event: add FeedOf[T] (#26310)
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
GitHub
parent
3315bad256
commit
3775e198df
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// Copyright 2022 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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//go:build go1.18
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// +build go1.18
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package event
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import (
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"reflect"
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"sync"
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)
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// FeedOf implements one-to-many subscriptions where the carrier of events is a channel.
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// Values sent to a Feed are delivered to all subscribed channels simultaneously.
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//
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// The zero value is ready to use.
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type FeedOf[T any] struct {
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once sync.Once // ensures that init only runs once
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sendLock chan struct{} // sendLock has a one-element buffer and is empty when held.It protects sendCases.
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removeSub chan chan<- T // interrupts Send
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sendCases caseList // the active set of select cases used by Send
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// The inbox holds newly subscribed channels until they are added to sendCases.
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mu sync.Mutex
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inbox caseList
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}
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func (f *FeedOf[T]) init() {
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f.removeSub = make(chan chan<- T)
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f.sendLock = make(chan struct{}, 1)
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f.sendLock <- struct{}{}
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f.sendCases = caseList{{Chan: reflect.ValueOf(f.removeSub), Dir: reflect.SelectRecv}}
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}
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// Subscribe adds a channel to the feed. Future sends will be delivered on the channel
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// until the subscription is canceled.
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//
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// The channel should have ample buffer space to avoid blocking other subscribers. Slow
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// subscribers are not dropped.
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func (f *FeedOf[T]) Subscribe(channel chan<- T) Subscription {
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f.once.Do(f.init)
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chanval := reflect.ValueOf(channel)
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sub := &feedOfSub[T]{feed: f, channel: channel, err: make(chan error, 1)}
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// Add the select case to the inbox.
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// The next Send will add it to f.sendCases.
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f.mu.Lock()
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defer f.mu.Unlock()
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cas := reflect.SelectCase{Dir: reflect.SelectSend, Chan: chanval}
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f.inbox = append(f.inbox, cas)
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return sub
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}
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func (f *FeedOf[T]) remove(sub *feedOfSub[T]) {
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// Delete from inbox first, which covers channels
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// that have not been added to f.sendCases yet.
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f.mu.Lock()
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index := f.inbox.find(sub.channel)
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if index != -1 {
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f.inbox = f.inbox.delete(index)
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f.mu.Unlock()
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return
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}
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f.mu.Unlock()
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select {
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case f.removeSub <- sub.channel:
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// Send will remove the channel from f.sendCases.
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case <-f.sendLock:
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// No Send is in progress, delete the channel now that we have the send lock.
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f.sendCases = f.sendCases.delete(f.sendCases.find(sub.channel))
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f.sendLock <- struct{}{}
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}
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}
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// Send delivers to all subscribed channels simultaneously.
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// It returns the number of subscribers that the value was sent to.
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func (f *FeedOf[T]) Send(value T) (nsent int) {
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rvalue := reflect.ValueOf(value)
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f.once.Do(f.init)
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<-f.sendLock
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// Add new cases from the inbox after taking the send lock.
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f.mu.Lock()
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f.sendCases = append(f.sendCases, f.inbox...)
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f.inbox = nil
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f.mu.Unlock()
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// Set the sent value on all channels.
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for i := firstSubSendCase; i < len(f.sendCases); i++ {
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f.sendCases[i].Send = rvalue
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}
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// Send until all channels except removeSub have been chosen. 'cases' tracks a prefix
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// of sendCases. When a send succeeds, the corresponding case moves to the end of
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// 'cases' and it shrinks by one element.
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cases := f.sendCases
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for {
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// Fast path: try sending without blocking before adding to the select set.
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// This should usually succeed if subscribers are fast enough and have free
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// buffer space.
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for i := firstSubSendCase; i < len(cases); i++ {
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if cases[i].Chan.TrySend(rvalue) {
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nsent++
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cases = cases.deactivate(i)
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i--
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}
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}
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if len(cases) == firstSubSendCase {
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break
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}
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// Select on all the receivers, waiting for them to unblock.
