diff --git a/tools/cache/BUILD b/tools/cache/BUILD
index 684d3dca..75d4ff07 100644
--- a/tools/cache/BUILD
+++ b/tools/cache/BUILD
@@ -15,6 +15,7 @@ go_test(
         "delta_fifo_test.go",
         "expiration_cache_test.go",
         "fifo_test.go",
+        "heap_test.go",
         "index_test.go",
         "mutation_detector_test.go",
         "processor_listener_test.go",
@@ -50,6 +51,7 @@ go_library(
         "expiration_cache_fakes.go",
         "fake_custom_store.go",
         "fifo.go",
+        "heap.go",
         "index.go",
         "listers.go",
         "listwatch.go",
diff --git a/tools/cache/heap.go b/tools/cache/heap.go
new file mode 100644
index 00000000..78e49245
--- /dev/null
+++ b/tools/cache/heap.go
@@ -0,0 +1,323 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// This file implements a heap data structure.
+
+package cache
+
+import (
+	"container/heap"
+	"fmt"
+	"sync"
+)
+
+const (
+	closedMsg = "heap is closed"
+)
+
+type LessFunc func(interface{}, interface{}) bool
+type heapItem struct {
+	obj   interface{} // The object which is stored in the heap.
+	index int         // The index of the object's key in the Heap.queue.
+}
+
+type itemKeyValue struct {
+	key string
+	obj interface{}
+}
+
+// heapData is an internal struct that implements the standard heap interface
+// and keeps the data stored in the heap.
+type heapData struct {
+	// items is a map from key of the objects to the objects and their index.
+	// We depend on the property that items in the map are in the queue and vice versa.
+	items map[string]*heapItem
+	// queue implements a heap data structure and keeps the order of elements
+	// according to the heap invariant. The queue keeps the keys of objects stored
+	// in "items".
+	queue []string
+
+	// keyFunc is used to make the key used for queued item insertion and retrieval, and
+	// should be deterministic.
+	keyFunc KeyFunc
+	// lessFunc is used to compare two objects in the heap.
+	lessFunc LessFunc
+}
+
+var (
+	_ = heap.Interface(&heapData{}) // heapData is a standard heap
+)
+
+// Less compares two objects and returns true if the first one should go
+// in front of the second one in the heap.
+func (h *heapData) Less(i, j int) bool {
+	if i > len(h.queue) || j > len(h.queue) {
+		return false
+	}
+	itemi, ok := h.items[h.queue[i]]
+	if !ok {
+		return false
+	}
+	itemj, ok := h.items[h.queue[j]]
+	if !ok {
+		return false
+	}
+	return h.lessFunc(itemi.obj, itemj.obj)
+}
+
+// Len returns the number of items in the Heap.
+func (h *heapData) Len() int { return len(h.queue) }
+
+// Swap implements swapping of two elements in the heap. This is a part of standard
+// heap interface and should never be called directly.
+func (h *heapData) Swap(i, j int) {
+	h.queue[i], h.queue[j] = h.queue[j], h.queue[i]
+	item := h.items[h.queue[i]]
+	item.index = i
+	item = h.items[h.queue[j]]
+	item.index = j
+}
+
+// Push is supposed to be called by heap.Push only.
+func (h *heapData) Push(kv interface{}) {
+	keyValue := kv.(*itemKeyValue)
+	n := len(h.queue)
+	h.items[keyValue.key] = &heapItem{keyValue.obj, n}
+	h.queue = append(h.queue, keyValue.key)
+}
+
+// Pop is supposed to be called by heap.Pop only.
+func (h *heapData) Pop() interface{} {
+	key := h.queue[len(h.queue)-1]
+	h.queue = h.queue[0 : len(h.queue)-1]
+	item, ok := h.items[key]
+	if !ok {
+		// This is an error
+		return nil
+	}
+	delete(h.items, key)
+	return item.obj
+}
+
+// Heap is a thread-safe producer/consumer queue that implements a heap data structure.
+// It can be used to implement priority queues and similar data structures.
+type Heap struct {
+	lock sync.RWMutex
+	cond sync.Cond
+
+	// data stores objects and has a queue that keeps their ordering according
+	// to the heap invariant.
+	data *heapData
+
+	// closed indicates that the queue is closed.
+	// It is mainly used to let Pop() exit its control loop while waiting for an item.
