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fragment_tree.rs
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fragment_tree.rs
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// Copyright (C) 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/>.
//! 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.
use std::{
borrow::Cow,
collections::{
hash_map::{Entry, HashMap},
BTreeMap, HashSet,
},
};
use super::LOG_TARGET;
use bitvec::prelude::*;
use polkadot_node_subsystem_util::inclusion_emulator::{
ConstraintModifications, Constraints, Fragment, ProspectiveCandidate, RelayChainBlockInfo,
};
use polkadot_primitives::{
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) {
entry.state = CandidateState::Backed;
}
}
/// 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 a candidate after the given `required_path` which passes
/// the predicate.
///
/// If there are multiple possibilities, this will select the first one.
///
/// This returns `None` if there is no candidate meeting those criteria.
///
/// The intention of the `required_path` is to allow queries on the basis of
/// one or more candidates which were previously pending availability becoming
/// available and opening up more room on the core.
pub(crate) fn select_child(
&self,
required_path: &[CandidateHash],
pred: impl Fn(&CandidateHash) -> bool,
) -> Option<CandidateHash> {
let base_node = {
// traverse the required path.
let mut node = NodePointer::Root;
for required_step in required_path {
node = self.node_candidate_child(node, &required_step)?;
}
node
};
// TODO [now]: taking the first selection might introduce bias
// or become gameable.
//
// For plausibly unique parachains, this shouldn't matter much.
// figure out alternative selection criteria?
match base_node {
NodePointer::Root => self
.nodes
.iter()
.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(|n| n.candidate_hash)
.next(),
NodePointer::Storage(ptr) => self.nodes[ptr]
.children
.iter()
.filter(|n| self.scope.get_pending_availability(&n.1).is_none())
.filter(|n| pred(&n.1))
.map(|n| n.1)
.next(),
}
}
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()),
NodePointer::Storage(ptr) => {
let node = &self.nodes[ptr];
let parent_rp = self
.scope
.ancestor_by_hash(&node.relay_parent())
.or_else(|| {
// if the relay-parent is out of scope _and_ it is in the tree,
// it must be a candidate pending availability.
self.scope
.get_pending_availability(&node.candidate_hash)
.map(|c| c.relay_parent.clone())
})
.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 > self.scope.max_depth {
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,
};
// Add nodes to tree wherever
// 1. parent hash is correct
// 2. relay-parent does not move backwards.
// 3. all non-pending-availability candidates have relay-parent in scope.
// 4. candidate outputs fulfill constraints
let required_head_hash = child_constraints.required_parent.hash();
for candidate in storage.iter_para_children(&required_head_hash) {
let pending = self.scope.get_pending_availability(&candidate.candidate_hash);
let relay_parent = pending
.map(|p| p.relay_parent.clone())
.or_else(|| self.scope.ancestor_by_hash(&candidate.relay_parent));
let relay_parent = match relay_parent {
Some(r) => r,
None => continue,
};
// require: pending availability candidates don't move backwards
// and only those can be out-of-scope.
//
// earliest_rp can be before the earliest relay parent in the scope
// when the parent is a pending availability candidate as well, but
// only other pending candidates can have a relay parent out of scope.
let min_relay_parent_number = pending
.map(|p| match parent_pointer {
NodePointer::Root => p.relay_parent.number,
NodePointer::Storage(_) => earliest_rp.number,
})
.unwrap_or_else(|| {
std::cmp::max(
earliest_rp.number,
self.scope.earliest_relay_parent().number,
)
});
if relay_parent.number < min_relay_parent_number {
continue // relay parent moved backwards.
}
// don't add candidates where the parent already has it as a child.
if self.node_has_candidate_child(parent_pointer, &candidate.candidate_hash) {
continue
}
let fragment = {
let mut constraints = child_constraints.clone();
if let Some(ref p) = pending {
// overwrite for candidates pending availability as a special-case.
constraints.min_relay_parent_number = p.relay_parent.number;
}
let f = Fragment::new(
relay_parent.clone(),
constraints,
candidate.candidate.partial_clone(),
);
match f {
Ok(f) => f.into_owned(),
Err(e) => {
gum::debug!(
target: LOG_TARGET,
err = ?e,
?relay_parent,
candidate_hash = ?candidate.candidate_hash,
"Failed to instantiate fragment",
);
continue
},
}
};
let mut cumulative_modifications = modifications.clone();
cumulative_modifications.stack(fragment.constraint_modifications());
let node = FragmentNode {
parent: parent_pointer,
fragment,
candidate_hash: candidate.candidate_hash,
depth: child_depth,
cumulative_modifications,
children: Vec::new(),
};
self.insert_node(node);
}
}
last_sweep_start = Some(sweep_start);
}
}
}
struct FragmentNode {
// A pointer to the parent node.
parent: NodePointer,
fragment: Fragment<'static>,
candidate_hash: CandidateHash,
depth: usize,
cumulative_modifications: ConstraintModifications,
children: Vec<(NodePointer, CandidateHash)>,
}
impl FragmentNode {
fn relay_parent(&self) -> Hash {
self.fragment.relay_parent().hash
}
fn candidate_child(&self, candidate_hash: &CandidateHash) -> Option<NodePointer> {
self.children.iter().find(|(_, c)| c == candidate_hash).map(|(p, _)| *p)
}
}
#[cfg(test)]
mod tests {
use super::*;
use assert_matches::assert_matches;
use polkadot_node_subsystem_util::inclusion_emulator::InboundHrmpLimitations;
use polkadot_primitives::{BlockNumber, CandidateCommitments, CandidateDescriptor, HeadData};
use polkadot_primitives_test_helpers as test_helpers;
fn make_constraints(
min_relay_parent_number: BlockNumber,
valid_watermarks: Vec<BlockNumber>,
required_parent: HeadData,
) -> Constraints {
Constraints {
min_relay_parent_number,
max_pov_size: 1_000_000,
max_code_size: 1_000_000,
ump_remaining: 10,
ump_remaining_bytes: 1_000,
max_ump_num_per_candidate: 10,
dmp_remaining_messages: [0; 10].into(),