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// Copyright (C) Parity Technologies (UK) Ltd.
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// 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/>.
//! A tree utility for managing parachain fragments not referenced by the relay-chain.
//!
//! # Overview
//!
//! This module exposes two main types: [`FragmentTree`] and [`CandidateStorage`] which are meant to
//! be used in close conjunction. Each fragment tree is associated with a particular relay-parent
//! and each node in the tree represents a candidate. Each parachain has a single candidate storage,
//! but can have multiple trees for each relay chain block in the view.
//!
//! A tree has an associated [`Scope`] which defines limits on candidates within the tree.
//! Candidates themselves have their own [`Constraints`] which are either the constraints from the
//! scope, or, if there are previous nodes in the tree, a modified version of the previous
//! candidate's constraints.
//!
//! This module also makes use of types provided by the Inclusion Emulator module, such as
//! [`Fragment`] and [`Constraints`]. These perform the actual job of checking for validity of
//! prospective fragments.
//!
//! # Usage
//!
//! It's expected that higher-level code will have a tree for each relay-chain block which might
//! reasonably have blocks built upon it.
//!
//! Because a para only has a single candidate storage, trees only store indices into the storage.
//! The storage is meant to be pruned when trees are dropped by higher-level code.
//!
//! # Cycles
//!
//! Nodes do not uniquely refer to a parachain block for two reasons.
//! 1. There's no requirement that head-data is unique for a parachain. Furthermore, a parachain
//! is under no obligation to be acyclic, and this is mostly just because it's totally
//! inefficient to enforce it. Practical use-cases are acyclic, but there is still more than
//! one way to reach the same head-data.
//! 2. and candidates only refer to their parent by its head-data. This whole issue could be
//! resolved by having candidates reference their parent by candidate hash.
//!
//! The implication is that when we receive a candidate receipt, there are actually multiple
//! possibilities for any candidates between the para-head recorded in the relay parent's state
//! and the candidate in question.
//!
//! This means that our candidates need to handle multiple parents and that depth is an
//! attribute of a node in a tree, not a candidate. Put another way, the same candidate might
//! have different depths in different parts of the tree.
//!
//! As an extreme example, a candidate which produces head-data which is the same as its parent
//! can correspond to multiple nodes within the same [`FragmentTree`]. Such cycles are bounded
//! by the maximum depth allowed by the tree. An example with `max_depth: 4`:
//!
//! ```text
//! committed head
//! |
//! depth 0: head_a
//! |
//! depth 1: head_b
//! |
//! depth 2: head_a
//! |
//! depth 3: head_b
//! |
//! depth 4: head_a
//! ```
//!
//! As long as the [`CandidateStorage`] has bounded input on the number of candidates supplied,
//! [`FragmentTree`] complexity is bounded. This means that higher-level code needs to be selective
//! about limiting the amount of candidates that are considered.
//!
//! The code in this module is not designed for speed or efficiency, but conceptual simplicity.
//! Our assumption is that the amount of candidates and parachains we consider will be reasonably
//! bounded and in practice will not exceed a few thousand at any time. This naive implementation
//! will still perform fairly well under these conditions, despite being somewhat wasteful of
//! memory.
#[cfg(test)]
mod tests;
use std::{
borrow::Cow,
collections::{
hash_map::{Entry, HashMap},
BTreeMap, HashSet,
},
};
use super::LOG_TARGET;
use bitvec::prelude::*;
use polkadot_node_subsystem::messages::Ancestors;
use polkadot_node_subsystem_util::inclusion_emulator::{
ConstraintModifications, Constraints, Fragment, ProspectiveCandidate, RelayChainBlockInfo,
};
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BlockNumber, CandidateHash, CommittedCandidateReceipt, Hash, HeadData, Id as ParaId,
PersistedValidationData,
};
/// Kinds of failures to import a candidate into storage.
#[derive(Debug, Clone, PartialEq)]
pub enum CandidateStorageInsertionError {
/// An error indicating that a supplied candidate didn't match the persisted
/// validation data provided alongside it.
PersistedValidationDataMismatch,
/// The candidate was already known.
