workqueue: make queue as configurable

The default queue implementation is mostly FIFO and it is not
exchangeable unless we implement the whole `workqueue.Interface` which
is less desirable as we have to duplicate a lot of code. There was one
attempt done in [kubernetes/kubernetes#109349][1] which tried to
implement a priority queue. That is really useful and [knative/pkg][2]
implemented something called two-lane-queue. While two lane queue is
great, but isn't perfect since a full slow queue can still slow down
items in fast queue.

This change proposes a swappable queue implementation while not adding
extra maintenance effort in kubernetes community. We are happy to
maintain our own queue implementation (similar to two-lane-queue) in
downstream.

[1]: https://github.com/kubernetes/kubernetes/pull/109349
[2]: https://github.com/knative/pkg/blob/main/controller/two_lane_queue.go

Kubernetes-commit: 87b4279e07349b3c68f16f69a349a02bddd12f25

util/workqueue/queue.go

@@ -33,6 +33,48 @@ type Interface interface {
 	ShuttingDown() bool
 }
 
+// Queue is the underlying storage for items. The functions below are always
+// called from the same goroutine.
+type Queue interface {
+	// Touch can be hooked when an existing item is added again. This may be
+	// useful if the implementation allows priority change for the given item.
+	Touch(item interface{})
+	// Push adds a new item.
+	Push(item interface{})
+	// Len tells the total number of items.
+	Len() int
+	// Pop retrieves an item.
+	Pop() (item interface{})
+}
+
+// DefaultQueue is a slice based FIFO queue.
+func DefaultQueue() Queue {
+	return new(queue)
+}
+
+// queue is a slice which implements Queue.
+type queue []interface{}
+
+func (q *queue) Touch(item interface{}) {}
+
+func (q *queue) Push(item interface{}) {
+	*q = append(*q, item)
+}
+
+func (q *queue) Len() int {
+	return len(*q)
+}
+
+func (q *queue) Pop() (item interface{}) {
+	item = (*q)[0]
+
+	// The underlying array still exists and reference this object, so the object will not be garbage collected.
+	(*q)[0] = nil
+	*q = (*q)[1:]
+
+	return item
+}
+
 // QueueConfig specifies optional configurations to customize an Interface.
 type QueueConfig struct {
 	// Name for the queue. If unnamed, the metrics will not be registered.
@@ -44,6 +86,9 @@ type QueueConfig struct {
 
 	// Clock ability to inject real or fake clock for testing purposes.
 	Clock clock.WithTicker
+
+	// Queue provides the underlying queue to use. It is optional and defaults to slice based FIFO queue.
+	Queue Queue
 }
 
 // New constructs a new work queue (see the package comment).
@@ -83,16 +128,22 @@ func newQueueWithConfig(config QueueConfig, updatePeriod time.Duration) *Type {
 		config.Clock = clock.RealClock{}
 	}
 
+	if config.Queue == nil {
+		config.Queue = DefaultQueue()
+	}
+
 	return newQueue(
 		config.Clock,
+		config.Queue,
 		metricsFactory.newQueueMetrics(config.Name, config.Clock),
 		updatePeriod,
 	)
 }
 
-func newQueue(c clock.WithTicker, metrics queueMetrics, updatePeriod time.Duration) *Type {
+func newQueue(c clock.WithTicker, queue Queue, metrics queueMetrics, updatePeriod time.Duration) *Type {
 	t := &Type{
 		clock:                      c,
+		queue:                      queue,
 		dirty:                      set{},
 		processing:                 set{},
 		cond:                       sync.NewCond(&sync.Mutex{}),
@@ -116,7 +167,7 @@ type Type struct {
 	// queue defines the order in which we will work on items. Every
 	// element of queue should be in the dirty set and not in the
 	// processing set.
-	queue []t
+	queue Queue
 
 	// dirty defines all of the items that need to be processed.
 	dirty set
@@ -167,6 +218,11 @@ func (q *Type) Add(item interface{}) {
 		return
 	}
 	if q.dirty.has(item) {
+		// the same item is added again before it is processed, call the Touch
+		// function if the queue cares about it (for e.g, reset its priority)
+		if !q.processing.has(item) {
+			q.queue.Touch(item)
+		}
 		return
 	}
 
@@ -177,7 +233,7 @@ func (q *Type) Add(item interface{}) {
 		return
 	}
 
-	q.queue = append(q.queue, item)
+	q.queue.Push(item)
 	q.cond.Signal()
 }
 
@@ -187,7 +243,7 @@ func (q *Type) Add(item interface{}) {
 func (q *Type) Len() int {
 	q.cond.L.Lock()
 	defer q.cond.L.Unlock()
-	return len(q.queue)
+	return q.queue.Len()
 }
 
 // Get blocks until it can return an item to be processed. If shutdown = true,
@@ -196,18 +252,15 @@ func (q *Type) Len() int {
 func (q *Type) Get() (item interface{}, shutdown bool) {
 	q.cond.L.Lock()
 	defer q.cond.L.Unlock()
-	for len(q.queue) == 0 && !q.shuttingDown {
+	for q.queue.Len() == 0 && !q.shuttingDown {
 		q.cond.Wait()
 	}
-	if len(q.queue) == 0 {
+	if q.queue.Len() == 0 {
 		// We must be shutting down.
 		return nil, true
 	}
 
-	item = q.queue[0]
-	// The underlying array still exists and reference this object, so the object will not be garbage collected.
-	q.queue[0] = nil
-	q.queue = q.queue[1:]
+	item = q.queue.Pop()
 
 	q.metrics.get(item)
 
@@ -228,7 +281,7 @@ func (q *Type) Done(item interface{}) {
 
 	q.processing.delete(item)
 	if q.dirty.has(item) {
-		q.queue = append(q.queue, item)
+		q.queue.Push(item)
 		q.cond.Signal()
 	} else if q.processing.len() == 0 {
 		q.cond.Signal()