prysm-pulse/beacon-chain/forkchoice/protoarray/store.go

package protoarray

import (
	"bytes"
	"context"
	"fmt"

	"github.com/pkg/errors"
	types "github.com/prysmaticlabs/eth2-types"
	fieldparams "github.com/prysmaticlabs/prysm/config/fieldparams"
	"github.com/prysmaticlabs/prysm/config/params"
	pbrpc "github.com/prysmaticlabs/prysm/proto/prysm/v1alpha1"
	"go.opencensus.io/trace"
)

// This defines the minimal number of block nodes that can be in the tree
// before getting pruned upon new finalization.
const defaultPruneThreshold = 256

// This tracks the last reported head root. Used for metrics.
var lastHeadRoot [32]byte

// New initializes a new fork choice store.
func New(justifiedEpoch, finalizedEpoch types.Epoch, finalizedRoot [32]byte) *ForkChoice {
	s := &Store{
		justifiedEpoch:    justifiedEpoch,
		finalizedEpoch:    finalizedEpoch,
		finalizedRoot:     finalizedRoot,
		proposerBoostRoot: [32]byte{},
		nodes:             make([]*Node, 0),
		nodesIndices:      make(map[[32]byte]uint64),
		canonicalNodes:    make(map[[32]byte]bool),
		pruneThreshold:    defaultPruneThreshold,
	}

	b := make([]uint64, 0)
	v := make([]Vote, 0)
	st := &optimisticStore{
		validatedTips: make(map[[32]byte]types.Slot),
	}
	return &ForkChoice{store: s, balances: b, votes: v, syncedTips: st}
}

// SetSyncedTips sets the synced and validated tips from the passed map
func (f *ForkChoice) SetSyncedTips(tips map[[32]byte]types.Slot) error {
	if len(tips) == 0 {
		return errInvalidSyncedTips
	}
	newTips := make(map[[32]byte]types.Slot, len(tips))
	for k, v := range tips {
		newTips[k] = v
	}
	f.syncedTips.Lock()
	defer f.syncedTips.Unlock()
	f.syncedTips.validatedTips = newTips
	return nil
}

// SyncedTips returns the synced and validated tips from the fork choice store.
func (f *ForkChoice) SyncedTips() map[[32]byte]types.Slot {
	f.syncedTips.RLock()
	defer f.syncedTips.RUnlock()

	m := make(map[[32]byte]types.Slot)
	for k, v := range f.syncedTips.validatedTips {
		m[k] = v
	}
	return m
}

// Head returns the head root from fork choice store.
// It firsts computes validator's balance changes then recalculates block tree from leaves to root.
func (f *ForkChoice) Head(
	ctx context.Context,
	justifiedEpoch types.Epoch,
	justifiedRoot [32]byte,
	justifiedStateBalances []uint64,
	finalizedEpoch types.Epoch,
) ([32]byte, error) {
	ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.Head")
	defer span.End()
	f.votesLock.Lock()
	defer f.votesLock.Unlock()

	calledHeadCount.Inc()

	newBalances := justifiedStateBalances

	// Using the write lock here because `updateCanonicalNodes` that gets called subsequently requires a write operation.
	f.store.nodesLock.Lock()
	defer f.store.nodesLock.Unlock()
	deltas, newVotes, err := computeDeltas(ctx, f.store.nodesIndices, f.votes, f.balances, newBalances)
	if err != nil {
		return [32]byte{}, errors.Wrap(err, "Could not compute deltas")
	}
	f.votes = newVotes

	if err := f.store.applyWeightChanges(ctx, justifiedEpoch, finalizedEpoch, newBalances, deltas); err != nil {
		return [32]byte{}, errors.Wrap(err, "Could not apply score changes")
	}
	f.balances = newBalances