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chosen, recv, _ := reflect.Select(cases)
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if chosen == 0 /* <-f.removeSub */ {
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index := f.sendCases.find(recv.Interface())
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f.sendCases = f.sendCases.delete(index)
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if index >= 0 && index < len(cases) {
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// Shrink 'cases' too because the removed case was still active.
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cases = f.sendCases[:len(cases)-1]
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}
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} else {
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cases = cases.deactivate(chosen)
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nsent++
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}
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}
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// Forget about the sent value and hand off the send lock.
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for i := firstSubSendCase; i < len(f.sendCases); i++ {
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f.sendCases[i].Send = reflect.Value{}
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}
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f.sendLock <- struct{}{}
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return nsent
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}
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type feedOfSub[T any] struct {
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feed *FeedOf[T]
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channel chan<- T
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errOnce sync.Once
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err chan error
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}
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func (sub *feedOfSub[T]) Unsubscribe() {
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sub.errOnce.Do(func() {
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sub.feed.remove(sub)
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close(sub.err)
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})
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}
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func (sub *feedOfSub[T]) Err() <-chan error {
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return sub.err
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}
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@ -0,0 +1,282 @@
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// Copyright 2016 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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//go:build go1.18
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// +build go1.18
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package event
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import (
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"sync"
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"testing"
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"time"
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)
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func TestFeedOf(t *testing.T) {
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var feed FeedOf[int]
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var done, subscribed sync.WaitGroup
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subscriber := func(i int) {
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defer done.Done()
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subchan := make(chan int)
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sub := feed.Subscribe(subchan)
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timeout := time.NewTimer(2 * time.Second)
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defer timeout.Stop()
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subscribed.Done()
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select {
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case v := <-subchan:
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if v != 1 {
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t.Errorf("%d: received value %d, want 1", i, v)
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}
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case <-timeout.C:
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t.Errorf("%d: receive timeout", i)
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}
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sub.Unsubscribe()
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select {
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case _, ok := <-sub.Err():
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if ok {
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t.Errorf("%d: error channel not closed after unsubscribe", i)
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}
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case <-timeout.C:
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t.Errorf("%d: unsubscribe timeout", i)
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}
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}
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const n = 1000
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done.Add(n)
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subscribed.Add(n)
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for i := 0; i < n; i++ {
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go subscriber(i)
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}
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subscribed.Wait()
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if nsent := feed.Send(1); nsent != n {
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t.Errorf("first send delivered %d times, want %d", nsent, n)
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}
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if nsent := feed.Send(2); nsent != 0 {
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t.Errorf("second send delivered %d times, want 0", nsent)
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}
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done.Wait()
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}
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func TestFeedOfSubscribeSameChannel(t *testing.T) {
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var (
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feed FeedOf[int]
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done sync.WaitGroup
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ch = make(chan int)
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sub1 = feed.Subscribe(ch)
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sub2 = feed.Subscribe(ch)
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_ = feed.Subscribe(ch)
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)
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expectSends := func(value, n int) {
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if nsent := feed.Send(value); nsent != n {
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t.Errorf("send delivered %d times, want %d", nsent, n)
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}
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done.Done()
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}
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expectRecv := func(wantValue, n int) {
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for i := 0; i < n; i++ {
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if v := <-ch; v != wantValue {
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t.Errorf("received %d, want %d", v, wantValue)
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}
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}
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}
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done.Add(1)
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go expectSends(1, 3)
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expectRecv(1, 3)
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done.Wait()
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sub1.Unsubscribe()
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done.Add(1)
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go expectSends(2, 2)
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expectRecv(2, 2)
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done.Wait()
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sub2.Unsubscribe()
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done.Add(1)
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go expectSends(3, 1)
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expectRecv(3, 1)
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done.Wait()
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}
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func TestFeedOfSubscribeBlockedPost(t *testing.T) {
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var (
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feed FeedOf[int]
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nsends = 2000
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ch1 = make(chan int)
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ch2 = make(chan int)
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wg sync.WaitGroup
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)
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defer wg.Wait()
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feed.Subscribe(ch1)
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wg.Add(nsends)
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for i := 0; i < nsends; i++ {
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go func() {
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feed.Send(99)
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wg.Done()
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}()
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}
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sub2 := feed.Subscribe(ch2)
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defer sub2.Unsubscribe()
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// We're done when ch1 has received N times.