CandidateAlreadyKnown(CandidateHash),
}
/// Stores candidates and information about them such as their relay-parents and their backing
/// states.
pub(crate) struct CandidateStorage {
// Index from head data hash to candidate hashes with that head data as a parent.
by_parent_head: HashMap<Hash, HashSet<CandidateHash>>,
// Index from head data hash to candidate hashes outputting that head data.
by_output_head: HashMap<Hash, HashSet<CandidateHash>>,
// Index from candidate hash to fragment node.
by_candidate_hash: HashMap<CandidateHash, CandidateEntry>,
}
impl CandidateStorage {
/// Create a new `CandidateStorage`.
pub fn new() -> Self {
CandidateStorage {
by_parent_head: HashMap::new(),
by_output_head: HashMap::new(),
by_candidate_hash: HashMap::new(),
}
}
/// Introduce a new candidate.
pub fn add_candidate(
&mut self,
candidate: CommittedCandidateReceipt,
persisted_validation_data: PersistedValidationData,
) -> Result<CandidateHash, CandidateStorageInsertionError> {
let candidate_hash = candidate.hash();
if self.by_candidate_hash.contains_key(&candidate_hash) {
return Err(CandidateStorageInsertionError::CandidateAlreadyKnown(candidate_hash))
}
if persisted_validation_data.hash() != candidate.descriptor.persisted_validation_data_hash {
return Err(CandidateStorageInsertionError::PersistedValidationDataMismatch)
}
let parent_head_hash = persisted_validation_data.parent_head.hash();
let output_head_hash = candidate.commitments.head_data.hash();
let entry = CandidateEntry {
candidate_hash,
relay_parent: candidate.descriptor.relay_parent,
state: CandidateState::Introduced,
candidate: ProspectiveCandidate {
commitments: Cow::Owned(candidate.commitments),
collator: candidate.descriptor.collator,
collator_signature: candidate.descriptor.signature,
persisted_validation_data,
pov_hash: candidate.descriptor.pov_hash,
validation_code_hash: candidate.descriptor.validation_code_hash,
},
};
self.by_parent_head.entry(parent_head_hash).or_default().insert(candidate_hash);
self.by_output_head.entry(output_head_hash).or_default().insert(candidate_hash);
// sanity-checked already.
self.by_candidate_hash.insert(candidate_hash, entry);
Ok(candidate_hash)
}
/// Remove a candidate from the store.
pub fn remove_candidate(&mut self, candidate_hash: &CandidateHash) {
if let Some(entry) = self.by_candidate_hash.remove(candidate_hash) {
let parent_head_hash = entry.candidate.persisted_validation_data.parent_head.hash();
if let Entry::Occupied(mut e) = self.by_parent_head.entry(parent_head_hash) {
e.get_mut().remove(&candidate_hash);
if e.get().is_empty() {
e.remove();
}
}
}
}
/// Note that an existing candidate has been seconded.
pub fn mark_seconded(&mut self, candidate_hash: &CandidateHash) {
if let Some(entry) = self.by_candidate_hash.get_mut(candidate_hash) {
if entry.state != CandidateState::Backed {
entry.state = CandidateState::Seconded;
}
}
}
/// Note that an existing candidate has been backed.
pub fn mark_backed(&mut self, candidate_hash: &CandidateHash) {
if let Some(entry) = self.by_candidate_hash.get_mut(candidate_hash) {
gum::trace!(target: LOG_TARGET, ?candidate_hash, "Candidate marked as backed");
entry.state = CandidateState::Backed;
} else {
gum::trace!(target: LOG_TARGET, ?candidate_hash, "Candidate not found while marking as backed");
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}
}
/// Whether a candidate is recorded as being backed.
pub fn is_backed(&self, candidate_hash: &CandidateHash) -> bool {
self.by_candidate_hash
.get(candidate_hash)
.map_or(false, |e| e.state == CandidateState::Backed)
}
/// Whether a candidate is contained within the storage already.
pub fn contains(&self, candidate_hash: &CandidateHash) -> bool {
self.by_candidate_hash.contains_key(candidate_hash)
}
/// Retain only candidates which pass the predicate.
pub(crate) fn retain(&mut self, pred: impl Fn(&CandidateHash) -> bool) {
self.by_candidate_hash.retain(|h, _v| pred(h));
self.by_parent_head.retain(|_parent, children| {
children.retain(|h| pred(h));
!children.is_empty()
});
self.by_output_head.retain(|_output, candidates| {
candidates.retain(|h| pred(h));
!candidates.is_empty()
});
}
/// Get head-data by hash.
pub(crate) fn head_data_by_hash(&self, hash: &Hash) -> Option<&HeadData> {
// First, search for candidates outputting this head data and extract the head data
// from their commitments if they exist.
