event: add new Subscription type and related utilities

This commit introduces a new Subscription type, which is synonymous with
ethereum.Subscription. It also adds a couple of utilities that make
working with Subscriptions easier. The mot complex utility is Feed, a
synchronisation device that implements broadcast subscriptions. Feed is
slightly faster than TypeMux and will replace uses of TypeMux across the
go-ethereum codebase in the future.

1 files changed, 240 insertions, 0 deletions

diff --git a/event/feed.go b/event/feed.go
new file mode 100644
index 000000000..bd8e26321
--- /dev/null
+++ b/event/feed.go
@@ -0,0 +1,240 @@
+// 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  chan struct{}    // one-element buffer, empty when held
+	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
+}
+
+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.
+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.sendLock
+	// Add new subscriptions from the inbox, then clear it.
+	f.sendCases = append(f.sendCases, f.inbox...)
+	for i := range f.inbox {
+		f.inbox[i] = reflect.SelectCase{}
+	}
+	f.inbox = f.inbox[:0]
+	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 := 1; 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 := 1; i < len(cases); i++ {
+			if cases[i].Chan.TrySend(rvalue) {
+				cases = cases.deactivate(i)
+				nsent++
+			}
+		}
+		if len(cases) == 1 {
+			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 := 1; 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 + "]"
+// }