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Exchange Head single-flight protection #229

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32 changes: 26 additions & 6 deletions p2p/exchange.go
Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ import (
"context"
"errors"
"fmt"
"golang.org/x/sync/singleflight"
"math/rand"
"sort"
"time"
Expand Down Expand Up @@ -35,6 +36,10 @@ var (
// chosen.
const minHeadResponses = 2

// syncKeyHeadOrigin represents the origin value used specifically for Head requests,
// serving as a sync key to prevent redundant queries.
const syncKeyHeadOrigin = "0"

// maxUntrustedHeadRequests is the number of head requests to be made to
// the network in order to determine the network head.
var maxUntrustedHeadRequests = 4
Expand All @@ -52,6 +57,8 @@ type Exchange[H header.Header[H]] struct {
peerTracker *peerTracker
metrics *exchangeMetrics

singleFlight *singleflight.Group

Params ClientParameters
}

Expand Down Expand Up @@ -81,11 +88,12 @@ func NewExchange[H header.Header[H]](
}

ex := &Exchange[H]{
host: host,
protocolID: protocolID(params.networkID),
peerTracker: newPeerTracker(host, gater, params.pidstore, metrics),
Params: params,
metrics: metrics,
host: host,
protocolID: protocolID(params.networkID),
peerTracker: newPeerTracker(host, gater, params.pidstore, metrics),
Params: params,
metrics: metrics,
singleFlight: &singleflight.Group{},
}

ex.trustedPeers = func() peer.IDSlice {
Expand Down Expand Up @@ -124,6 +132,19 @@ func (ex *Exchange[H]) Head(ctx context.Context, opts ...header.HeadOption[H]) (
ctx, span := tracerClient.Start(ctx, "head")
defer span.End()

head, err, _ := ex.singleFlight.Do(syncKeyHeadOrigin, func() (interface{}, error) {
return ex.head(ctx, span, opts...)
})
ex.singleFlight.Forget(syncKeyHeadOrigin)
if err != nil {
span.SetStatus(codes.Error, err.Error())
return head.(H), err
}
span.SetStatus(codes.Ok, "")
return head.(H), nil
}

func (ex *Exchange[H]) head(ctx context.Context, span trace.Span, opts ...header.HeadOption[H]) (H, error) {
reqCtx := ctx
startTime := time.Now()
if deadline, ok := ctx.Deadline(); ok {
Expand Down Expand Up @@ -244,7 +265,6 @@ func (ex *Exchange[H]) Head(ctx context.Context, opts ...header.HeadOption[H]) (
}

ex.metrics.head(ctx, time.Since(startTime), len(headers), headType, headStatusOk)
span.SetStatus(codes.Ok, "")
return head, nil
}

Expand Down
59 changes: 59 additions & 0 deletions p2p/exchange_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ package p2p
import (
"context"
"strconv"
sync2 "sync"
"testing"
"time"

Expand Down Expand Up @@ -156,6 +157,64 @@ func TestExchange_RequestHead_UnresponsivePeer(t *testing.T) {
assert.NotNil(t, head)
}

func TestExchange_RequestHeadFlightProtection(t *testing.T) {
ctx, cancel := context.WithCancel(context.Background())
t.Cleanup(cancel)

hosts := createMocknet(t, 3)
exchg, trustedStore := createP2PExAndServer(t, hosts[0], hosts[1])

// create the same requests
tests := []struct {
requestFromTrusted bool
lastHeader *headertest.DummyHeader
expectedHeight uint64
expectedHash header.Hash
}{
{
requestFromTrusted: true,
lastHeader: trustedStore.Headers[trustedStore.HeadHeight-1],
expectedHeight: trustedStore.HeadHeight,
expectedHash: trustedStore.Headers[trustedStore.HeadHeight].Hash(),
},
{
// request from untrusted peer should be the same as trusted bc of single-preflight
requestFromTrusted: false,
lastHeader: trustedStore.Headers[trustedStore.HeadHeight-1],
expectedHeight: trustedStore.HeadHeight,
expectedHash: trustedStore.Headers[trustedStore.HeadHeight].Hash(),
},
}

var wg sync2.WaitGroup
// run over goroutine
for i, tt := range tests {
wg.Add(1)
go func(testStruct struct {
requestFromTrusted bool
lastHeader *headertest.DummyHeader
expectedHeight uint64
expectedHash header.Hash
}, it int) {
defer wg.Done()
var opts []header.HeadOption[*headertest.DummyHeader]
if !testStruct.requestFromTrusted {
opts = append(opts, header.WithTrustedHead[*headertest.DummyHeader](testStruct.lastHeader))
}

h, errG := exchg.Head(ctx, opts...)
require.NoError(t, errG)

assert.Equal(t, testStruct.expectedHeight, h.Height())
assert.Equal(t, testStruct.expectedHash, h.Hash())

}(tt, i)
// ensure first Head will be locked by request from trusted peer
time.Sleep(time.Microsecond)
}
wg.Wait()
}

func TestExchange_RequestHeader(t *testing.T) {
hosts := createMocknet(t, 2)
exchg, store := createP2PExAndServer(t, hosts[0], hosts[1])
Expand Down
4 changes: 2 additions & 2 deletions sync/sync.go
Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,7 @@ var log = logging.Logger("header/sync")
type Syncer[H header.Header[H]] struct {
sub header.Subscriber[H] // to subscribe for new Network Heads
store syncStore[H] // to store all the headers to
getter syncGetter[H] // to fetch headers from
getter header.Getter[H] // to fetch headers from
metrics *metrics

// stateLk protects state which represents the current or latest sync
Expand Down Expand Up @@ -80,7 +80,7 @@ func NewSyncer[H header.Header[H]](
return &Syncer[H]{
sub: sub,
store: syncStore[H]{Store: store},
getter: syncGetter[H]{Getter: getter},
getter: getter,
metrics: metrics,
triggerSync: make(chan struct{}, 1), // should be buffered
Params: &params,
Expand Down
52 changes: 0 additions & 52 deletions sync/sync_getter.go

This file was deleted.

72 changes: 0 additions & 72 deletions sync/sync_getter_test.go

This file was deleted.

16 changes: 0 additions & 16 deletions sync/sync_head.go
Original file line number Diff line number Diff line change
Expand Up @@ -29,14 +29,6 @@ func (s *Syncer[H]) Head(ctx context.Context, _ ...header.HeadOption[H]) (H, err
return sbjHead, nil
}

// single-flight protection ensure only one Head is requested at the time
if !s.getter.Lock() {
// means that other routine held the lock and set the subjective head for us,
// so just recursively get it
return s.Head(ctx)
}
defer s.getter.Unlock()

s.metrics.outdatedHead(s.ctx)

reqCtx, cancel := context.WithTimeout(ctx, headRequestTimeout)
Expand Down Expand Up @@ -80,14 +72,6 @@ func (s *Syncer[H]) subjectiveHead(ctx context.Context) (H, error) {
}
// otherwise, request head from a trusted peer
log.Infow("stored head header expired", "height", storeHead.Height())
// single-flight protection
// ensure only one Head is requested at the time
if !s.getter.Lock() {
// means that other routine held the lock and set the subjective head for us,
// so just recursively get it
return s.subjectiveHead(ctx)
}
defer s.getter.Unlock()

trustHead, err := s.getter.Head(ctx)
if err != nil {
Expand Down