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.

Author: fjl

event/feedof.go

@@ -0,0 +1,167 @@
+// 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
+}