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tka: implement synchronization mechanics
This PR implements the synchronization mechanics for TKA: generating a SyncOffer, processing a SyncOffer to find an intersection, and computing the set of AUMs that should be transmitted to effect convergence. This is the final PR implementing core mechanics for TKA. Signed-off-by: Tom DNetto <tom@tailscale.com>
This commit is contained in:
Tom DNetto
251
tka/sync.go
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251
tka/sync.go
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// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package tka
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import (
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"errors"
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"fmt"
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"os"
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)
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const (
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// Max iterations searching for any intersection.
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maxSyncIter = 2000
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// Max iterations searching for a head intersection.
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maxSyncHeadIntersectionIter = 400
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)
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// ErrNoIntersection is returned when a shared AUM could
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// not be determined when evaluating a remote sync offer.
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var ErrNoIntersection = errors.New("no intersection")
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// SyncOffer conveys information about the current head & ancestor AUMs,
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// for the purpose of synchronization with some remote end.
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//
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// Ancestors should contain a subset of the ancestors of the chain.
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// The last entry in that slice is the oldest-known AUM in the chain.
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type SyncOffer struct {
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Head AUMHash
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Ancestors []AUMHash
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}
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const (
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// The starting number of AUMs to skip when listing
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// ancestors in a SyncOffer.
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ancestorsSkipStart = 4
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// How many bits to advance the skip count when listing
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// ancestors in a SyncOffer.
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//
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// 2 bits, so (4<<2), so after skipping 4 it skips 16.
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ancestorsSkipShift = 2
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)
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func (a *Authority) syncOffer() (SyncOffer, error) {
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oldest := a.oldestAncestor.Hash()
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out := SyncOffer{
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Head: a.Head(),
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Ancestors: make([]AUMHash, 0, 6), // 6 chosen arbitrarily.
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}
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// We send some subset of our ancestors to help the remote
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// find a more-recent 'head intersection'.
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// The number of AUMs between each ancestor entry gets
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// exponentially larger.
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var (
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skipAmount uint64 = ancestorsSkipStart
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curs AUMHash = a.Head()
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)
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for i := uint64(0); i < maxSyncHeadIntersectionIter; i++ {
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if i > 0 && (i%skipAmount) == 0 {
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out.Ancestors = append(out.Ancestors, curs)
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skipAmount = skipAmount << ancestorsSkipShift
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}
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parent, err := a.storage.AUM(curs)
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if err != nil {
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if err != os.ErrNotExist {
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return SyncOffer{}, err
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}
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break
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}
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// We add the oldest later on, so don't duplicate.
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if parent.Hash() == oldest {
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break
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}
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copy(curs[:], parent.PrevAUMHash)
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}
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out.Ancestors = append(out.Ancestors, oldest)
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return out, nil
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}
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// SyncOffer returns an abbreviated description of the current AUM
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// chain, which can be used to synchronize with another (untrusted)
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// Authority instance.
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//
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// The returned SyncOffer structure should be transmitted to the remote
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// Authority, which should call MissingAUMs() using it to determine
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// AUMs which need to be transmitted. This list of AUMs from the remote
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// can then be applied locally with Inform().
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//
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// This SyncOffer + AUM exchange should be performed by both ends,
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// because its possible that either end has AUMs that the other needs
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// to find out about.
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func (a *Authority) SyncOffer() (SyncOffer, error) {
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return a.syncOffer()
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}
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// intersection describes how to synchronize AUMs with a remote
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// authority.
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type intersection struct {
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// if true, no exchange of AUMs is needed.
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upToDate bool
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// headIntersection is the latest common AUM on the remote. In other
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// words, we need to send all AUMs since this one.
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headIntersection *AUMHash
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// tailIntersection is the oldest common AUM on the remote. In other
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// words, we diverge with the remote after this AUM, so we both need
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// to transmit our AUM chain starting here.
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tailIntersection *AUMHash
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}
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// computeSyncIntersection determines the common AUMs between a local and
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// remote SyncOffer. This intersection can be used to synchronize both
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// sides.