//
// Otherwise, search for candidates building upon this head data and extract the head data
// from their persisted validation data if they exist.
self.by_output_head
.get(hash)
.and_then(|m| m.iter().next())
.and_then(|a_candidate| self.by_candidate_hash.get(a_candidate))
.map(|e| &e.candidate.commitments.head_data)
.or_else(|| {
self.by_parent_head
.get(hash)
.and_then(|m| m.iter().next())
.and_then(|a_candidate| self.by_candidate_hash.get(a_candidate))
.map(|e| &e.candidate.persisted_validation_data.parent_head)
})
}
/// Returns candidate's relay parent, if present.
pub(crate) fn relay_parent_by_candidate_hash(
&self,
candidate_hash: &CandidateHash,
) -> Option<Hash> {
self.by_candidate_hash.get(candidate_hash).map(|entry| entry.relay_parent)
}
fn iter_para_children<'a>(
&'a self,
parent_head_hash: &Hash,
) -> impl Iterator<Item = &'a CandidateEntry> + 'a {
let by_candidate_hash = &self.by_candidate_hash;
self.by_parent_head
.get(parent_head_hash)
.into_iter()
.flat_map(|hashes| hashes.iter())
.filter_map(move |h| by_candidate_hash.get(h))
}
fn get(&'_ self, candidate_hash: &CandidateHash) -> Option<&'_ CandidateEntry> {
self.by_candidate_hash.get(candidate_hash)
}
#[cfg(test)]
pub fn len(&self) -> (usize, usize) {
(self.by_parent_head.len(), self.by_candidate_hash.len())
}
}
/// The state of a candidate.
///
/// Candidates aren't even considered until they've at least been seconded.
#[derive(Debug, PartialEq)]
enum CandidateState {
/// The candidate has been introduced in a spam-protected way but
/// is not necessarily backed.
Introduced,
/// The candidate has been seconded.
Seconded,
/// The candidate has been completely backed by the group.
Backed,
}
#[derive(Debug)]
struct CandidateEntry {
candidate_hash: CandidateHash,
relay_parent: Hash,
candidate: ProspectiveCandidate<'static>,
state: CandidateState,
}
/// A candidate existing on-chain but pending availability, for special treatment
/// in the [`Scope`].
#[derive(Debug, Clone)]
pub(crate) struct PendingAvailability {
/// The candidate hash.
pub candidate_hash: CandidateHash,
/// The block info of the relay parent.
pub relay_parent: RelayChainBlockInfo,
}
/// The scope of a [`FragmentTree`].
#[derive(Debug)]
pub(crate) struct Scope {
para: ParaId,
relay_parent: RelayChainBlockInfo,
ancestors: BTreeMap<BlockNumber, RelayChainBlockInfo>,
ancestors_by_hash: HashMap<Hash, RelayChainBlockInfo>,
pending_availability: Vec<PendingAvailability>,
base_constraints: Constraints,
max_depth: usize,
}
/// An error variant indicating that ancestors provided to a scope
/// had unexpected order.
#[derive(Debug)]
pub struct UnexpectedAncestor {
/// The block number that this error occurred at.
pub number: BlockNumber,
/// The previous seen block number, which did not match `number`.
pub prev: BlockNumber,
}
impl Scope {
/// Define a new [`Scope`].
///
/// All arguments are straightforward except the ancestors.
///
/// Ancestors should be in reverse order, starting with the parent
/// of the `relay_parent`, and proceeding backwards in block number
/// increments of 1. Ancestors not following these conditions will be
/// rejected.
///
/// This function will only consume ancestors up to the `min_relay_parent_number` of
/// the `base_constraints`.
