queue polling not correctly handles client disconnect

queue/builtin/queue.go

@@ -82,6 +82,24 @@ func (q *Queue) Pull() *queue.Work {
 	return work
 }
 
+// PullClose retrieves and removes the head of this queue,
+// waiting if necessary until work becomes available. The
+// CloseNotifier should be provided to clone the channel
+// if the subscribing client terminates its connection.
+func (q *Queue) PullClose(cn queue.CloseNotifier) *queue.Work {
+	for {
+		select {
+		case <-cn.CloseNotify():
+			return nil
+		case work := <-q.itemc:
+			q.Lock()
+			delete(q.items, work)
+			q.Unlock()
+			return work
+		}
+	}
+}
+
 // PullAck retrieves and removes the head of this queue, waiting
 // if necessary until work becomes available. Items pull from the
 // queue that aren't acknowledged will be pushed back to the queue

queue/queue.go

@@ -13,6 +13,12 @@ type Queue interface {
 	// if necessary until work becomes available.
 	Pull() *Work
 
+	// PullClose retrieves and removes the head of this queue,
+	// waiting if necessary until work becomes available. The
+	// CloseNotifier should be provided to clone the channel
+	// if the subscribing client terminates its connection.
+	PullClose(CloseNotifier) *Work
+
 	// PullAck retrieves and removes the head of this queue, waiting
 	// if necessary until work becomes available. Items pull from the
 	// queue that aren't acknowledged will be pushed back to the queue
@@ -26,3 +32,9 @@ type Queue interface {
 	// queue, in proper sequence.
 	Items() []*Work
 }
+
+type CloseNotifier interface {
+	// CloseNotify returns a channel that receives a single value
+	// when the client connection has gone away.
+	CloseNotify() <-chan bool
+}

server/queue.go

@@ -19,8 +19,12 @@ import (
 // GET /queue/pull
 func PollBuild(c *gin.Context) {
 	queue := ToQueue(c)
-	work := queue.Pull()
+	work := queue.PullClose(c.Writer)
+	if work == nil {
+		c.AbortWithStatus(500)
+	} else {
 		c.JSON(200, work)
+	}
 }
 
 // GET /queue/push/:owner/:repo