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func computeSyncIntersection(authority *Authority, localOffer, remoteOffer SyncOffer) (*intersection, error) {
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// Simple case: up to date.
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if remoteOffer.Head == localOffer.Head {
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return &intersection{upToDate: true, headIntersection: &localOffer.Head}, nil
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}
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// Case: 'head intersection'
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// If we have the remote's head, its more likely than not that
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// we have updates that build on that head. To confirm this,
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// we iterate backwards through our chain to see if the given
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// head is an ancestor of our current chain.
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//
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// In other words:
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// <Us> A -> B -> C
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// <Them> A -> B
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// ∴ their head intersects with our chain, we need to send C
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var hasRemoteHead bool
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_, err := authority.storage.AUM(remoteOffer.Head)
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if err != nil {
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if err != os.ErrNotExist {
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return nil, err
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}
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} else {
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hasRemoteHead = true
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}
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if hasRemoteHead {
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curs := localOffer.Head
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for i := 0; i < maxSyncHeadIntersectionIter; i++ {
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parent, err := authority.storage.AUM(curs)
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if err != nil {
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if err != os.ErrNotExist {
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return nil, err
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}
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break
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}
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if parent.Hash() == remoteOffer.Head {
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h := parent.Hash()
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return &intersection{headIntersection: &h}, nil
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}
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copy(curs[:], parent.PrevAUMHash)
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}
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}
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// Case: 'tail intersection'
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// So we don't have a clue what the remote's head is, but
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// if one of the ancestors they gave us is part of our chain,
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// then theres an intersection, which is a starting point for
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// the remote to send us AUMs from.
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//
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// We iterate the list of ancestors in order because the remote
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// ordered them such that the newer ones are earlier, so with
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// a bit of luck we can use an earlier one and hence do less work /
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// transmit fewer AUMs.
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for _, a := range remoteOffer.Ancestors {
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state, err := computeStateAt(authority.storage, maxSyncIter, a)
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if err != nil {
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if err != os.ErrNotExist {
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return nil, fmt.Errorf("computeStateAt: %v", err)
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}
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continue
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}
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end, _, err := fastForward(authority.storage, maxSyncIter, state, func(curs AUM, _ State) bool {
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return curs.Hash() == localOffer.Head
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})
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if err != nil {
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return nil, err
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}
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// fastForward can terminate before the done condition if there are
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// no more children left, so we check again before considering this
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// an intersection.
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if end.Hash() == localOffer.Head {
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return &intersection{tailIntersection: &a}, nil
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}
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}
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return nil, ErrNoIntersection
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}
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// MissingAUMs returns AUMs a remote may be missing based on the
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// remotes' SyncOffer.
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func (a *Authority) MissingAUMs(remoteOffer SyncOffer) ([]AUM, error) {
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localOffer, err := a.syncOffer()
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if err != nil {
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return nil, fmt.Errorf("local syncOffer: %v", err)
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}
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intersection, err := computeSyncIntersection(a, localOffer, remoteOffer)
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if err != nil {
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return nil, fmt.Errorf("intersection: %v", err)
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}
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if intersection.upToDate {
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return nil, nil
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}
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out := make([]AUM, 0, 12) // 12 chosen arbitrarily.
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if intersection.headIntersection != nil {
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state, err := computeStateAt(a.storage, maxSyncIter, *intersection.headIntersection)
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if err != nil {
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return nil, err
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}
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_, _, err = fastForward(a.storage, maxSyncIter, state, func(curs AUM, _ State) bool {
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if curs.Hash() != *intersection.headIntersection {
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out = append(out, curs)
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}
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return false
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})
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return out, err
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}
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if intersection.tailIntersection != nil {
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state, err := computeStateAt(a.storage, maxSyncIter, *intersection.tailIntersection)
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if err != nil {
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return nil, err
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}
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_, _, err = fastForward(a.storage, maxSyncIter, state, func(curs AUM, _ State) bool {
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if curs.Hash() != *intersection.tailIntersection {
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out = append(out, curs)
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}
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return false
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})
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return out, err
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}
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panic("unreachable")
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}
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