mirror of
https://github.com/tailscale/tailscale.git
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4f1374ec9e
Signed-off-by: Tom DNetto <tom@tailscale.com>
339 lines
11 KiB
Go
339 lines
11 KiB
Go
// 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 (WIP) implements the Tailnet Key Authority.
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package tka
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import (
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"bytes"
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"errors"
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"fmt"
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"os"
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"sort"
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)
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// A chain describes a linear sequence of updates from Oldest to Head,
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// resulting in some State at Head.
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type chain struct {
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Oldest AUM
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Head AUM
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state State
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// Set to true if the AUM chain intersects with the active
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// chain from a previous run.
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chainsThroughActive bool
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}
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// computeChainCandidates returns all possible chains based on AUMs stored
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// in the given tailchonk. A chain is defined as a unique (oldest, newest)
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// AUM tuple. chain.state is not yet populated in returned chains.
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//
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// If lastKnownOldest is provided, any chain that includes the given AUM
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// has the chainsThroughActive field set to true. This bit is leveraged
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// in computeActiveAncestor() to filter out irrelevant chains when determining
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// the active ancestor from a list of distinct chains.
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func computeChainCandidates(storage Chonk, lastKnownOldest *AUMHash, maxIter int) ([]chain, error) {
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heads, err := storage.Heads()
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if err != nil {
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return nil, fmt.Errorf("reading heads: %v", err)
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}
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candidates := make([]chain, len(heads))
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for i := range heads {
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// Oldest is iteratively computed below.
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candidates[i] = chain{Oldest: heads[i], Head: heads[i]}
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}
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// Not strictly necessary, but simplifies checks in tests.
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sort.Slice(candidates, func(i, j int) bool {
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ih, jh := candidates[i].Oldest.Hash(), candidates[j].Oldest.Hash()
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return bytes.Compare(ih[:], jh[:]) < 0
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})
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// candidates.Oldest needs to be computed by working backwards from
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// head as far as we can.
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iterAgain := true // if theres still work to be done.
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for i := 0; iterAgain; i++ {
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if i >= maxIter {
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return nil, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
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}
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iterAgain = false
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for j := range candidates {
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parent, hasParent := candidates[j].Oldest.Parent()
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if hasParent {
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parent, err := storage.AUM(parent)
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if err != nil {
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if err == os.ErrNotExist {
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continue
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}
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return nil, fmt.Errorf("reading parent: %v", err)
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}
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candidates[j].Oldest = parent
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if lastKnownOldest != nil && *lastKnownOldest == parent.Hash() {
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candidates[j].chainsThroughActive = true
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}
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iterAgain = true
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}
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}
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}
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return candidates, nil
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}
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// pickNextAUM returns the AUM which should be used as the next
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// AUM in the chain, possibly applying fork resolution logic.
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//
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// In other words: given an AUM with 3 children like this:
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// / - 1
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// P - 2
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// \ - 3
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//
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// pickNextAUM will determine and return the correct branch.
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//
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// This method takes ownership of the provided slice.
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func pickNextAUM(state State, candidates []AUM) AUM {
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switch len(candidates) {
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case 0:
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panic("pickNextAUM called with empty candidate set")
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case 1:
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return candidates[0]
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}
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// Oooof, we have some forks in the chain. We need to pick which
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// one to use by applying the Fork Resolution Algorithm ✨
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//
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// The rules are this:
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// 1. The child with the highest signature weight is chosen.
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// 2. If equal, the child which is a RemoveKey AUM is chosen.
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// 3. If equal, the child with the lowest AUM hash is chosen.
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sort.Slice(candidates, func(j, i int) bool {
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// Rule 1.
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iSigWeight, jSigWeight := candidates[i].Weight(state), candidates[j].Weight(state)
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if iSigWeight != jSigWeight {
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return iSigWeight < jSigWeight
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}
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// Rule 2.