	return f.store.head(ctx, justifiedRoot)
}

// ProcessAttestation processes attestation for vote accounting, it iterates around validator indices
// and update their votes accordingly.
func (f *ForkChoice) ProcessAttestation(ctx context.Context, validatorIndices []uint64, blockRoot [32]byte, targetEpoch types.Epoch) {
	_, span := trace.StartSpan(ctx, "protoArrayForkChoice.ProcessAttestation")
	defer span.End()
	f.votesLock.Lock()
	defer f.votesLock.Unlock()

	for _, index := range validatorIndices {
		// Validator indices will grow the vote cache.
		for index >= uint64(len(f.votes)) {
			f.votes = append(f.votes, Vote{currentRoot: params.BeaconConfig().ZeroHash, nextRoot: params.BeaconConfig().ZeroHash})
		}

		// Newly allocated vote if the root fields are untouched.
		newVote := f.votes[index].nextRoot == params.BeaconConfig().ZeroHash &&
			f.votes[index].currentRoot == params.BeaconConfig().ZeroHash

		// Vote gets updated if it's newly allocated or high target epoch.
		if newVote || targetEpoch > f.votes[index].nextEpoch {
			f.votes[index].nextEpoch = targetEpoch
			f.votes[index].nextRoot = blockRoot
		}
	}

	processedAttestationCount.Inc()
}

// NodeCount returns the current number of nodes in the Store
func (f *ForkChoice) NodeCount() int {
	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()
	return len(f.store.nodes)
}

// ProposerBoost returns the proposerBoost of the store
func (f *ForkChoice) ProposerBoost() [fieldparams.RootLength]byte {
	return f.store.proposerBoost()
}

// ProcessBlock processes a new block by inserting it to the fork choice store.
func (f *ForkChoice) ProcessBlock(
	ctx context.Context,
	slot types.Slot,
	blockRoot, parentRoot [32]byte,
	justifiedEpoch, finalizedEpoch types.Epoch, optimistic bool) error {
	ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.ProcessBlock")
	defer span.End()

	if err := f.store.insert(ctx, slot, blockRoot, parentRoot, justifiedEpoch, finalizedEpoch); err != nil {
		return err
	}

	if !optimistic {
		return f.SetOptimisticToValid(ctx, blockRoot)
	}
	return nil
}

// Prune prunes the fork choice store with the new finalized root. The store is only pruned if the input
// root is different than the current store finalized root, and the number of the store has met prune threshold.
func (f *ForkChoice) Prune(ctx context.Context, finalizedRoot [32]byte) error {
	return f.store.prune(ctx, finalizedRoot, f.syncedTips)
}

// HasNode returns true if the node exists in fork choice store,
// false else wise.
func (f *ForkChoice) HasNode(root [32]byte) bool {
	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()

	_, ok := f.store.nodesIndices[root]
	return ok
}

// HasParent returns true if the node parent exists in fork choice store,
// false else wise.
func (f *ForkChoice) HasParent(root [32]byte) bool {
	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()

	i, ok := f.store.nodesIndices[root]
	if !ok || i >= uint64(len(f.store.nodes)) {
		return false
	}

	return f.store.nodes[i].parent != NonExistentNode
}

// IsCanonical returns true if the given root is part of the canonical chain.
func (f *ForkChoice) IsCanonical(root [32]byte) bool {
	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()

	return f.store.canonicalNodes[root]
}

// AncestorRoot returns the ancestor root of input block root at a given slot.
func (f *ForkChoice) AncestorRoot(ctx context.Context, root [32]byte, slot types.Slot) ([]byte, error) {
	ctx, span := trace.StartSpan(ctx, "protoArray.AncestorRoot")
	defer span.End()

	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()

	i, ok := f.store.nodesIndices[root]
	if !ok {
		return nil, errors.New("node does not exist")
	}
	if i >= uint64(len(f.store.nodes)) {
		return nil, errors.New("node index out of range")
	}

	for f.store.nodes[i].slot > slot {
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}

		i = f.store.nodes[i].parent

		if i >= uint64(len(f.store.nodes)) {
			return nil, errors.New("node index out of range")
		}
	}

	return f.store.nodes[i].root[:], nil
}

// PruneThreshold of fork choice store.
func (s *Store) PruneThreshold() uint64 {
	return s.pruneThreshold
}