+	closed bool
+}
+
+// Close the Heap and signals condition variables that may be waiting to pop
+// items from the heap.
+func (h *Heap) Close() {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	h.closed = true
+	h.cond.Broadcast()
+}
+
+// Add inserts an item, and puts it in the queue. The item is updated if it
+// already exists.
+func (h *Heap) Add(obj interface{}) error {
+	key, err := h.data.keyFunc(obj)
+	if err != nil {
+		return KeyError{obj, err}
+	}
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	if h.closed {
+		return fmt.Errorf(closedMsg)
+	}
+	if _, exists := h.data.items[key]; exists {
+		h.data.items[key].obj = obj
+		heap.Fix(h.data, h.data.items[key].index)
+	} else {
+		h.addIfNotPresentLocked(key, obj)
+	}
+	h.cond.Broadcast()
+	return nil
+}
+
+// Adds all the items in the list to the queue and then signals the condition
+// variable. It is useful when the caller would like to add all of the items
+// to the queue before consumer starts processing them.
+func (h *Heap) BulkAdd(list []interface{}) error {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	if h.closed {
+		return fmt.Errorf(closedMsg)
+	}
+	for _, obj := range list {
+		key, err := h.data.keyFunc(obj)
+		if err != nil {
+			return KeyError{obj, err}
+		}
+		if _, exists := h.data.items[key]; exists {
+			h.data.items[key].obj = obj
+			heap.Fix(h.data, h.data.items[key].index)
+		} else {
+			h.addIfNotPresentLocked(key, obj)
+		}
+	}
+	h.cond.Broadcast()
+	return nil
+}
+
+// AddIfNotPresent inserts an item, and puts it in the queue. If an item with
+// the key is present in the map, no changes is made to the item.
+//
+// This is useful in a single producer/consumer scenario so that the consumer can
+// safely retry items without contending with the producer and potentially enqueueing
+// stale items.
+func (h *Heap) AddIfNotPresent(obj interface{}) error {
+	id, err := h.data.keyFunc(obj)
+	if err != nil {
+		return KeyError{obj, err}
+	}
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	if h.closed {
+		return fmt.Errorf(closedMsg)
+	}
+	h.addIfNotPresentLocked(id, obj)
+	h.cond.Broadcast()
+	return nil
+}
+
+// addIfNotPresentLocked assumes the lock is already held and adds the the provided
+// item to the queue if it does not already exist.
+func (h *Heap) addIfNotPresentLocked(key string, obj interface{}) {
+	if _, exists := h.data.items[key]; exists {
+		return
+	}
+	heap.Push(h.data, &itemKeyValue{key, obj})
+}
+
+// Update is the same as Add in this implementation. When the item does not
+// exist, it is added.
+func (h *Heap) Update(obj interface{}) error {
+	return h.Add(obj)
+}
+
+// Delete removes an item.
+func (h *Heap) Delete(obj interface{}) error {
+	key, err := h.data.keyFunc(obj)
+	if err != nil {
+		return KeyError{obj, err}
+	}
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	if item, ok := h.data.items[key]; ok {
+		heap.Remove(h.data, item.index)
+		return nil
+	}
+	return fmt.Errorf("object not found")
+}
+
+// Pop waits until an item is ready. If multiple items are
+// ready, they are returned in the order given by Heap.data.lessFunc.
+func (h *Heap) Pop() (interface{}, error) {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+	for len(h.data.queue) == 0 {
+		// When the queue is empty, invocation of Pop() is blocked until new item is enqueued.
+		// When Close() is called, the h.closed is set and the condition is broadcast,
+		// which causes this loop to continue and return from the Pop().
+		if h.closed {
+			return nil, fmt.Errorf("heap is closed")
+		}
+		h.cond.Wait()
+	}
+	obj := heap.Pop(h.data)
+	if obj != nil {
+		return obj, nil
+	} else {
+		return nil, fmt.Errorf("object was removed from heap data")
+	}
+}
+
+// List returns a list of all the items.
+func (h *Heap) List() []interface{} {
+	h.lock.RLock()
+	defer h.lock.RUnlock()
+	list := make([]interface{}, 0, len(h.data.items))
+	for _, item := range h.data.items {
+		list = append(list, item.obj)
+	}
+	return list
+}
+
+// ListKeys returns a list of all the keys of the objects currently in the Heap.
