Make close concurrent safe

subscription.go

@@ -2,6 +2,7 @@ package pubsub
 
 import (
 	"context"
+	"sync"
 )
 
 // Subscription handles the details of a particular Topic subscription.
@@ -10,9 +11,9 @@ type Subscription struct {
 	topic    string
 	ch       chan *Message
 	cancelCh chan<- *Subscription
-	closed   bool
 	ctx      context.Context
 	err      error
+	once     sync.Once
 }
 
 // Topic returns the topic string associated with the Subscription
@@ -44,8 +45,7 @@ func (sub *Subscription) Cancel() {
 }
 
 func (sub *Subscription) close() {
-	if !sub.closed {
+	sub.once.Do(func() {
 		close(sub.ch)
-		sub.closed = true
-	}
+	})
 }