///
/// Only ancestors whose children have the same session as the relay-parent's
/// children should be provided.
///
/// It is allowed to provide zero ancestors.
pub fn with_ancestors(
para: ParaId,
relay_parent: RelayChainBlockInfo,
base_constraints: Constraints,
pending_availability: Vec<PendingAvailability>,
max_depth: usize,
ancestors: impl IntoIterator<Item = RelayChainBlockInfo>,
) -> Result<Self, UnexpectedAncestor> {
let mut ancestors_map = BTreeMap::new();
let mut ancestors_by_hash = HashMap::new();
{
let mut prev = relay_parent.number;
for ancestor in ancestors {
if prev == 0 {
return Err(UnexpectedAncestor { number: ancestor.number, prev })
} else if ancestor.number != prev - 1 {
return Err(UnexpectedAncestor { number: ancestor.number, prev })
} else if prev == base_constraints.min_relay_parent_number {
break
} else {
prev = ancestor.number;
ancestors_by_hash.insert(ancestor.hash, ancestor.clone());
ancestors_map.insert(ancestor.number, ancestor);
}
}
}
Ok(Scope {
para,
relay_parent,
base_constraints,
pending_availability,
max_depth,
ancestors: ancestors_map,
ancestors_by_hash,
})
}
/// Get the earliest relay-parent allowed in the scope of the fragment tree.
pub fn earliest_relay_parent(&self) -> RelayChainBlockInfo {
self.ancestors
.iter()
.next()
.map(|(_, v)| v.clone())
.unwrap_or_else(|| self.relay_parent.clone())
}
/// Get the ancestor of the fragment tree by hash.
pub fn ancestor_by_hash(&self, hash: &Hash) -> Option<RelayChainBlockInfo> {
if hash == &self.relay_parent.hash {
return Some(self.relay_parent.clone())
}
self.ancestors_by_hash.get(hash).map(|info| info.clone())
}
/// Whether the candidate in question is one pending availability in this scope.
pub fn get_pending_availability(
&self,
candidate_hash: &CandidateHash,
) -> Option<&PendingAvailability> {
self.pending_availability.iter().find(|c| &c.candidate_hash == candidate_hash)
}
/// Get the base constraints of the scope
pub fn base_constraints(&self) -> &Constraints {
&self.base_constraints
}
}
/// We use indices into a flat vector to refer to nodes in the tree.
/// Every tree also has an implicit root.
#[derive(Debug, Clone, Copy, PartialEq)]
enum NodePointer {
Root,
Storage(usize),
}
/// A hypothetical candidate, which may or may not exist in
/// the fragment tree already.
pub(crate) enum HypotheticalCandidate<'a> {
Complete {
receipt: Cow<'a, CommittedCandidateReceipt>,
persisted_validation_data: Cow<'a, PersistedValidationData>,
},
Incomplete {
relay_parent: Hash,
parent_head_data_hash: Hash,
},
}
impl<'a> HypotheticalCandidate<'a> {
fn parent_head_data_hash(&self) -> Hash {
match *self {
HypotheticalCandidate::Complete { ref persisted_validation_data, .. } =>
persisted_validation_data.as_ref().parent_head.hash(),
HypotheticalCandidate::Incomplete { ref parent_head_data_hash, .. } =>
*parent_head_data_hash,
}
}
fn relay_parent(&self) -> Hash {
match *self {
HypotheticalCandidate::Complete { ref receipt, .. } =>
receipt.descriptor().relay_parent,
HypotheticalCandidate::Incomplete { ref relay_parent, .. } => *relay_parent,
}
}
}
/// This is a tree of candidates based on some underlying storage of candidates and a scope.
///
/// All nodes in the tree must be either pending availability or within the scope. Within the scope
/// means it's built off of the relay-parent or an ancestor.
pub(crate) struct FragmentTree {
scope: Scope,
// Invariant: a contiguous prefix of the 'nodes' storage will contain
// the top-level children.
nodes: Vec<FragmentNode>,
// The candidates stored in this tree, mapped to a bitvec indicating the depths
// where the candidate is stored.
candidates: HashMap<CandidateHash, BitVec<u16, Msb0>>,
}
impl FragmentTree {
/// Create a new [`FragmentTree`] with given scope and populated from the storage.
