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event: add FeedOf[T] (#26310)

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This PR adds a new type event.FeedOf[T], which is like event.Feed but parameterized
over the channel element type. Performance is unchanged, and it still uses reflect. But
unlike Feed, the generic version doesn't need to type-check interface{} arguments.
All panic cases are gone from the API.
This commit is contained in:
Felix Lange 2022-12-09 16:20:46 +01:00 committed by GitHub
parent 3315bad256
commit 3775e198df
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2 changed files with 449 additions and 0 deletions
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167
event/feedof.go Normal file
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// Copyright 2022 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
//go:build go1.18
// +build go1.18
package event
import (
"reflect"
"sync"
)
// FeedOf implements one-to-many subscriptions where the carrier of events is a channel.
// Values sent to a Feed are delivered to all subscribed channels simultaneously.
//
// The zero value is ready to use.
type FeedOf[T any] struct {
once sync.Once // ensures that init only runs once
sendLock chan struct{} // sendLock has a one-element buffer and is empty when held.It protects sendCases.
removeSub chan chan<- T // interrupts Send
sendCases caseList // the active set of select cases used by Send
// The inbox holds newly subscribed channels until they are added to sendCases.
mu sync.Mutex
inbox caseList
}
func (f *FeedOf[T]) init() {
f.removeSub = make(chan chan<- T)
f.sendLock = make(chan struct{}, 1)
f.sendLock <- struct{}{}
f.sendCases = caseList{{Chan: reflect.ValueOf(f.removeSub), Dir: reflect.SelectRecv}}
}
// Subscribe adds a channel to the feed. Future sends will be delivered on the channel
// until the subscription is canceled.
//
// The channel should have ample buffer space to avoid blocking other subscribers. Slow
// subscribers are not dropped.
func (f *FeedOf[T]) Subscribe(channel chan<- T) Subscription {
f.once.Do(f.init)
chanval := reflect.ValueOf(channel)
sub := &feedOfSub[T]{feed: f, channel: channel, err: make(chan error, 1)}
// Add the select case to the inbox.
// The next Send will add it to f.sendCases.
f.mu.Lock()
defer f.mu.Unlock()
cas := reflect.SelectCase{Dir: reflect.SelectSend, Chan: chanval}
f.inbox = append(f.inbox, cas)
return sub
}
func (f *FeedOf[T]) remove(sub *feedOfSub[T]) {
// Delete from inbox first, which covers channels
// that have not been added to f.sendCases yet.
f.mu.Lock()
index := f.inbox.find(sub.channel)
if index != -1 {
f.inbox = f.inbox.delete(index)
f.mu.Unlock()
return
}
f.mu.Unlock()
select {
case f.removeSub <- sub.channel:
// Send will remove the channel from f.sendCases.
case <-f.sendLock:
// No Send is in progress, delete the channel now that we have the send lock.
f.sendCases = f.sendCases.delete(f.sendCases.find(sub.channel))
f.sendLock <- struct{}{}
}
}
// Send delivers to all subscribed channels simultaneously.
// It returns the number of subscribers that the value was sent to.
func (f *FeedOf[T]) Send(value T) (nsent int) {
rvalue := reflect.ValueOf(value)
f.once.Do(f.init)
<-f.sendLock
// Add new cases from the inbox after taking the send lock.
f.mu.Lock()
f.sendCases = append(f.sendCases, f.inbox...)
f.inbox = nil
f.mu.Unlock()
// Set the sent value on all channels.
for i := firstSubSendCase; i < len(f.sendCases); i++ {
f.sendCases[i].Send = rvalue
}
// Send until all channels except removeSub have been chosen. 'cases' tracks a prefix
// of sendCases. When a send succeeds, the corresponding case moves to the end of
// 'cases' and it shrinks by one element.
cases := f.sendCases
for {
// Fast path: try sending without blocking before adding to the select set.
