From a4442608e6fd20a04e48b9d4a17178d880304612 Mon Sep 17 00:00:00 2001
From: Conner Fromknecht <conner@lightning.engineering>
Date: Fri, 30 Nov 2018 16:33:40 -0800
Subject: [PATCH] queue/queue: wait group inner go routines, add idempotency

---
 queue/queue.go | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/queue/queue.go b/queue/queue.go
index 497e6f7e..07eca3fa 100644
--- a/queue/queue.go
+++ b/queue/queue.go
@@ -2,6 +2,8 @@ package queue
 
 import (
 	"container/list"
+	"sync"
+	"sync/atomic"
 )
 
 // ConcurrentQueue is a concurrent-safe FIFO queue with unbounded capacity.
@@ -10,10 +12,15 @@ import (
 // items from the in channel to the out channel in the correct order that must
 // be started by calling Start().
 type ConcurrentQueue struct {
+	started uint32 // to be used atomically
+	stopped uint32 // to be used atomically
+
 	chanIn   chan interface{}
 	chanOut  chan interface{}
-	quit     chan struct{}
 	overflow *list.List
+
+	wg   sync.WaitGroup
+	quit chan struct{}
 }
 
 // NewConcurrentQueue constructs a ConcurrentQueue. The bufferSize parameter is
@@ -24,8 +31,8 @@ func NewConcurrentQueue(bufferSize int) *ConcurrentQueue {
 	return &ConcurrentQueue{
 		chanIn:   make(chan interface{}),
 		chanOut:  make(chan interface{}, bufferSize),
-		quit:     make(chan struct{}),
 		overflow: list.New(),
+		quit:     make(chan struct{}),
 	}
 }
 
@@ -44,7 +51,14 @@ func (cq *ConcurrentQueue) ChanOut() <-chan interface{} {
 // minimize overhead, but if the out channel is full it pushes items to an
 // overflow queue. This must be called before using the queue.
 func (cq *ConcurrentQueue) Start() {
+	if !atomic.CompareAndSwapUint32(&cq.started, 0, 1) {
+		return
+	}
+
+	cq.wg.Add(1)
 	go func() {
+		defer cq.wg.Done()
+
 		for {
 			nextElement := cq.overflow.Front()
 			if nextElement == nil {
@@ -82,5 +96,10 @@ func (cq *ConcurrentQueue) Start() {
 // channel. This does not clear the queue state, so the queue can be restarted
 // without dropping items.
 func (cq *ConcurrentQueue) Stop() {
-	cq.quit <- struct{}{}
+	if !atomic.CompareAndSwapUint32(&cq.stopped, 0, 1) {
+		return
+	}
+
+	close(cq.quit)
+	cq.wg.Wait()
 }