///
/// Can be populated recursively (i.e. `populate` will pick up candidates that build on other
/// candidates).
pub fn populate(scope: Scope, storage: &CandidateStorage) -> Self {
gum::trace!(
target: LOG_TARGET,
relay_parent = ?scope.relay_parent.hash,
relay_parent_num = scope.relay_parent.number,
para_id = ?scope.para,
ancestors = scope.ancestors.len(),
"Instantiating Fragment Tree",
);
let mut tree = FragmentTree { scope, nodes: Vec::new(), candidates: HashMap::new() };
tree.populate_from_bases(storage, vec![NodePointer::Root]);
tree
}
/// Get the scope of the Fragment Tree.
pub fn scope(&self) -> &Scope {
&self.scope
}
// Inserts a node and updates child references in a non-root parent.
fn insert_node(&mut self, node: FragmentNode) {
let pointer = NodePointer::Storage(self.nodes.len());
let parent_pointer = node.parent;
let candidate_hash = node.candidate_hash;
let max_depth = self.scope.max_depth;
self.candidates
.entry(candidate_hash)
.or_insert_with(|| bitvec![u16, Msb0; 0; max_depth + 1])
.set(node.depth, true);
match parent_pointer {
NodePointer::Storage(ptr) => {
self.nodes.push(node);
self.nodes[ptr].children.push((pointer, candidate_hash))
},
NodePointer::Root => {
// Maintain the invariant of node storage beginning with depth-0.
if self.nodes.last().map_or(true, |last| last.parent == NodePointer::Root) {
self.nodes.push(node);
} else {
let pos =
self.nodes.iter().take_while(|n| n.parent == NodePointer::Root).count();
self.nodes.insert(pos, node);
}
},
}
}
fn node_has_candidate_child(
&self,
pointer: NodePointer,
candidate_hash: &CandidateHash,
) -> bool {
self.node_candidate_child(pointer, candidate_hash).is_some()
}
fn node_candidate_child(
&self,
pointer: NodePointer,
candidate_hash: &CandidateHash,
) -> Option<NodePointer> {
match pointer {
NodePointer::Root => self
.nodes
.iter()
.take_while(|n| n.parent == NodePointer::Root)
.enumerate()
.find(|(_, n)| &n.candidate_hash == candidate_hash)
.map(|(i, _)| NodePointer::Storage(i)),
NodePointer::Storage(ptr) =>
self.nodes.get(ptr).and_then(|n| n.candidate_child(candidate_hash)),
}
}
/// Returns an O(n) iterator over the hashes of candidates contained in the
/// tree.
pub(crate) fn candidates(&self) -> impl Iterator<Item = CandidateHash> + '_ {
self.candidates.keys().cloned()
}
/// Whether the candidate exists and at what depths.
pub(crate) fn candidate(&self, candidate: &CandidateHash) -> Option<Vec<usize>> {
self.candidates.get(candidate).map(|d| d.iter_ones().collect())
}
/// Add a candidate and recursively populate from storage.
///
/// Candidates can be added either as children of the root or children of other candidates.
pub(crate) fn add_and_populate(&mut self, hash: CandidateHash, storage: &CandidateStorage) {
let candidate_entry = match storage.get(&hash) {
None => return,
Some(e) => e,
};
let candidate_parent = &candidate_entry.candidate.persisted_validation_data.parent_head;
// Select an initial set of bases, whose required relay-parent matches that of the
// candidate.
let root_base = if &self.scope.base_constraints.required_parent == candidate_parent {
Some(NodePointer::Root)
} else {
None
};
let non_root_bases = self
.nodes
.iter()
.enumerate()
.filter(|(_, n)| {
n.cumulative_modifications.required_parent.as_ref() == Some(candidate_parent)
})
.map(|(i, _)| NodePointer::Storage(i));
let bases = root_base.into_iter().chain(non_root_bases).collect();
// Pass this into the population function, which will sanity-check stuff like depth,
// fragments, etc. and then recursively populate.
self.populate_from_bases(storage, bases);
}
/// Returns `true` if the path from the root to the node's parent (inclusive)
/// only contains backed candidates, `false` otherwise.
