseCodeAt::Current), // nothing pruned, use current
				|t| if t >= &para_at { None } else { Some(UseCodeAt::Current) },
			)
		}
	}

	// The block at which the most recently tracked code change occurred, from the perspective
	// of the para.
	fn most_recent_change(&self) -> Option<N> {
		self.upgrade_times.last().map(|x| x.expected_at.clone())
	}

	// prunes all code upgrade logs occurring at or before `max`.
	// note that code replaced at `x` is the code used to validate all blocks before
	// `x`. Thus, `max` should be outside of the slashing window when this is invoked.
	//
	// Since we don't want to prune anything inside the acceptance period, and the parablock only
	// enters the acceptance period after being included, we prune based on the activation height of
	// the code change, not the expected height of the code change.
	//
	// returns an iterator of block numbers at which code was replaced, where the replaced
	// code should be now pruned, in ascending order.
	fn prune_up_to(&'_ mut self, max: N) -> impl Iterator<Item=N> + '_ {
		let to_prune = self.upgrade_times.iter().take_while(|t| t.activated_at <= max).count();
		let drained = if to_prune == 0 {
			// no-op prune.
			self.upgrade_times.drain(self.upgrade_times.len()..)
		} else {
			// if we are actually pruning something, update the last_pruned member.
			self.last_pruned = Some(self.upgrade_times[to_prune - 1].expected_at);
			self.upgrade_times.drain(..to_prune)
		};

		drained.map(|times| times.expected_at)
	}
}

/// Arguments for initializing a para.
#[derive(Encode, Decode)]
#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
pub struct ParaGenesisArgs {
	/// The initial head data to use.
	pub genesis_head: HeadData,
	/// The initial validation code to use.
	pub validation_code: ValidationCode,
	/// True if parachain, false if parathread.
	pub parachain: bool,
}


decl_storage! {
	trait Store for Module<T: Trait> as Paras {
		/// All parachains. Ordered ascending by ParaId. Parathreads are not included.
		Parachains get(fn parachains): Vec<ParaId>;
		/// All parathreads.
		Parathreads: map hasher(twox_64_concat) ParaId => Option<()>;
		/// The head-data of every registered para.
		Heads get(fn para_head): map hasher(twox_64_concat) ParaId => Option<HeadData>;
		/// The validation code of every live para.
		CurrentCode get(fn current_code): map hasher(twox_64_concat) ParaId => Option<ValidationCode>;
		/// Actual past code, indicated by the para id as well as the block number at which it became outdated.
		PastCode: map hasher(twox_64_concat) (ParaId, T::BlockNumber) => Option<ValidationCode>;
		/// Past code of parachains. The parachains themselves may not be registered anymore,
		/// but we also keep their code on-chain for the same amount of time as outdated code
		/// to keep it available for secondary checkers.
		PastCodeMeta get(fn past_code_meta):
			map hasher(twox_64_concat) ParaId => ParaPastCodeMeta<T::BlockNumber>;
		/// Which paras have past code that needs pruning and the relay-chain block at which the code was replaced.
		/// Note that this is the actual height of the included block, not the expected height at which the
		/// code upgrade would be applied, although they may be equal.
		/// This is to ensure the entire acceptance period is covered, not an offset acceptance period starting
		/// from the time at which the parachain perceives a code upgrade as having occurred.
		/// Multiple entries for a single para are permitted. Ordered ascending by block number.
		PastCodePruning: Vec<(ParaId, T::BlockNumber)>;
		/// The block number at which the planned code change is expected for a para.
		/// The change will be applied after the first parablock for this ID included which executes
		/// in the context of a relay chain block with a number >= `expected_at`.
		FutureCodeUpgrades get(fn future_code_upgrade_at): map hasher(twox_64_concat) ParaId => Option<T::BlockNumber>;
		/// The actual future code of a para.
		FutureCode: map hasher(twox_64_concat) ParaId => Option<ValidationCode>;