// This should usually succeed if subscribers are fast enough and have free
// buffer space.
for i := firstSubSendCase; i < len(cases); i++ {
if cases[i].Chan.TrySend(rvalue) {
nsent++
cases = cases.deactivate(i)
i--
}
}
if len(cases) == firstSubSendCase {
break
}
// Select on all the receivers, waiting for them to unblock.
chosen, recv, _ := reflect.Select(cases)
if chosen == 0 /* <-f.removeSub */ {
index := f.sendCases.find(recv.Interface())
f.sendCases = f.sendCases.delete(index)
if index >= 0 && index < len(cases) {
// Shrink 'cases' too because the removed case was still active.
cases = f.sendCases[:len(cases)-1]
}
} else {
cases = cases.deactivate(chosen)
nsent++
}
}
// Forget about the sent value and hand off the send lock.
for i := firstSubSendCase; i < len(f.sendCases); i++ {
f.sendCases[i].Send = reflect.Value{}
}
f.sendLock <- struct{}{}
return nsent
}
type feedOfSub[T any] struct {
feed *FeedOf[T]
channel chan<- T
errOnce sync.Once
err chan error
}
func (sub *feedOfSub[T]) Unsubscribe() {
sub.errOnce.Do(func() {
sub.feed.remove(sub)
close(sub.err)
})
}
func (sub *feedOfSub[T]) Err() <-chan error {
return sub.err
}

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event/feedof_test.go Normal file
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// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
//go:build go1.18
// +build go1.18
package event
import (
"sync"
"testing"
"time"
)
func TestFeedOf(t *testing.T) {
var feed FeedOf[int]
var done, subscribed sync.WaitGroup
subscriber := func(i int) {
defer done.Done()
subchan := make(chan int)
sub := feed.Subscribe(subchan)
timeout := time.NewTimer(2 * time.Second)
defer timeout.Stop()
subscribed.Done()
select {
case v := <-subchan:
if v != 1 {
t.Errorf("%d: received value %d, want 1", i, v)
}
case <-timeout.C:
t.Errorf("%d: receive timeout", i)
}
sub.Unsubscribe()
select {
case _, ok := <-sub.Err():
if ok {
t.Errorf("%d: error channel not closed after unsubscribe", i)
}
case <-timeout.C:
t.Errorf("%d: unsubscribe timeout", i)
}
}
const n = 1000
done.Add(n)
subscribed.Add(n)
for i := 0; i < n; i++ {
go subscriber(i)
}
subscribed.Wait()
if nsent := feed.Send(1); nsent != n {
t.Errorf("first send delivered %d times, want %d", nsent, n)
}
if nsent := feed.Send(2); nsent != 0 {
t.Errorf("second send delivered %d times, want 0", nsent)
}
done.Wait()
}
func TestFeedOfSubscribeSameChannel(t *testing.T) {
var (
feed FeedOf[int]
done sync.WaitGroup
ch = make(chan int)
sub1 = feed.Subscribe(ch)
sub2 = feed.Subscribe(ch)
_ = feed.Subscribe(ch)
)
expectSends := func(value, n int) {
if nsent := feed.Send(value); nsent != n {
t.Errorf("send delivered %d times, want %d", nsent, n)
}
done.Done()
}
expectRecv := func(wantValue, n int) {
for i := 0; i < n; i++ {
if v := <-ch; v != wantValue {
t.Errorf("received %d, want %d", v, wantValue)
}
}
}
done.Add(1)
go expectSends(1, 3)
expectRecv(1, 3)
done.Wait()
sub1.Unsubscribe()
done.Add(1)
go expectSends(2, 2)
expectRecv(2, 2)
done.Wait()
sub2.Unsubscribe()
done.Add(1)
go expectSends(3, 1)
expectRecv(3, 1)
done.Wait()
}
func TestFeedOfSubscribeBlockedPost(t *testing.T) {
var (
feed FeedOf[int]
nsends = 2000
ch1 = make(chan int)
ch2 = make(chan int)
wg sync.WaitGroup
)
defer wg.Wait()
feed.Subscribe(ch1)
wg.Add(nsends)
for i := 0; i < nsends; i++ {
go func() {
feed.Send(99)
wg.Done()
}()
}
sub2 := feed.Subscribe(ch2)
defer sub2.Unsubscribe()
// We're done when ch1 has received N times.