fn path_contains_backed_only_candidates(
&self,
mut parent_pointer: NodePointer,
candidate_storage: &CandidateStorage,
) -> bool {
while let NodePointer::Storage(ptr) = parent_pointer {
let node = &self.nodes[ptr];
let candidate_hash = &node.candidate_hash;
if candidate_storage.get(candidate_hash).map_or(true, |candidate_entry| {
!matches!(candidate_entry.state, CandidateState::Backed)
}) {
return false
}
parent_pointer = node.parent;
}
true
}
/// Returns the hypothetical depths where a candidate with the given hash and parent head data
/// would be added to the tree, without applying other candidates recursively on top of it.
///
/// If the candidate is already known, this returns the actual depths where this
/// candidate is part of the tree.
///
/// Setting `backed_in_path_only` to `true` ensures this function only returns such membership
/// that every candidate in the path from the root is backed.
pub(crate) fn hypothetical_depths(
&self,
hash: CandidateHash,
candidate: HypotheticalCandidate,
candidate_storage: &CandidateStorage,
backed_in_path_only: bool,
) -> Vec<usize> {
// if `true`, we always have to traverse the tree.
if !backed_in_path_only {
// if known.
if let Some(depths) = self.candidates.get(&hash) {
return depths.iter_ones().collect()
}
}
// if out of scope.
let candidate_relay_parent = candidate.relay_parent();
let candidate_relay_parent = if self.scope.relay_parent.hash == candidate_relay_parent {
self.scope.relay_parent.clone()
} else if let Some(info) = self.scope.ancestors_by_hash.get(&candidate_relay_parent) {
info.clone()
} else {
return Vec::new()
};
let max_depth = self.scope.max_depth;
let mut depths = bitvec![u16, Msb0; 0; max_depth + 1];
// iterate over all nodes where parent head-data matches,
// relay-parent number is <= candidate, and depth < max_depth.
let node_pointers = (0..self.nodes.len()).map(NodePointer::Storage);
for parent_pointer in std::iter::once(NodePointer::Root).chain(node_pointers) {
let (modifications, child_depth, earliest_rp) = match parent_pointer {
NodePointer::Root =>
(ConstraintModifications::identity(), 0, self.scope.earliest_relay_parent()),
NodePointer::Storage(ptr) => {
let node = &self.nodes[ptr];
let parent_rp = self
.scope
.ancestor_by_hash(&node.relay_parent())
.or_else(|| {
self.scope
.get_pending_availability(&node.candidate_hash)
.map(|_| self.scope.earliest_relay_parent())
})
.expect("All nodes in tree are either pending availability or within scope; qed");
(node.cumulative_modifications.clone(), node.depth + 1, parent_rp)
},
};
if child_depth > max_depth {
continue
}
if earliest_rp.number > candidate_relay_parent.number {
continue
}
let child_constraints =
match self.scope.base_constraints.apply_modifications(&modifications) {
Err(e) => {
gum::debug!(
target: LOG_TARGET,
new_parent_head = ?modifications.required_parent,
err = ?e,
"Failed to apply modifications",
);
continue
},
Ok(c) => c,
};
let parent_head_hash = candidate.parent_head_data_hash();
if parent_head_hash != child_constraints.required_parent.hash() {
continue
}
// We do additional checks for complete candidates.
if let HypotheticalCandidate::Complete { ref receipt, ref persisted_validation_data } =
candidate
{
let prospective_candidate = ProspectiveCandidate {
commitments: Cow::Borrowed(&receipt.commitments),
collator: receipt.descriptor().collator.clone(),
collator_signature: receipt.descriptor().signature.clone(),
persisted_validation_data: persisted_validation_data.as_ref().clone(),
pov_hash: receipt.descriptor().pov_hash,
validation_code_hash: receipt.descriptor().validation_code_hash,
};
if Fragment::new(
candidate_relay_parent.clone(),
child_constraints,
prospective_candidate,
)
.is_err()
{
continue
}
}
// Check that the path only contains backed candidates, if necessary.
if !backed_in_path_only ||
self.path_contains_backed_only_candidates(parent_pointer, candidate_storage)
{
depths.set(child_depth, true);
}
}
depths.iter_ones().collect()
}
/// Select `count` candidates after the given `ancestors` which pass
/// the predicate and have not already been backed on chain.
