From fc73cc96785159571b360e5b761c3070a9201bd6 Mon Sep 17 00:00:00 2001
From: Wilmer Paulino <wilmer.paulino@gmail.com>
Date: Thu, 21 Jun 2018 17:38:23 -0700
Subject: [PATCH] chain: add concurrent unbounded queue implementation

---
 chain/queue.go | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 chain/queue.go

diff --git a/chain/queue.go b/chain/queue.go
new file mode 100644
index 0000000..b30515b
--- /dev/null
+++ b/chain/queue.go
@@ -0,0 +1,88 @@
+package chain
+
+import (
+	"container/list"
+)
+
+// ConcurrentQueue is a concurrent-safe FIFO queue with unbounded capacity.
+// Clients interact with the queue by pushing items into the in channel and
+// popping items from the out channel. There is a goroutine that manages moving
+// items from the in channel to the out channel in the correct order that must
+// be started by calling Start().
+type ConcurrentQueue struct {
+	chanIn   chan interface{}
+	chanOut  chan interface{}
+	quit     chan struct{}
+	overflow *list.List
+}
+
+// NewConcurrentQueue constructs a ConcurrentQueue. The bufferSize parameter is
+// the capacity of the output channel. When the size of the queue is below this
+// threshold, pushes do not incur the overhead of the less efficient overflow
+// structure.
+func NewConcurrentQueue(bufferSize int) *ConcurrentQueue {
+	return &ConcurrentQueue{
+		chanIn:   make(chan interface{}),
+		chanOut:  make(chan interface{}, bufferSize),
+		quit:     make(chan struct{}),
+		overflow: list.New(),
+	}
+}
+
+// ChanIn returns a channel that can be used to push new items into the queue.
+func (cq *ConcurrentQueue) ChanIn() chan<- interface{} {
+	return cq.chanIn
+}
+
+// ChanOut returns a channel that can be used to pop items from the queue.
+func (cq *ConcurrentQueue) ChanOut() <-chan interface{} {
+	return cq.chanOut
+}
+
+// Start begins a goroutine that manages moving items from the in channel to
+// the out channel. The queue tries to move items directly to the out channel
+// 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() {
+	go func() {
+		for {
+			nextElement := cq.overflow.Front()
+			if nextElement == nil {
+				// The overflow queue is empty, so incoming
+				// items can be pushed directly to the output
+				// channel. However, if output channel is full,
+				// we'll push to the overflow list instead.
+				select {
+				case item := <-cq.chanIn:
+					select {
+					case cq.chanOut <- item:
+					case <-cq.quit:
+						return
+					default:
+						cq.overflow.PushBack(item)
+					}
+				case <-cq.quit:
+					return
+				}
+			} else {
+				// The overflow queue is not empty, so any new
+				// items get pushed to the back to preserve
+				// order.
+				select {
+				case item := <-cq.chanIn:
+					cq.overflow.PushBack(item)
+				case cq.chanOut <- nextElement.Value:
+					cq.overflow.Remove(nextElement)
+				case <-cq.quit:
+					return
+				}
+			}
+		}
+	}()
+}
+
+// Stop ends the goroutine that moves items from the in channel to the out
+// channel.
+func (cq *ConcurrentQueue) Stop() {
+	close(cq.quit)
+}