// JustifiedEpoch of fork choice store.
func (f *ForkChoice) JustifiedEpoch() types.Epoch {
	return f.store.justifiedEpoch
}

// FinalizedEpoch of fork choice store.
func (f *ForkChoice) FinalizedEpoch() types.Epoch {
	return f.store.finalizedEpoch
}

// proposerBoost of fork choice store.
func (s *Store) proposerBoost() [fieldparams.RootLength]byte {
	s.proposerBoostLock.RLock()
	defer s.proposerBoostLock.RUnlock()
	return s.proposerBoostRoot
}

// head starts from justified root and then follows the best descendant links
// to find the best block for head.
func (s *Store) head(ctx context.Context, justifiedRoot [32]byte) ([32]byte, error) {
	ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.head")
	defer span.End()

	// Justified index has to be valid in node indices map, and can not be out of bound.
	justifiedIndex, ok := s.nodesIndices[justifiedRoot]
	if !ok {
		return [32]byte{}, errUnknownJustifiedRoot
	}
	if justifiedIndex >= uint64(len(s.nodes)) {
		return [32]byte{}, errInvalidJustifiedIndex
	}

	justifiedNode := s.nodes[justifiedIndex]
	bestDescendantIndex := justifiedNode.bestDescendant
	// If the justified node doesn't have a best descendant,
	// the best node is itself.
	if bestDescendantIndex == NonExistentNode {
		bestDescendantIndex = justifiedIndex
	}
	if bestDescendantIndex >= uint64(len(s.nodes)) {
		return [32]byte{}, errInvalidBestDescendantIndex
	}

	bestNode := s.nodes[bestDescendantIndex]

	if !s.viableForHead(bestNode) {
		return [32]byte{}, fmt.Errorf("head at slot %d with weight %d is not eligible, finalizedEpoch %d != %d, justifiedEpoch %d != %d",
			bestNode.slot, bestNode.weight/10e9, bestNode.finalizedEpoch, s.finalizedEpoch, bestNode.justifiedEpoch, s.justifiedEpoch)
	}

	// Update metrics.
	if bestNode.root != lastHeadRoot {
		headChangesCount.Inc()
		headSlotNumber.Set(float64(bestNode.slot))
		lastHeadRoot = bestNode.root
	}

	// Update canonical mapping given the head root.
	if err := s.updateCanonicalNodes(ctx, bestNode.root); err != nil {
		return [32]byte{}, err
	}

	return bestNode.root, nil
}

// updateCanonicalNodes updates the canonical nodes mapping given the input block root.
func (s *Store) updateCanonicalNodes(ctx context.Context, root [32]byte) error {
	ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.updateCanonicalNodes")
	defer span.End()

	// Set the input node to canonical.
	i := s.nodesIndices[root]
	var newCanonicalRoots [][32]byte
	var n *Node
	for i != NonExistentNode {
		if ctx.Err() != nil {
			return ctx.Err()
		}

		// Get the parent node, if the node is already in canonical mapping,
		// we can be sure rest of the ancestors are canonical. Exit early.
		n = s.nodes[i]
		if s.canonicalNodes[n.root] {
			break
		}

		// Set parent node to canonical. Repeat until parent node index is undefined.
		newCanonicalRoots = append(newCanonicalRoots, n.root)
		i = n.parent
	}