/// Does an exhaustive search into the tree after traversing the ancestors path.
/// If the ancestors draw out a path that can be traversed in multiple ways, no
/// candidates will be returned.
/// If the ancestors do not draw out a full path (the path contains holes), candidates will be
/// suggested that may fill these holes.
/// If the ancestors don't draw out a valid path, no candidates will be returned. If there are
/// multiple possibilities of the same size, this will select the first one. If there is no
/// chain of size `count` that matches the criteria, this will return the largest chain it could
/// find with the criteria. If there are no candidates meeting those criteria, returns an empty
/// `Vec`.
/// Cycles are accepted, but this code expects that the runtime will deduplicate
/// identical candidates when occupying the cores (when proposing to back A->B->A, only A will
/// be backed on chain).
/// The intention of the `ancestors` is to allow queries on the basis of
/// one or more candidates which were previously pending availability becoming
/// available or candidates timing out.
pub(crate) fn find_backable_chain(
ancestors: Ancestors,
count: u32,
pred: impl Fn(&CandidateHash) -> bool,
) -> Vec<CandidateHash> {
if count == 0 {
return vec![]
}
// First, we need to order the ancestors.
// The node returned is the one from which we can start finding new backable candidates.
let Some(base_node) = self.find_ancestor_path(ancestors) else { return vec![] };
self.find_backable_chain_inner(
base_node,
count,
count,
&pred,
&mut Vec::with_capacity(count as usize),
)
}
// Try finding a candidate chain starting from `base_node` of length `expected_count`.
// If not possible, return the longest one we could find.
// Does a depth-first search, since we're optimistic that there won't be more than one such
// chains (parachains shouldn't usually have forks). So in the usual case, this will conclude
// in `O(expected_count)`.
// Cycles are accepted, but this doesn't allow for infinite execution time, because the maximum
// depth we'll reach is `expected_count`.
//
// Worst case performance is `O(num_forks ^ expected_count)`, the same as populating the tree.
// Although an exponential function, this is actually a constant that can only be altered via
// sudo/governance, because:
// 1. `num_forks` at a given level is at most `max_candidate_depth * max_validators_per_core`
// (because each validator in the assigned group can second `max_candidate_depth`
// candidates). The prospective-parachains subsystem assumes that the number of para forks is
// limited by collator-protocol and backing subsystems. In practice, this is a constant which
// can only be altered by sudo or governance.
// 2. `expected_count` is equal to the number of cores a para is scheduled on (in an elastic
// scaling scenario). For non-elastic-scaling, this is just 1. In practice, this should be a
// small number (1-3), capped by the total number of available cores (a constant alterable
// only via governance/sudo).
fn find_backable_chain_inner(
&self,
base_node: NodePointer,
expected_count: u32,
remaining_count: u32,
pred: &dyn Fn(&CandidateHash) -> bool,
accumulator: &mut Vec<CandidateHash>,
) -> Vec<CandidateHash> {
if remaining_count == 0 {
// The best option is the chain we've accumulated so far.
return accumulator.to_vec();
}
let children: Vec<_> = match base_node {
NodePointer::Root => self
.nodes
.iter()
.enumerate()
.take_while(|(_, n)| n.parent == NodePointer::Root)
.filter(|(_, n)| self.scope.get_pending_availability(&n.candidate_hash).is_none())
.filter(|(_, n)| pred(&n.candidate_hash))
.map(|(ptr, n)| (NodePointer::Storage(ptr), n.candidate_hash))
.collect(),
NodePointer::Storage(base_node_ptr) => {
let base_node = &self.nodes[base_node_ptr];
base_node
.children
.iter()
.filter(|(_, hash)| self.scope.get_pending_availability(&hash).is_none())
.filter(|(_, hash)| pred(&hash))
.map(|(ptr, hash)| (*ptr, *hash))
.collect()
},
};
let mut best_result = accumulator.clone();
for (child_ptr, child_hash) in children {
accumulator.push(child_hash);
let result = self.find_backable_chain_inner(
child_ptr,
expected_count,
remaining_count - 1,
&pred,
accumulator,
);
accumulator.pop();
// Short-circuit the search if we've found the right length. Otherwise, we'll
// search for a max.
