event: use sync.Once for init for faster/cleaner locking

Author: karalabe

event/feed.go

@@ -33,9 +33,8 @@ var errBadChannel = errors.New("event: Subscribe argument does not have sendable
 //
 // 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{}
+	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
 
@@ -60,9 +59,6 @@ func (e feedTypeError) Error() string {
 }
 
 func (f *Feed) init() {
-	if f.sendLock != nil {
-		return
-	}
 	f.removeSub = make(chan interface{})
 	f.sendLock = make(chan struct{}, 1)
 	f.sendLock <- struct{}{}
@@ -75,6 +71,8 @@ func (f *Feed) init() {
 // 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 {
@@ -84,7 +82,6 @@ func (f *Feed) Subscribe(channel interface{}) Subscription {
 
 	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)})
 	}
@@ -130,10 +127,7 @@ func (f *Feed) remove(sub *feedSub) {
 // 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.once.Do(f.init)
 	<-f.sendLock
 
 	// Add new cases from the inbox after taking the send lock.