	// i is either NonExistentNode or has the index of the last canonical
	// node before the last head update.
	if i == NonExistentNode {
		s.canonicalNodes = make(map[[fieldparams.RootLength]byte]bool)
	} else {
		for j := i + 1; j < uint64(len(s.nodes)); j++ {
			delete(s.canonicalNodes, s.nodes[j].root)
		}
	}

	for _, canonicalRoot := range newCanonicalRoots {
		s.canonicalNodes[canonicalRoot] = true
	}

	return nil
}

// insert registers a new block node to the fork choice store's node list.
// It then updates the new node's parent with best child and descendant node.
func (s *Store) insert(ctx context.Context,
	slot types.Slot,
	root, parent [32]byte,
	justifiedEpoch, finalizedEpoch types.Epoch) error {
	_, span := trace.StartSpan(ctx, "protoArrayForkChoice.insert")
	defer span.End()

	s.nodesLock.Lock()
	defer s.nodesLock.Unlock()

	// Return if the block has been inserted into Store before.
	if _, ok := s.nodesIndices[root]; ok {
		return nil
	}

	index := uint64(len(s.nodes))
	parentIndex, ok := s.nodesIndices[parent]
	// Mark genesis block's parent as non-existent.
	if !ok {
		parentIndex = NonExistentNode
	}

	n := &Node{
		slot:           slot,
		root:           root,
		parent:         parentIndex,
		justifiedEpoch: justifiedEpoch,
		finalizedEpoch: finalizedEpoch,
		bestChild:      NonExistentNode,
		bestDescendant: NonExistentNode,
		weight:         0,
	}

	s.nodesIndices[root] = index
	s.nodes = append(s.nodes, n)

	// Update parent with the best child and descendant only if it's available.
	if n.parent != NonExistentNode {
		if err := s.updateBestChildAndDescendant(parentIndex, index); err != nil {
			return err
		}
	}

	// Update metrics.
	processedBlockCount.Inc()
	nodeCount.Set(float64(len(s.nodes)))

	return nil
}

// applyWeightChanges iterates backwards through the nodes in store. It checks all nodes parent
// and its best child. For each node, it updates the weight with input delta and
// back propagate the nodes' delta to its parents' delta. After scoring changes,
// the best child is then updated along with the best descendant.
func (s *Store) applyWeightChanges(
	ctx context.Context, justifiedEpoch, finalizedEpoch types.Epoch, newBalances []uint64, delta []int,
) error {
	_, span := trace.StartSpan(ctx, "protoArrayForkChoice.applyWeightChanges")
	defer span.End()

	// The length of the nodes can not be different than length of the delta.
	if len(s.nodes) != len(delta) {
		return errInvalidDeltaLength
	}

	// Update the justified / finalized epochs in store if necessary.
	if s.justifiedEpoch != justifiedEpoch || s.finalizedEpoch != finalizedEpoch {
		s.justifiedEpoch = justifiedEpoch
		s.finalizedEpoch = finalizedEpoch
	}

	// Proposer score defaults to 0.
	proposerScore := uint64(0)

	// Iterate backwards through all index to node in store.
	var err error
	for i := len(s.nodes) - 1; i >= 0; i-- {
		n := s.nodes[i]

		// There is no need to adjust the balances or manage parent of the zero hash, it
		// is an alias to the genesis block.
		if n.root == params.BeaconConfig().ZeroHash {
			continue
		}

		nodeDelta := delta[i]

		// If we have a node where the proposer boost was previously applied,
		// we then decrease the delta by the required score amount.
		s.proposerBoostLock.Lock()
		if s.previousProposerBoostRoot != params.BeaconConfig().ZeroHash && s.previousProposerBoostRoot == n.root {
			nodeDelta -= int(s.previousProposerBoostScore)
		}

		if s.proposerBoostRoot != params.BeaconConfig().ZeroHash && s.proposerBoostRoot == n.root {
			proposerScore, err = computeProposerBoostScore(newBalances)
			if err != nil {
				s.proposerBoostLock.Unlock()
				return err
			}
			nodeDelta = nodeDelta + int(proposerScore)
		}
		s.proposerBoostLock.Unlock()

		// A node's weight can not be negative but the delta can be negative.
		if nodeDelta < 0 {
			d := uint64(-nodeDelta)
			if n.weight < d {
				n.weight = 0
			} else {
				n.weight -= d
			}
		} else {
			n.weight += uint64(nodeDelta)
		}

