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statemachine.go
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statemachine.go
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package votecollector
import (
"errors"
"fmt"
"sync"
"github.com/rs/zerolog"
"go.uber.org/atomic"
"github.com/onflow/flow-go/consensus/hotstuff"
"github.com/onflow/flow-go/consensus/hotstuff/model"
"github.com/onflow/flow-go/consensus/hotstuff/voteaggregator"
)
var (
ErrDifferentCollectorState = errors.New("different state")
)
// VerifyingVoteProcessorFactory generates hotstuff.VerifyingVoteCollector instances
type VerifyingVoteProcessorFactory = func(log zerolog.Logger, proposal *model.Proposal) (hotstuff.VerifyingVoteProcessor, error)
// VoteCollector implements a state machine for transition between different states of vote collector
type VoteCollector struct {
sync.Mutex
log zerolog.Logger
workers hotstuff.Workers
notifier hotstuff.Consumer
createVerifyingProcessor VerifyingVoteProcessorFactory
votesCache VotesCache
votesProcessor atomic.Value
}
var _ hotstuff.VoteCollector = (*VoteCollector)(nil)
func (m *VoteCollector) atomicLoadProcessor() hotstuff.VoteProcessor {
return m.votesProcessor.Load().(*atomicValueWrapper).processor
}
// atomic.Value doesn't allow storing interfaces as atomic values,
// it requires that stored type is always the same, so we need a wrapper that will mitigate this restriction
// https://github.com/golang/go/issues/22550
type atomicValueWrapper struct {
processor hotstuff.VoteProcessor
}
func NewStateMachineFactory(
log zerolog.Logger,
notifier hotstuff.Consumer,
verifyingVoteProcessorFactory VerifyingVoteProcessorFactory,
) voteaggregator.NewCollectorFactoryMethod {
return func(view uint64, workers hotstuff.Workers) (hotstuff.VoteCollector, error) {
return NewStateMachine(view, log, workers, notifier, verifyingVoteProcessorFactory), nil
}
}
func NewStateMachine(
view uint64,
log zerolog.Logger,
workers hotstuff.Workers,
notifier hotstuff.Consumer,
verifyingVoteProcessorFactory VerifyingVoteProcessorFactory,
) *VoteCollector {
log = log.With().
Str("hotstuff", "VoteCollector").
Uint64("view", view).
Logger()
sm := &VoteCollector{
log: log,
workers: workers,
notifier: notifier,
createVerifyingProcessor: verifyingVoteProcessorFactory,
votesCache: *NewVotesCache(view),
}
// without a block, we don't process votes (only cache them)
sm.votesProcessor.Store(&atomicValueWrapper{
processor: NewNoopCollector(hotstuff.VoteCollectorStatusCaching),
})
return sm
}
// AddVote adds a vote to current vote collector
// All expected errors are handled via callbacks to notifier.
// Under normal execution only exceptions are propagated to caller.
func (m *VoteCollector) AddVote(vote *model.Vote) error {
// Cache vote
err := m.votesCache.AddVote(vote)
if err != nil {
if errors.Is(err, RepeatedVoteErr) {
return nil
}
if doubleVoteErr, isDoubleVoteErr := model.AsDoubleVoteError(err); isDoubleVoteErr {
m.notifier.OnDoubleVotingDetected(doubleVoteErr.FirstVote, doubleVoteErr.ConflictingVote)
return nil
}
return fmt.Errorf("internal error adding vote %v to cache for block %v: %w",
vote.ID(), vote.BlockID, err)
}
err = m.processVote(vote)
if err != nil {
if errors.Is(err, VoteForIncompatibleBlockError) {
// For honest nodes, there should be only a single proposal per view and all votes should
// be for this proposal. However, byzantine nodes might deviate from this happy path:
// * A malicious leader might create multiple (individually valid) conflicting proposals for the
// same view. Honest replicas will send correct votes for whatever proposal they see first.
// We only accept the first valid block and reject any other conflicting blocks that show up later.
// * Alternatively, malicious replicas might send votes with the expected view, but for blocks that
// don't exist.
// In either case, receiving votes for the same view but for different block IDs is a symptom
// of malicious consensus participants. Hence, we log it here as a warning:
m.log.Warn().
Err(err).
Msg("received vote for incompatible block")
return nil
}
return fmt.Errorf("internal error processing vote %v for block %v: %w",
vote.ID(), vote.BlockID, err)
}
return nil
}
// processVote uses compare-and-repeat pattern to process vote with underlying vote processor
func (m *VoteCollector) processVote(vote *model.Vote) error {
for {
processor := m.atomicLoadProcessor()
currentState := processor.Status()
err := processor.Process(vote)
if err != nil {
if model.IsInvalidVoteError(err) {
m.notifier.OnInvalidVoteDetected(vote)
return nil
}
// ATTENTION: due to how our logic is designed this situation is only possible
// where we receive the same vote twice, this is not a case of double voting.
// This scenario is possible if leader submits his vote additionally to the vote in proposal.
if model.IsDuplicatedSignerError(err) {
return nil
}
return err
}
if currentState != m.Status() {
continue
}
return nil
}
}
// Status returns the status of underlying vote processor
func (m *VoteCollector) Status() hotstuff.VoteCollectorStatus {
return m.atomicLoadProcessor().Status()
}
// View returns view associated with this collector
func (m *VoteCollector) View() uint64 {
return m.votesCache.View()
}
// ProcessBlock performs validation of block signature and processes block with respected collector.
