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-rw-r--r--event/feed.go249
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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 {
+ // sendLock has a one-element buffer and is empty when held.
+ // It protects sendCases.
+ sendLock chan struct{}
+ 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() {
+ if f.sendLock != nil {
+ return
+ }
+ 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 {
+ 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()
+ f.init()
+ 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) {
+ f.mu.Lock()
+ f.init()
+ f.mu.Unlock()
+
+ <-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.
+ rvalue := reflect.ValueOf(value)
+ if !f.typecheck(rvalue.Type()) {
+ f.sendLock <- struct{}{}
+ panic(feedTypeError{op: "Send", got: rvalue.Type(), want: f.etype})
+ }
+ for i := firstSubSendCase; i < len(f.sendCases); i++ {
+ f.sendCases[i].Send = rvalue
+ }
+
+ // Send until all channels except removeSub have been chosen.
+ 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) {
+ 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 + "]"
+// }