		// Update parent's best child and descendant if the node has a known parent.
		if n.parent != NonExistentNode {
			// Protection against node parent index out of bound. This should not happen.
			if int(n.parent) >= len(delta) {
				return errInvalidParentDelta
			}
			// Back propagate the nodes' delta to its parent.
			delta[n.parent] += nodeDelta
		}
	}

	// Set the previous boosted root and score.
	s.proposerBoostLock.Lock()
	s.previousProposerBoostRoot = s.proposerBoostRoot
	s.previousProposerBoostScore = proposerScore
	s.proposerBoostLock.Unlock()

	for i := len(s.nodes) - 1; i >= 0; i-- {
		n := s.nodes[i]
		if n.parent != NonExistentNode {
			if int(n.parent) >= len(delta) {
				return errInvalidParentDelta
			}
			if err := s.updateBestChildAndDescendant(n.parent, uint64(i)); err != nil {
				return err
			}
		}
	}

	return nil
}

// updateBestChildAndDescendant updates parent node's best child and descendant.
// It looks at input parent node and input child node and potentially modifies parent's best
// child and best descendant indices.
// There are four outcomes:
// 1.)  The child is already the best child, but it's now invalid due to a FFG change and should be removed.
// 2.)  The child is already the best child and the parent is updated with the new best descendant.
// 3.)  The child is not the best child but becomes the best child.
// 4.)  The child is not the best child and does not become the best child.
func (s *Store) updateBestChildAndDescendant(parentIndex, childIndex uint64) error {

	// Protection against parent index out of bound, this should not happen.
	if parentIndex >= uint64(len(s.nodes)) {
		return errInvalidNodeIndex
	}
	parent := s.nodes[parentIndex]

	// Protection against child index out of bound, again this should not happen.
	if childIndex >= uint64(len(s.nodes)) {
		return errInvalidNodeIndex
	}
	child := s.nodes[childIndex]

	// Is the child viable to become head? Based on justification and finalization rules.
	childLeadsToViableHead, err := s.leadsToViableHead(child)
	if err != nil {
		return err
	}

	// Define 3 variables for the 3 outcomes mentioned above. This is to
	// set `parent.bestChild` and `parent.bestDescendant` to. These
	// aliases are to assist readability.
	changeToNone := []uint64{NonExistentNode, NonExistentNode}
	bestDescendant := child.bestDescendant
	if bestDescendant == NonExistentNode {
		bestDescendant = childIndex
	}
	changeToChild := []uint64{childIndex, bestDescendant}
	noChange := []uint64{parent.bestChild, parent.bestDescendant}
	var newParentChild []uint64

	if parent.bestChild != NonExistentNode {
		if parent.bestChild == childIndex && !childLeadsToViableHead {
			// If the child is already the best child of the parent, but it's not viable for head,
			// we should remove it. (Outcome 1)
			newParentChild = changeToNone
		} else if parent.bestChild == childIndex {
			// If the child is already the best child of the parent, set it again to ensure the best
			// descendant of the parent is updated. (Outcome 2)
			newParentChild = changeToChild
		} else {
			// Protection against parent's best child going out of bound.
			if parent.bestChild > uint64(len(s.nodes)) {
				return errInvalidBestDescendantIndex
			}
			bestChild := s.nodes[parent.bestChild]
			// Is current parent's best child viable to be head? Based on justification and finalization rules.
			bestChildLeadsToViableHead, err := s.leadsToViableHead(bestChild)
			if err != nil {
				return err
			}