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if iKind, jKind := candidates[i].MessageKind, candidates[j].MessageKind; iKind != jKind &&
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(iKind == AUMRemoveKey || jKind == AUMRemoveKey) {
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return jKind == AUMRemoveKey
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}
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// Rule 3.
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iHash, jHash := candidates[i].Hash(), candidates[j].Hash()
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return bytes.Compare(iHash[:], jHash[:]) > 0
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})
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return candidates[0]
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}
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// advanceChain computes the next AUM to advance with based on all child
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// AUMs, returning the chosen AUM & the state obtained by applying that
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// AUM.
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//
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// The return value for next is nil if there are no children AUMs, hence
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// the provided state is at head (up to date).
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func advanceChain(state State, candidates []AUM) (next *AUM, out State, err error) {
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if len(candidates) == 0 {
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return nil, state, nil
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}
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aum := pickNextAUM(state, candidates)
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if state, err = state.applyVerifiedAUM(aum); err != nil {
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return nil, State{}, fmt.Errorf("advancing state: %v", err)
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}
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return &aum, state, nil
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}
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// fastForward iteratively advances the current state based on known AUMs until
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// the given termination function returns true or there is no more progress possible.
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//
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// The last-processed AUM, and the state computed after applying the last AUM,
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// are returned.
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func fastForward(storage Chonk, maxIter int, startState State, done func(curAUM AUM, curState State) bool) (AUM, State, error) {
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if startState.LastAUMHash == nil {
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return AUM{}, State{}, errors.New("invalid initial state")
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}
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nextAUM, err := storage.AUM(*startState.LastAUMHash)
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if err != nil {
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return AUM{}, State{}, fmt.Errorf("reading next: %v", err)
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}
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curs := nextAUM
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state := startState
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for i := 0; i < maxIter; i++ {
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if done != nil && done(curs, state) {
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return curs, state, nil
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}
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children, err := storage.ChildAUMs(curs.Hash())
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if err != nil {
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return AUM{}, State{}, fmt.Errorf("getting children of %X: %v", curs.Hash(), err)
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}
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next, nextState, err := advanceChain(state, children)
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if err != nil {
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return AUM{}, State{}, fmt.Errorf("advance %X: %v", curs.Hash(), err)
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}
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if next == nil {
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// There were no more children, we are at 'head'.
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return curs, state, nil
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}
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curs = *next
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state = nextState
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}
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return AUM{}, State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
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}
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// computeStateAt returns the State at wantHash.
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func computeStateAt(storage Chonk, maxIter int, wantHash AUMHash) (State, error) {
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// TODO(tom): This is going to get expensive for really long
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// chains. We should make nodes emit a checkpoint every
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// X updates or something.
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topAUM, err := storage.AUM(wantHash)
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if err != nil {
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return State{}, err
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}
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// Iterate backwards till we find a starting point to compute
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// the state from.
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//
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// Valid starting points are either a checkpoint AUM, or a
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// genesis AUM.
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curs := topAUM
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var state State
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for i := 0; true; i++ {
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if i > maxIter {
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return State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
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}
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// Checkpoints encapsulate the state at that point, dope.
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if curs.MessageKind == AUMCheckpoint {
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state = curs.State.cloneForUpdate(&curs)
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break
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}
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parent, hasParent := curs.Parent()
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if !hasParent {
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// This is a 'genesis' update: there are none before it, so
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// this AUM can be applied to the empty state to determine
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// the state at this AUM.
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//
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// It is only valid for NoOp, AddKey, and Checkpoint AUMs
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// to be a genesis update. Checkpoint was handled earlier.
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if mk := curs.MessageKind; mk == AUMNoOp || mk == AUMAddKey {
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var err error
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if state, err = (State{}).applyVerifiedAUM(curs); err != nil {
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return State{}, fmt.Errorf("applying genesis (%+v): %v", curs, err)
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}
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break
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}
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return State{}, fmt.Errorf("invalid genesis update: %+v", curs)
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}
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// If we got here, the current state is dependent on the previous.
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// Keep iterating backwards till thats not the case.
