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nodes.go
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nodes.go
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package protoarray
import (
"bytes"
"context"
"errors"
"fmt"
"math"
"github.com/prysmaticlabs/prysm/shared/params"
"go.opencensus.io/trace"
)
// 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.nodeIndices[justifiedRoot]
if !ok {
return [32]byte{}, errUnknownJustifiedRoot
}
if justifiedIndex >= uint64(len(s.nodes)) {
return [32]byte{}, errInvalidJustifiedIndex
}
justifiedNode := s.nodes[justifiedIndex]
bestDescendantIndex := justifiedNode.BestDescendent
// If the justified node doesn't have a best descendent,
// 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
}
return bestNode.root, 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 uint64,
root [32]byte,
parent [32]byte,
justifiedEpoch uint64, finalizedEpoch uint64) error {
ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.insert")
defer span.End()
s.nodeIndicesLock.Lock()
defer s.nodeIndicesLock.Unlock()
// Return if the block has been inserted into Store before.
if _, ok := s.nodeIndices[root]; ok {
return nil
}
index := len(s.nodes)
parentIndex, ok := s.nodeIndices[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,
BestDescendent: nonExistentNode,
Weight: 0,
}
s.nodeIndices[root] = uint64(index)
s.nodes = append(s.nodes, n)
// Update parent with the best child and descendent only if it's available.
if n.Parent != nonExistentNode {
if err := s.updateBestChildAndDescendant(parentIndex, uint64(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 best descendant.
func (s *Store) applyWeightChanges(ctx context.Context, justifiedEpoch uint64, finalizedEpoch uint64, delta []int) error {
ctx, 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
}
// Iterate backwards through all index to node in store.
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 nodeDelta < 0 {
// A node's weight can not be negative but the delta can be negative.
if int(n.Weight)+nodeDelta < 0 {
n.Weight = 0
} else {
// Subtract node's weight.
n.Weight -= uint64(math.Abs(float64(nodeDelta)))
}
} else {
// Add node's weight.
n.Weight += uint64(nodeDelta)
}
s.nodes[i] = n
// Update parent's best child and descendent 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
if err := s.updateBestChildAndDescendant(n.Parent, uint64(i)); err != nil {
return err
}
}
}
return nil
}
// updateBestChildAndDescendant updates parent node's best child and descendent.
// It looks at input parent node and input child node and potentially modifies parent's best
// child and best descendent 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 best child.
func (s *Store) updateBestChildAndDescendant(parentIndex uint64, 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.bestDescendent` to. These
// aliases are to assist readability.
changeToNone := []uint64{nonExistentNode, nonExistentNode}
bestDescendant := child.BestDescendent
if bestDescendant == nonExistentNode {
bestDescendant = childIndex
}
changeToChild := []uint64{childIndex, bestDescendant}
noChange := []uint64{parent.bestChild, parent.BestDescendent}
newParentChild := make([]uint64, 0)
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 best
// descendent 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 doesnt.
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.BestDescendent = 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) error {
ctx, span := trace.StartSpan(ctx, "protoArrayForkChoice.prune")
defer span.End()
s.nodeIndicesLock.Lock()
defer s.nodeIndicesLock.Unlock()
// The node would have seen finalized root or else it'd
// be able to prune it.
finalizedIndex, ok := s.nodeIndices[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
}
// Remove the key/values from indices mapping on to be pruned nodes.
// These nodes are before the finalized index.
for i := uint64(0); i < finalizedIndex; i++ {
if int(i) >= len(s.nodes) {
return errInvalidNodeIndex
}
delete(s.nodeIndices, s.nodes[i].root)
}
// Finalized index can not be greater than the length of the node.
if int(finalizedIndex) >= len(s.nodes) {
return errors.New("invalid finalized index")
}
s.nodes = s.nodes[finalizedIndex:]
// Adjust indices to node mapping.
for k, v := range s.nodeIndices {
s.nodeIndices[k] = v - finalizedIndex
}
// Iterate through existing nodes and adjust its parent/child indices with the newly pruned layout.
for i, node := range s.nodes {
if node.Parent != nonExistentNode {
// If the node's parent is less than finalized index, set it to non existent.
if node.Parent >= finalizedIndex {
node.Parent -= finalizedIndex
} else {
node.Parent = nonExistentNode
}
}
if node.bestChild != nonExistentNode {
if node.bestChild < finalizedIndex {
return errInvalidBestChildIndex
}
node.bestChild -= finalizedIndex
}
if node.BestDescendent != nonExistentNode {
if node.BestDescendent < finalizedIndex {
return errInvalidBestDescendantIndex
}
node.BestDescendent -= finalizedIndex
}
s.nodes[i] = node
}
prunedCount.Inc()
return nil
}
// leadsToViableHead returns true if the node or the best descendent 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 bestDescendentViable bool
bestDescendentIndex := node.BestDescendent
// If the best descendant is not part of the leaves.
if bestDescendentIndex != nonExistentNode {
// Protection against out of bound, best descendent index can not be
// exceeds length of nodes list.
if bestDescendentIndex >= uint64(len(s.nodes)) {
return false, errInvalidBestDescendantIndex
}
bestDescendentNode := s.nodes[bestDescendentIndex]
bestDescendentViable = s.viableForHead(bestDescendentNode)
}
// The node is viable as long as the best descendent is viable.
return bestDescendentViable || 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
}