			if childLeadsToViableHead && !bestChildLeadsToViableHead {
				// The child leads to a viable head, but the current parent's best child doesn't.
				newParentChild = changeToChild
			} else if !childLeadsToViableHead && bestChildLeadsToViableHead {
				// The child doesn't lead to a viable head, the current parent's best child does.
				newParentChild = noChange
			} else if child.weight == bestChild.weight {
				// If both are viable, compare their weights.
				// Tie-breaker of equal weights by root.
				if bytes.Compare(child.root[:], bestChild.root[:]) > 0 {
					newParentChild = changeToChild
				} else {
					newParentChild = noChange
				}
			} else {
				// Choose winner by weight.
				if child.weight > bestChild.weight {
					newParentChild = changeToChild
				} else {
					newParentChild = noChange
				}
			}
		}
	} else {
		if childLeadsToViableHead {
			// If parent doesn't have a best child and the child is viable.
			newParentChild = changeToChild
		} else {
			// If parent doesn't have a best child and the child is not viable.
			newParentChild = noChange
		}
	}

	// Update parent with the outcome.
	parent.bestChild = newParentChild[0]
	parent.bestDescendant = newParentChild[1]
	s.nodes[parentIndex] = parent

	return nil
}

// prune prunes the store with the new finalized root. The tree is only
// pruned if the input finalized root are different than the one in stored and
// the number of the nodes in store has met prune threshold.
func (s *Store) prune(ctx context.Context, finalizedRoot [32]byte, syncedTips *optimisticStore) error {
	_, span := trace.StartSpan(ctx, "protoArrayForkChoice.prune")
	defer span.End()

	s.nodesLock.Lock()
	defer s.nodesLock.Unlock()

	// The node would have seen finalized root or else it
	// wouldn't be able to prune it.
	finalizedIndex, ok := s.nodesIndices[finalizedRoot]
	if !ok {
		return errUnknownFinalizedRoot
	}

	// The number of the nodes has not met the prune threshold.
	// Pruning at small numbers incurs more cost than benefit.
	if finalizedIndex < s.pruneThreshold {
		return nil
	}

	// Traverse through the node list starting from the finalized node at index 0.
	// Nodes that are not branching off from the finalized node will be removed.
	syncedTips.Lock()
	defer syncedTips.Unlock()

	canonicalNodesMap := make(map[uint64]uint64, uint64(len(s.nodes))-finalizedIndex)
	canonicalNodes := make([]*Node, 1, uint64(len(s.nodes))-finalizedIndex)
	finalizedNode := s.nodes[finalizedIndex]
	finalizedTipIndex, err := s.findSyncedTip(ctx, finalizedNode, syncedTips)
	if err != nil {
		return err
	}
	finalizedNode.parent = NonExistentNode
	canonicalNodes[0] = finalizedNode
	canonicalNodesMap[finalizedIndex] = uint64(0)

	for idx := uint64(0); idx < uint64(len(s.nodes)); idx++ {
		node := copyNode(s.nodes[idx])
		parentIdx, ok := canonicalNodesMap[node.parent]
		if ok {
			s.nodesIndices[node.root] = uint64(len(canonicalNodes))
			canonicalNodesMap[idx] = uint64(len(canonicalNodes))
			node.parent = parentIdx
			canonicalNodes = append(canonicalNodes, node)
		} else {
			// Remove node and synced tip that is not part of finalized branch.
			delete(s.nodesIndices, node.root)
			_, ok := syncedTips.validatedTips[node.root]
			if ok && idx != finalizedTipIndex {
				delete(syncedTips.validatedTips, node.root)
			}
		}
	}
	s.nodesIndices[finalizedRoot] = uint64(0)

	// Recompute the best child and descendant for each canonical nodes.
	for _, node := range canonicalNodes {
		if node.bestChild != NonExistentNode {
			node.bestChild = canonicalNodesMap[node.bestChild]
		}
		if node.bestDescendant != NonExistentNode {
			node.bestDescendant = canonicalNodesMap[node.bestDescendant]
		}
	}

	s.nodes = canonicalNodes
	prunedCount.Inc()
	syncedTipsCount.Set(float64(len(syncedTips.validatedTips)))
	return nil
}

// leadsToViableHead returns true if the node or the best descendant of the node is viable for head.
// Any node with diff finalized or justified epoch than the ones in fork choice store
// should not be viable to head.
func (s *Store) leadsToViableHead(node *Node) (bool, error) {
	var bestDescendantViable bool
	bestDescendantIndex := node.bestDescendant