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if curs, err = storage.AUM(parent); err != nil {
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return State{}, fmt.Errorf("reading parent: %v", err)
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}
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}
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// We now know some starting point state. Iterate forward till we
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// are at the AUM we want state for.
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_, state, err = fastForward(storage, maxIter, state, func(curs AUM, _ State) bool {
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return curs.Hash() == wantHash
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})
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// fastForward only terminates before the done condition if it
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// doesnt have any later AUMs to process. This cant be the case
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// as we've already iterated through them above so they must exist,
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// but we check anyway to be super duper sure.
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if err == nil && *state.LastAUMHash != wantHash {
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panic("unexpected fastForward outcome")
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}
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return state, err
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}
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// computeActiveAncestor determines which ancestor AUM to use as the
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// ancestor of the valid chain.
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//
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// If all the chains end up having the same ancestor, then thats the
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// only possible ancestor, ezpz. However if there are multiple distinct
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// ancestors, that means there are distinct chains, and we need some
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// hint to choose what to use. For that, we rely on the chainsThroughActive
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// bit, which signals to us that that ancestor was part of the
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// chain in a previous run.
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func computeActiveAncestor(storage Chonk, chains []chain) (AUMHash, error) {
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// Dedupe possible ancestors, tracking if they were part of
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// the active chain on a previous run.
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ancestors := make(map[AUMHash]bool, len(chains))
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for _, c := range chains {
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ancestors[c.Oldest.Hash()] = c.chainsThroughActive
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}
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if len(ancestors) == 1 {
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// There's only one. DOPE.
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for k, _ := range ancestors {
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return k, nil
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}
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}
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// Theres more than one, so we need to use the ancestor that was
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// part of the active chain in a previous iteration.
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// Note that there can only be one distinct ancestor that was
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// formerly part of the active chain, because AUMs can only have
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// one parent and would have converged to a common ancestor.
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for k, chainsThroughActive := range ancestors {
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if chainsThroughActive {
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return k, nil
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}
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}
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return AUMHash{}, errors.New("multiple distinct chains")
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}
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// computeActiveChain bootstraps the runtime state of the Authority when
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// starting entirely off stored state.
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//
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// TODO(tom): Don't look at head states, just iterate forward from
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// the ancestor.
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//
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// The algorithm is as follows:
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// 1. Determine all possible 'head' (like in git) states.
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// 2. Filter these possible chains based on whether the ancestor was
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// formerly (in a previous run) part of the chain.
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// 3. Compute the state of the state machine at this ancestor. This is
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// needed for fast-forward, as each update operates on the state of
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// the update preceeding it.
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// 4. Iteratively apply updates till we reach head ('fast forward').
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func computeActiveChain(storage Chonk, lastKnownOldest *AUMHash, maxIter int) (chain, error) {
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chains, err := computeChainCandidates(storage, lastKnownOldest, maxIter)
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if err != nil {
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return chain{}, fmt.Errorf("computing candidates: %v", err)
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}
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// Find the right ancestor.
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oldestHash, err := computeActiveAncestor(storage, chains)
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if err != nil {
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return chain{}, fmt.Errorf("computing ancestor: %v", err)
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}
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ancestor, err := storage.AUM(oldestHash)
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if err != nil {
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return chain{}, err
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}
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// At this stage we know the ancestor AUM, so we have excluded distinct
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// chains but we might still have forks (so we don't know the head AUM).
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//
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// We iterate forward from the ancestor AUM, handling any forks as we go
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// till we arrive at a head.
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out := chain{Oldest: ancestor, Head: ancestor}
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if out.state, err = computeStateAt(storage, maxIter, oldestHash); err != nil {
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return chain{}, fmt.Errorf("bootstrapping state: %v", err)
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}
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out.Head, out.state, err = fastForward(storage, maxIter, out.state, nil)
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if err != nil {
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return chain{}, fmt.Errorf("fast forward: %v", err)
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}
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return out, nil
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}
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