Fix line-jumping bug in FIFO implementation

Keep the FIFO's internal set in sync with the queue during Add/Update
operations to prevent a queue line-jumping scenario (described in a
new unit test).

pkg/client/cache/fifo.go

@@ -28,28 +28,28 @@ import (
 // processed once, and when it is processed, the most recent version will be
 // processed. This can't be done with a channel.
 type FIFO struct {
-	lock  sync.RWMutex
-	cond  sync.Cond
+	lock sync.RWMutex
+	cond sync.Cond
+	// We depend on the property that items in the set are in the queue and vice versa.
 	items map[string]interface{}
 	queue []string
 }
 
-// Add inserts an item, and puts it in the queue.
+// Add inserts an item, and puts it in the queue. The item is only enqueued
+// if it doesn't already exist in the set.
 func (f *FIFO) Add(id string, obj interface{}) {
 	f.lock.Lock()
 	defer f.lock.Unlock()
+	if _, exists := f.items[id]; !exists {
+		f.queue = append(f.queue, id)
+	}
 	f.items[id] = obj
-	f.queue = append(f.queue, id)
 	f.cond.Broadcast()
 }
 
-// Update updates an item, and adds it to the queue.
+// Update is the same as Add in this implementation.
 func (f *FIFO) Update(id string, obj interface{}) {
-	f.lock.Lock()
-	defer f.lock.Unlock()
-	f.items[id] = obj
-	f.queue = append(f.queue, id)
-	f.cond.Broadcast()
+	f.Add(id, obj)
 }
 
 // Delete removes an item. It doesn't add it to the queue, because

pkg/client/cache/fifo_test.go

@@ -107,3 +107,31 @@ func TestFIFO_addReplace(t *testing.T) {
 		t.Errorf("item did not get removed")
 	}
 }
+
+func TestFIFO_detectLineJumpers(t *testing.T) {
+	f := NewFIFO()
+
+	f.Add("foo", 10)
+	f.Add("bar", 1)
+	f.Add("foo", 11)
+	f.Add("foo", 13)
+	f.Add("zab", 30)
+
+	if e, a := 13, f.Pop().(int); a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+
+	f.Add("foo", 14) // ensure foo doesn't jump back in line
+
+	if e, a := 1, f.Pop().(int); a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+
+	if e, a := 30, f.Pop().(int); a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+
+	if e, a := 14, f.Pop().(int); a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+}