	// If the best descendant is not part of the leaves.
	if bestDescendantIndex != NonExistentNode {
		// Protection against out of bound, the best descendant index can not be
		// exceeds length of nodes list.
		if bestDescendantIndex >= uint64(len(s.nodes)) {
			return false, errInvalidBestDescendantIndex
		}

		bestDescendantNode := s.nodes[bestDescendantIndex]
		bestDescendantViable = s.viableForHead(bestDescendantNode)
	}

	// The node is viable as long as the best descendant is viable.
	return bestDescendantViable || s.viableForHead(node), nil
}

// viableForHead returns true if the node is viable to head.
// Any node with diff finalized or justified epoch than the ones in fork choice store
// should not be viable to head.
func (s *Store) viableForHead(node *Node) bool {
	// `node` is viable if its justified epoch and finalized epoch are the same as the one in `Store`.
	// It's also viable if we are in genesis epoch.
	justified := s.justifiedEpoch == node.justifiedEpoch || s.justifiedEpoch == 0
	finalized := s.finalizedEpoch == node.finalizedEpoch || s.finalizedEpoch == 0

	return justified && finalized
}

// Returns the list of leaves in the Fork Choice store.
// These are all the nodes that have NonExistentNode as best child.
// This internal method assumes that the caller holds a lock in s.nodesLock.
func (s *Store) leaves() ([]uint64, error) {
	var leaves []uint64
	for i := uint64(0); i < uint64(len(s.nodes)); i++ {
		node := s.nodes[i]
		if node.bestChild == NonExistentNode {
			leaves = append(leaves, i)
		}
	}
	return leaves, nil
}

// Tips returns all possible chain heads (leaves of fork choice tree).
// Heads roots and heads slots are returned.
func (f *ForkChoice) Tips() ([][32]byte, []types.Slot) {

	// Deliberate choice to not preallocate space for below.
	// Heads cant be more than 2-3 in the worst case where pre-allocation will be 64 to begin with.
	headsRoots := make([][32]byte, 0)
	headsSlots := make([]types.Slot, 0)

	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()
	for _, node := range f.store.nodes {
		// Possible heads have no children.
		if node.BestDescendant() == NonExistentNode && node.BestChild() == NonExistentNode {
			headsRoots = append(headsRoots, node.Root())
			headsSlots = append(headsSlots, node.Slot())
		}
	}
	return headsRoots, headsSlots
}

func (f *ForkChoice) ForkChoiceNodes() []*pbrpc.ForkChoiceNode {
	f.store.nodesLock.RLock()
	defer f.store.nodesLock.RUnlock()
	ret := make([]*pbrpc.ForkChoiceNode, len(f.store.nodes))
	var parentRoot [32]byte
	for i, node := range f.store.nodes {
		root := node.Root()
		parentIdx := node.parent
		if parentIdx == NonExistentNode {
			parentRoot = params.BeaconConfig().ZeroHash
		} else {
			parent := f.store.nodes[parentIdx]
			parentRoot = parent.Root()
		}
		bestDescendantIdx := node.BestDescendant()
		var bestDescendantRoot [32]byte
		if bestDescendantIdx == NonExistentNode {
			bestDescendantRoot = params.BeaconConfig().ZeroHash
		} else {
			bestDescendantNode := f.store.nodes[bestDescendantIdx]
			bestDescendantRoot = bestDescendantNode.Root()
		}

		ret[i] = &pbrpc.ForkChoiceNode{
			Slot:           node.Slot(),
			Root:           root[:],
			Parent:         parentRoot[:],
			JustifiedEpoch: node.JustifiedEpoch(),
			FinalizedEpoch: node.FinalizedEpoch(),
			Weight:         node.Weight(),
			BestDescendant: bestDescendantRoot[:],
		}
	}
	return ret
}