github.com/gochain-io/gochain@v2.2.26+incompatible/event/feed.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/>.

package event

import (
	"context"
	"errors"
	"fmt"
	"reflect"
	"sync"
	"time"

	"go.opencensus.io/trace"

	"github.com/gochain-io/gochain/log"
)

var errBadChannel = errors.New("event: Subscribe argument does not have sendable channel type")

const feedTimeout = 100 * time.Millisecond

// Feed 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.
//
// Feeds can only be used with a single type. The type is determined by the first Send or
// Subscribe operation. Subsequent calls to these methods panic if the type does not
// match.
//
// The zero value is ready to use.
type Feed 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 interface{} // 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
	etype  reflect.Type
	closed bool
}

// This is the index of the first actual subscription channel in sendCases.
// sendCases[0] is a SelectRecv case for the removeSub channel.
const firstSubSendCase = 1

type feedTypeError struct {
	got, want reflect.Type
	op        string
}

func (e feedTypeError) Error() string {
	return "event: wrong type in " + e.op + " got " + e.got.String() + ", want " + e.want.String()
}

func (f *Feed) init() {
	f.removeSub = make(chan interface{})
	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. All channels added must have the same element type.
//
// The channel should have ample buffer space to avoid blocking other subscribers.
// Slow subscribers are not dropped.
func (f *Feed) Subscribe(channel interface{}) Subscription {
	f.once.Do(f.init)

	chanval := reflect.ValueOf(channel)
	chantyp := chanval.Type()
	if chantyp.Kind() != reflect.Chan || chantyp.ChanDir()&reflect.SendDir == 0 {
		panic(errBadChannel)
	}
	sub := &feedSub{feed: f, channel: chanval, err: make(chan error, 1)}

	f.mu.Lock()
	defer f.mu.Unlock()
	if !f.typecheck(chantyp.Elem()) {
		panic(feedTypeError{op: "Subscribe", got: chantyp, want: reflect.ChanOf(reflect.SendDir, f.etype)})
	}
	// Add the select case to the inbox.
	// The next Send will add it to f.sendCases.
	cas := reflect.SelectCase{Dir: reflect.SelectSend, Chan: chanval}
	f.inbox = append(f.inbox, cas)
	return sub
}

// note: callers must hold f.mu
func (f *Feed) typecheck(typ reflect.Type) bool {
	if f.etype == nil {
		f.etype = typ
		return true
	}
	return f.etype == typ
}

func (f *Feed) remove(sub *feedSub) {
	// Delete from inbox first, which covers channels
	// that have not been added to f.sendCases yet.
	ch := sub.channel.Interface()
	f.mu.Lock()
	index := f.inbox.find(ch)
	if index != -1 {
		f.inbox = f.inbox.delete(index)
		f.mu.Unlock()
		return
	}
	f.mu.Unlock()

	select {
	case f.removeSub <- ch:
		// 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.
		if i := f.sendCases.find(ch); i != -1 {
			f.sendCases = f.sendCases.delete(i)
		}
		f.sendLock <- struct{}{}
	}
}

// Send delivers to all subscribed channels simultaneously.
// It returns the number of subscribers that the value was sent to.
func (f *Feed) Send(value interface{}) (nsent int) {
	return f.SendCtx(context.Background(), value)
}

func (f *Feed) SendCtx(ctx context.Context, value interface{}) (nsent int) {
	ctx, span := trace.StartSpan(ctx, "Feed.Send")
	defer span.End()

	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

	if !f.typecheck(rvalue.Type()) {
		f.sendLock <- struct{}{}
		panic(feedTypeError{op: "Send", got: rvalue.Type(), want: f.etype})
	}
	f.mu.Unlock()

	// Set the sent value on all channels.
	for i := firstSubSendCase; i < len(f.sendCases); i++ {
		if f.sendCases[i].Dir == reflect.SelectSend {
			f.sendCases[i].Send = rvalue
		}
	}

	// Create timer & select-case.
	timer := time.NewTimer(feedTimeout)
	defer timer.Stop()
	timerSelectCase := reflect.SelectCase{
		Dir:  reflect.SelectRecv,
		Chan: reflect.ValueOf(timer.C),
	}

	// Send until all channels except removeSub & timer have been chosen.
	minSelectCaseN := firstSubSendCase + 1 // removeSub + timeout.C
	cases := make(caseList, len(f.sendCases)+1)
	copy(cases, f.sendCases)
	cases[len(cases)-1] = timerSelectCase
	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].Dir == reflect.SelectSend && cases[i].Chan.TrySend(rvalue) {
				nsent++
				cases = cases.deactivate(i)
				i--
			}
		}
		if len(cases) == minSelectCaseN {
			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) {
				cases = f.sendCases[:len(cases)-1]
			}
		} else if cases[chosen].Chan == timerSelectCase.Chan {
			log.Warn("Feed channel send timeout, value dropped", "data", fmt.Sprintf("%T", value))
			break
		} 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 feedSub struct {
	feed    *Feed
	channel reflect.Value
	errOnce sync.Once
	err     chan error
}

func (sub *feedSub) Unsubscribe() {
	sub.errOnce.Do(func() {
		sub.feed.remove(sub)
		close(sub.err)
	})
}

func (sub *feedSub) Err() <-chan error {
	return sub.err
}

type caseList []reflect.SelectCase

// find returns the index of a case containing the given channel.
func (cs caseList) find(channel interface{}) int {
	for i, cas := range cs {
		if cas.Chan.Interface() == channel {
			return i
		}
	}
	return -1
}

// delete removes the given case from cs.
func (cs caseList) delete(index int) caseList {
	return append(cs[:index], cs[index+1:]...)
}

// deactivate moves the case at index into the non-accessible portion of the cs slice.
func (cs caseList) deactivate(index int) caseList {
	last := len(cs) - 1
	cs[index], cs[last] = cs[last], cs[index]
	return cs[:last]
}

// func (cs caseList) String() string {
//     s := "["
//     for i, cas := range cs {
//             if i != 0 {
//                     s += ", "
//             }
//             switch cas.Dir {
//             case reflect.SelectSend:
//                     s += fmt.Sprintf("%v<-", cas.Chan.Interface())
//             case reflect.SelectRecv:
//                     s += fmt.Sprintf("<-%v", cas.Chan.Interface())
//             }
//     }
//     return s + "]"
// }