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 pallet acts as the main registry of paras.
//!
//! # Tracking State of Paras
//!
//! The most important responsibility of this module is to track which parachains and parathreads
//! are active and what their current state is. The current state of a para consists of the current
//! head data and the current validation code (AKA Parachain Validation Function (PVF)).
//!
//! A para is not considered live until it is registered and activated in this pallet.
//!
//! The set of parachains and parathreads cannot change except at session boundaries. This is
//! primarily to ensure that the number and meaning of bits required for the availability bitfields
//! does not change except at session boundaries.
//!
//! # Validation Code Upgrades
//!
//! When a para signals the validation code upgrade it will be processed by this module. This can
//! be in turn split into more fine grained items:
//!
//! - Part of the acceptance criteria checks if the para can indeed signal an upgrade,
//!
//! - When the candidate is enacted, this module schedules code upgrade, storing the prospective
//!   validation code.
//!
//! - Actually assign the prospective validation code to be the current one after all conditions are
//!   fulfilled.
//!
//! The conditions that must be met before the para can use the new validation code are:
//!
//! 1. The validation code should have been "soaked" in the storage for a given number of blocks. That
//!    is, the validation code should have been stored in on-chain storage for some time, so that in
//!    case of a revert with a non-extreme height difference, that validation code can still be
//!    found on-chain.
//!
//! 2. The validation code was vetted by the validators and declared as non-malicious in a processes
//!    known as PVF pre-checking.
//!
//! # Validation Code Management
//!
//! Potentially, one validation code can be used by several different paras. For example, during
//! initial stages of deployment several paras can use the same "shell" validation code, or
//! there can be shards of the same para that use the same validation code.
//!
//! In case a validation code ceases to have any users it must be pruned from the on-chain storage.
//!
//! # Para Lifecycle Management
//!
//! A para can be in one of the two stable states: it is either a parachain or a parathread.
//!
//! However, in order to get into one of those two states, it must first be onboarded. Onboarding
//! can be only enacted at session boundaries. Onboarding must take at least one full session.
//! Moreover, a brand new validation code should go through the PVF pre-checking process.
//!
//! Once the para is in one of the two stable states, it can switch to the other stable state or to
//! initiate offboarding process. The result of offboarding is removal of all data related to that
//! para.
//!
//! # PVF Pre-checking
//!
//! As was mentioned above, a brand new validation code should go through a process of approval.
//! As part of this process, validators from the active set will take the validation code and
//! check if it is malicious. Once they did that and have their judgement, either accept or reject,
//! they issue a statement in a form of an unsigned extrinsic. This extrinsic is processed by this
//! pallet. Once supermajority is gained for accept, then the process that initiated the check
//! is resumed (as mentioned before this can be either upgrading of validation code or onboarding).
//! If supermajority is gained for reject, then the process is canceled.
//!
//! Below is a state diagram that depicts states of a single PVF pre-checking vote.
//!
//! ```text
//!                                            ┌──────────┐
//!                        supermajority       │          │
//!                    ┌────────for───────────▶│ accepted │
//!        vote────┐   │                       │          │
//!         │      │   │                       └──────────┘
//!         │      │   │
//!         │  ┌───────┐
//!         │  │       │
//!         └─▶│ init  │────supermajority      ┌──────────┐
//!            │       │       against         │          │
//!            └───────┘           └──────────▶│ rejected │
//!             ▲  │                           │          │
//!             │  │ session                   └──────────┘
//!             │  └──change
//!             │     │
//!             │     ▼
//!             ┌─────┐
//! start──────▶│reset│
//!             └─────┘
//! ```
//!

use crate::{configuration, initializer::SessionChangeNotification, shared};
use bitvec::{order::Lsb0 as BitOrderLsb0, vec::BitVec};
use frame_support::{pallet_prelude::*, traits::EstimateNextSessionRotation};
use frame_system::pallet_prelude::*;
use parity_scale_codec::{Decode, Encode};
use primitives::v1::{
	ConsensusLog, HeadData, Id as ParaId, PvfCheckStatement, SessionIndex, UpgradeGoAhead,
	UpgradeRestriction, ValidationCode, ValidationCodeHash, ValidatorSignature,
};
use scale_info::TypeInfo;
use sp_core::RuntimeDebug;
use sp_runtime::{
	traits::{AppVerify, One},
	DispatchResult, SaturatedConversion,
};
use sp_std::{cmp, convert::TryInto, mem, prelude::*};

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

pub use crate::Origin as ParachainOrigin;

#[cfg(feature = "runtime-benchmarks")]
pub(crate) mod benchmarking;

pub use pallet::*;

const LOG_TARGET: &str = "runtime::paras";

// the two key times necessary to track for every code replacement.
#[derive(Default, Encode, Decode, TypeInfo)]
#[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, TypeInfo)]
#[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 `PastCodeHash` 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 `activated_at` value,
	/// not the `expected_at` value.
	last_pruned: Option<N>,
}

/// The possible states of a para, to take into account delayed lifecycle changes.
///
/// If the para is in a "transition state", it is expected that the parachain is
/// queued in the `ActionsQueue` to transition it into a stable state. Its lifecycle
/// state will be used to determine the state transition to apply to the para.
#[derive(PartialEq, Eq, Clone, Encode, Decode, RuntimeDebug, TypeInfo)]
pub enum ParaLifecycle {
	/// Para is new and is onboarding as a Parathread or Parachain.
	Onboarding,
	/// Para is a Parathread.
	Parathread,
	/// Para is a Parachain.
	Parachain,
	/// Para is a Parathread which is upgrading to a Parachain.
	UpgradingParathread,
	/// Para is a Parachain which is downgrading to a Parathread.
	DowngradingParachain,
	/// Parathread is queued to be offboarded.
	OffboardingParathread,
	/// Parachain is queued to be offboarded.
	OffboardingParachain,
}

impl ParaLifecycle {
	/// Returns true if parachain is currently onboarding. To learn if the
	/// parachain is onboarding as a parachain or parathread, look at the
	/// `UpcomingGenesis` storage item.
	pub fn is_onboarding(&self) -> bool {
		matches!(self, ParaLifecycle::Onboarding)
	}

	/// Returns true if para is in a stable state, i.e. it is currently
	/// a parachain or parathread, and not in any transition state.