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.

event/feedof.go

+// 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
+}

event/feedof_test.go

+// 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()
+}