// Taking the first best selection doesn't introduce bias or become gameable,
// because `find_ancestor_path` uses a `HashSet` to track the ancestors, which
// makes the order in which ancestors are visited non-deterministic.
if result.len() == expected_count as usize {
return result
} else if best_result.len() < result.len() {
best_result = result;
}
best_result
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
// Orders the ancestors into a viable path from root to the last one.
// Returns a pointer to the last node in the path.
// We assume that the ancestors form a chain (that the
// av-cores do not back parachain forks), None is returned otherwise.
// If we cannot use all ancestors, stop at the first found hole in the chain. This usually
// translates to a timed out candidate.
fn find_ancestor_path(&self, mut ancestors: Ancestors) -> Option<NodePointer> {
// The number of elements in the path we've processed so far.
let mut depth = 0;
let mut last_node = NodePointer::Root;
let mut next_node: Option<NodePointer> = Some(NodePointer::Root);
while let Some(node) = next_node {
if depth > self.scope.max_depth {
return None;
}
last_node = node;
next_node = match node {
NodePointer::Root => {
let children = self
.nodes
.iter()
.enumerate()
.take_while(|n| n.1.parent == NodePointer::Root)
.map(|(index, node)| (NodePointer::Storage(index), node.candidate_hash))
.collect::<Vec<_>>();
self.find_valid_child(&mut ancestors, children.iter()).ok()?
},
NodePointer::Storage(ptr) => {
let children = self.nodes.get(ptr).and_then(|n| Some(n.children.iter()));
if let Some(children) = children {
self.find_valid_child(&mut ancestors, children).ok()?
} else {
None
}
},
};
depth += 1;
}
Some(last_node)
}
// Find a node from the given iterator which is present in the ancestors
// collection. If there are multiple such nodes, return an error and log a warning. We don't
// accept forks in a parachain to be backed. The supplied ancestors should all form a chain.
// If there is no such node, return None.
fn find_valid_child<'a>(
&self,
ancestors: &'a mut Ancestors,
nodes: impl Iterator<Item = &'a (NodePointer, CandidateHash)> + 'a,
) -> Result<Option<NodePointer>, ()> {
let mut possible_children =
nodes.filter_map(|(node_ptr, hash)| match ancestors.remove(&hash) {
true => Some(node_ptr),
false => None,
});
// We don't accept forks in a parachain to be backed. The supplied ancestors
// should all form a chain.
let next = possible_children.next();
if let Some(second_child) = possible_children.next() {
if let (Some(NodePointer::Storage(first_child)), NodePointer::Storage(second_child)) =
(next, second_child)
{
gum::error!(
target: LOG_TARGET,
para_id = ?self.scope.para,
relay_parent = ?self.scope.relay_parent,
"Trying to find new backable candidates for a parachain for which we've backed a fork.\
This is a bug and the runtime should not have allowed it.\n\
Backed candidates with the same parent: {}, {}",
self.nodes[*first_child].candidate_hash,
self.nodes[*second_child].candidate_hash,
);
}
Err(())
} else {
Ok(next.copied())
}
}
fn populate_from_bases(&mut self, storage: &CandidateStorage, initial_bases: Vec<NodePointer>) {
// Populate the tree breadth-first.
let mut last_sweep_start = None;
loop {
let sweep_start = self.nodes.len();
if Some(sweep_start) == last_sweep_start {
break
}
let parents: Vec<NodePointer> = if let Some(last_start) = last_sweep_start {
(last_start..self.nodes.len()).map(NodePointer::Storage).collect()
} else {
initial_bases.clone()
};
// 1. get parent head and find constraints
// 2. iterate all candidates building on the right head and viable relay parent
// 3. add new node
for parent_pointer in parents {
let (modifications, child_depth, earliest_rp) = match parent_pointer {
NodePointer::Root =>
(ConstraintModifications::identity(), 0, self.scope.earliest_relay_parent()),