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// The number of receives on ch2 depends on scheduling.
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for i := 0; i < nsends; {
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select {
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case <-ch1:
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i++
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case <-ch2:
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}
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}
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}
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func TestFeedOfUnsubscribeBlockedPost(t *testing.T) {
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var (
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feed FeedOf[int]
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nsends = 200
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chans = make([]chan int, 2000)
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subs = make([]Subscription, len(chans))
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bchan = make(chan int)
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bsub = feed.Subscribe(bchan)
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wg sync.WaitGroup
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)
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for i := range chans {
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chans[i] = make(chan int, nsends)
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}
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// Queue up some Sends. None of these can make progress while bchan isn't read.
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wg.Add(nsends)
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for i := 0; i < nsends; i++ {
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go func() {
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feed.Send(99)
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wg.Done()
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}()
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}
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// Subscribe the other channels.
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for i, ch := range chans {
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subs[i] = feed.Subscribe(ch)
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}
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// Unsubscribe them again.
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for _, sub := range subs {
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sub.Unsubscribe()
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}
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// Unblock the Sends.
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bsub.Unsubscribe()
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wg.Wait()
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}
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// Checks that unsubscribing a channel during Send works even if that
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// channel has already been sent on.
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func TestFeedOfUnsubscribeSentChan(t *testing.T) {
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var (
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feed FeedOf[int]
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ch1 = make(chan int)
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ch2 = make(chan int)
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sub1 = feed.Subscribe(ch1)
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sub2 = feed.Subscribe(ch2)
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wg sync.WaitGroup
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)
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defer sub2.Unsubscribe()
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wg.Add(1)
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go func() {
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feed.Send(0)
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wg.Done()
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}()
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// Wait for the value on ch1.
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<-ch1
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// Unsubscribe ch1, removing it from the send cases.
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sub1.Unsubscribe()
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// Receive ch2, finishing Send.
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<-ch2
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wg.Wait()
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// Send again. This should send to ch2 only, so the wait group will unblock
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// as soon as a value is received on ch2.
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wg.Add(1)
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go func() {
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feed.Send(0)
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wg.Done()
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}()
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<-ch2
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wg.Wait()
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}
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func TestFeedOfUnsubscribeFromInbox(t *testing.T) {
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var (
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feed FeedOf[int]
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ch1 = make(chan int)
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ch2 = make(chan int)
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sub1 = feed.Subscribe(ch1)
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sub2 = feed.Subscribe(ch1)
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sub3 = feed.Subscribe(ch2)
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)
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if len(feed.inbox) != 3 {
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t.Errorf("inbox length != 3 after subscribe")
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}
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if len(feed.sendCases) != 1 {
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t.Errorf("sendCases is non-empty after unsubscribe")
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}
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sub1.Unsubscribe()
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sub2.Unsubscribe()
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sub3.Unsubscribe()
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if len(feed.inbox) != 0 {
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t.Errorf("inbox is non-empty after unsubscribe")
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}
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if len(feed.sendCases) != 1 {
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t.Errorf("sendCases is non-empty after unsubscribe")
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}
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}
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func BenchmarkFeedOfSend1000(b *testing.B) {
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var (
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done sync.WaitGroup
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feed FeedOf[int]
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nsubs = 1000
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)
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subscriber := func(ch <-chan int) {
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for i := 0; i < b.N; i++ {
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<-ch
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}
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done.Done()
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}
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done.Add(nsubs)
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for i := 0; i < nsubs; i++ {
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ch := make(chan int, 200)
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feed.Subscribe(ch)
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go subscriber(ch)
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}
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// The actual benchmark.
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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if feed.Send(i) != nsubs {
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panic("wrong number of sends")
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}
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}
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b.StopTimer()
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done.Wait()
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}
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