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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/>.
package event
import (
"errors"
"reflect"
"sync"
)
var errBadChannel = errors.New("event: Subscribe argument does not have sendable channel type")
// 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
}
// 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.
f.sendCases = f.sendCases.delete(f.sendCases.find(ch))
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) {
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++ {
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 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 + "]"
// }
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