// The number of receives on ch2 depends on scheduling.
for i := 0; i < nsends; {
select {
case <-ch1:
i++
case <-ch2:
}
}
}
func TestFeedOfUnsubscribeBlockedPost(t *testing.T) {
var (
feed FeedOf[int]
nsends = 200
chans = make([]chan int, 2000)
subs = make([]Subscription, len(chans))
bchan = make(chan int)
bsub = feed.Subscribe(bchan)
wg sync.WaitGroup
)
for i := range chans {
chans[i] = make(chan int, nsends)
}
// Queue up some Sends. None of these can make progress while bchan isn't read.
wg.Add(nsends)
for i := 0; i < nsends; i++ {
go func() {
feed.Send(99)
wg.Done()
}()
}
// Subscribe the other channels.
for i, ch := range chans {
subs[i] = feed.Subscribe(ch)
}
// Unsubscribe them again.
for _, sub := range subs {
sub.Unsubscribe()
}
// Unblock the Sends.
bsub.Unsubscribe()
wg.Wait()
}
// Checks that unsubscribing a channel during Send works even if that
// channel has already been sent on.
func TestFeedOfUnsubscribeSentChan(t *testing.T) {
var (
feed FeedOf[int]
ch1 = make(chan int)
ch2 = make(chan int)
sub1 = feed.Subscribe(ch1)
sub2 = feed.Subscribe(ch2)
wg sync.WaitGroup
)
defer sub2.Unsubscribe()
wg.Add(1)
go func() {
feed.Send(0)
wg.Done()
}()
// Wait for the value on ch1.
<-ch1
// Unsubscribe ch1, removing it from the send cases.
sub1.Unsubscribe()
// Receive ch2, finishing Send.
<-ch2
wg.Wait()
// Send again. This should send to ch2 only, so the wait group will unblock
// as soon as a value is received on ch2.
wg.Add(1)
go func() {
feed.Send(0)
wg.Done()
}()
<-ch2
wg.Wait()
}
func TestFeedOfUnsubscribeFromInbox(t *testing.T) {
var (
feed FeedOf[int]
ch1 = make(chan int)
ch2 = make(chan int)
sub1 = feed.Subscribe(ch1)
sub2 = feed.Subscribe(ch1)
sub3 = feed.Subscribe(ch2)
)
if len(feed.inbox) != 3 {
t.Errorf("inbox length != 3 after subscribe")
}
if len(feed.sendCases) != 1 {
t.Errorf("sendCases is non-empty after unsubscribe")
}
sub1.Unsubscribe()
sub2.Unsubscribe()
sub3.Unsubscribe()
if len(feed.inbox) != 0 {
t.Errorf("inbox is non-empty after unsubscribe")
}
if len(feed.sendCases) != 1 {
t.Errorf("sendCases is non-empty after unsubscribe")
}
}
func BenchmarkFeedOfSend1000(b *testing.B) {
var (
done sync.WaitGroup
feed FeedOf[int]
nsubs = 1000
)
subscriber := func(ch <-chan int) {
for i := 0; i < b.N; i++ {
<-ch
}
done.Done()
}
done.Add(nsubs)
for i := 0; i < nsubs; i++ {
ch := make(chan int, 200)
feed.Subscribe(ch)
go subscriber(ch)
}
// The actual benchmark.
b.ResetTimer()
for i := 0; i < b.N; i++ {
if feed.Send(i) != nsubs {
panic("wrong number of sends")
}
}
b.StopTimer()
done.Wait()
}