// In case we have received double proposal, we will stop attempting to build a QC for this view,
// because we don't want to build on any proposal from an equivocating primary. Note: slashing challenges
// for proposal equivocation are triggered by hotstuff.Forks, so we don't have to do anything else here.
//
// The internal state change is implemented as an atomic compare-and-swap, i.e.
// the state transition is only executed if VoteCollector's internal state is
// equal to `expectedValue`. The implementation only allows the transitions
// CachingVotes -> VerifyingVotes
// CachingVotes -> Invalid
// VerifyingVotes -> Invalid
func (m *VoteCollector) ProcessBlock(proposal *model.Proposal) error {
if proposal.Block.View != m.View() {
return fmt.Errorf("this VoteCollector requires a proposal for view %d but received block %v with view %d",
m.votesCache.View(), proposal.Block.BlockID, proposal.Block.View)
}
for {
proc := m.atomicLoadProcessor()
switch proc.Status() {
// first valid block for this view: commence state transition from caching to verifying
case hotstuff.VoteCollectorStatusCaching:
err := m.caching2Verifying(proposal)
if errors.Is(err, ErrDifferentCollectorState) {
continue // concurrent state update by other thread => restart our logic
}
if err != nil {
return fmt.Errorf("internal error updating VoteProcessor's status from %s to %s for block %v: %w",
proc.Status().String(), hotstuff.VoteCollectorStatusVerifying.String(), proposal.Block.BlockID, err)
}
m.log.Info().
Hex("block_id", proposal.Block.BlockID[:]).
Msg("vote collector status changed from caching to verifying")
m.processCachedVotes(proposal.Block)
// We already received a valid block for this view. Check whether the proposer is
// equivocating and terminate vote processing in this case. Note: proposal equivocation
// is handled by hotstuff.Forks, so we don't have to do anything else here.
case hotstuff.VoteCollectorStatusVerifying:
verifyingProc, ok := proc.(hotstuff.VerifyingVoteProcessor)
if !ok {
return fmt.Errorf("while processing block %v, found that VoteProcessor reports status %s but has an incompatible implementation type %T",
proposal.Block.BlockID, proc.Status(), verifyingProc)
}
if verifyingProc.Block().BlockID != proposal.Block.BlockID {
m.terminateVoteProcessing()
}
// Vote processing for this view has already been terminated. Note: proposal equivocation
// is handled by hotstuff.Forks, so we don't have anything to do here.
case hotstuff.VoteCollectorStatusInvalid: /* no op */
default:
return fmt.Errorf("while processing block %v, found that VoteProcessor reported unknown status %s", proposal.Block.BlockID, proc.Status())
}
return nil
}
}
// RegisterVoteConsumer registers a VoteConsumer. Upon registration, the collector
// feeds all cached votes into the consumer in the order they arrived.
// CAUTION, VoteConsumer implementations must be
// * NON-BLOCKING and consume the votes without noteworthy delay, and
// * CONCURRENCY SAFE
func (m *VoteCollector) RegisterVoteConsumer(consumer hotstuff.VoteConsumer) {
m.votesCache.RegisterVoteConsumer(consumer)
}
// caching2Verifying ensures that the VoteProcessor is currently in state `VoteCollectorStatusCaching`
// and replaces it by a newly-created VerifyingVoteProcessor.
// Error returns:
// * ErrDifferentCollectorState if the VoteCollector's state is _not_ `CachingVotes`
// * all other errors are unexpected and potential symptoms of internal bugs or state corruption (fatal)
func (m *VoteCollector) caching2Verifying(proposal *model.Proposal) error {
blockID := proposal.Block.BlockID
newProc, err := m.createVerifyingProcessor(m.log, proposal)
if err != nil {
return fmt.Errorf("failed to create VerifyingVoteProcessor for block %v: %w", blockID, err)
}
newProcWrapper := &atomicValueWrapper{processor: newProc}
m.Lock()
defer m.Unlock()
proc := m.atomicLoadProcessor()
if proc.Status() != hotstuff.VoteCollectorStatusCaching {
return fmt.Errorf("processors's current state is %s: %w", proc.Status().String(), ErrDifferentCollectorState)
}
m.votesProcessor.Store(newProcWrapper)
return nil
}
func (m *VoteCollector) terminateVoteProcessing() {
if m.Status() == hotstuff.VoteCollectorStatusInvalid {
return
}
newProcWrapper := &atomicValueWrapper{
processor: NewNoopCollector(hotstuff.VoteCollectorStatusInvalid),
}
m.Lock()
defer m.Unlock()
m.votesProcessor.Store(newProcWrapper)
}
// processCachedVotes feeds all cached votes into the VoteProcessor
func (m *VoteCollector) processCachedVotes(block *model.Block) {
for _, vote := range m.votesCache.All() {
if vote.BlockID != block.BlockID {
continue
}
blockVote := vote
voteProcessingTask := func() {
err := m.processVote(blockVote)
if err != nil {
m.log.Fatal().Err(err).Msg("internal error processing cached vote")
}
}
m.workers.Submit(voteProcessingTask)
}
}