+func (h *Heap) ListKeys() []string {
+	h.lock.RLock()
+	defer h.lock.RUnlock()
+	list := make([]string, 0, len(h.data.items))
+	for key := range h.data.items {
+		list = append(list, key)
+	}
+	return list
+}
+
+// Get returns the requested item, or sets exists=false.
+func (h *Heap) Get(obj interface{}) (interface{}, bool, error) {
+	key, err := h.data.keyFunc(obj)
+	if err != nil {
+		return nil, false, KeyError{obj, err}
+	}
+	return h.GetByKey(key)
+}
+
+// GetByKey returns the requested item, or sets exists=false.
+func (h *Heap) GetByKey(key string) (interface{}, bool, error) {
+	h.lock.RLock()
+	defer h.lock.RUnlock()
+	item, exists := h.data.items[key]
+	if !exists {
+		return nil, false, nil
+	}
+	return item.obj, true, nil
+}
+
+// IsClosed returns true if the queue is closed.
+func (h *Heap) IsClosed() bool {
+	h.lock.RLock()
+	defer h.lock.RUnlock()
+	if h.closed {
+		return true
+	}
+	return false
+}
+
+// NewHeap returns a Heap which can be used to queue up items to process.
+func NewHeap(keyFn KeyFunc, lessFn LessFunc) *Heap {
+	h := &Heap{
+		data: &heapData{
+			items:    map[string]*heapItem{},
+			queue:    []string{},
+			keyFunc:  keyFn,
+			lessFunc: lessFn,
+		},
+	}
+	h.cond.L = &h.lock
+	return h
+}
diff --git a/tools/cache/heap_test.go b/tools/cache/heap_test.go
new file mode 100644
index 00000000..c2e47698
--- /dev/null
+++ b/tools/cache/heap_test.go
@@ -0,0 +1,382 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package cache
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func testHeapObjectKeyFunc(obj interface{}) (string, error) {
+	return obj.(testHeapObject).name, nil
+}
+
+type testHeapObject struct {
+	name string
+	val  interface{}
+}
+
+func mkHeapObj(name string, val interface{}) testHeapObject {
+	return testHeapObject{name: name, val: val}
+}
+
+func compareInts(val1 interface{}, val2 interface{}) bool {
+	first := val1.(testHeapObject).val.(int)
+	second := val2.(testHeapObject).val.(int)
+	return first < second
+}
+
+// TestHeapBasic tests Heap invariant and synchronization.
+func TestHeapBasic(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	var wg sync.WaitGroup
+	wg.Add(2)
+	const amount = 500
+	var i, u int
+	// Insert items in the heap in opposite orders in two go routines.
+	go func() {
+		for i = amount; i > 0; i-- {
+			h.Add(mkHeapObj(string([]rune{'a', rune(i)}), i))
+		}
+		wg.Done()
+	}()
+	go func() {
+		for u = 0; u < amount; u++ {
+			h.Add(mkHeapObj(string([]rune{'b', rune(u)}), u+1))
+		}
+		wg.Done()
+	}()
+	// Wait for the two go routines to finish.
+	wg.Wait()
+	// Make sure that the numbers are popped in ascending order.
+	prevNum := 0
+	for i := 0; i < amount*2; i++ {
+		obj, err := h.Pop()
+		num := obj.(testHeapObject).val.(int)
+		// All the items must be sorted.
+		if err != nil || prevNum > num {
+			t.Errorf("got %v out of order, last was %v", obj, prevNum)
+		}
+		prevNum = num
+	}
+}
+
+// Tests Heap.Add and ensures that heap invariant is preserved after adding items.
+func TestHeap_Add(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+	h.Add(mkHeapObj("baz", 11))
+	h.Add(mkHeapObj("zab", 30))
+	h.Add(mkHeapObj("foo", 13)) // This updates "foo".
+
+	item, err := h.Pop()
+	if e, a := 1, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	item, err = h.Pop()
+	if e, a := 11, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	h.Delete(mkHeapObj("baz", 11)) // Nothing is deleted.
+	h.Add(mkHeapObj("foo", 14))    // foo is updated.
+	item, err = h.Pop()
+	if e, a := 14, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	item, err = h.Pop()
+	if e, a := 30, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+}
+
+// TestHeap_BulkAdd tests Heap.BulkAdd functionality and ensures that all the
+// items given to BulkAdd are added to the queue before Pop reads them.
+func TestHeap_BulkAdd(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	const amount = 500
+	// Insert items in the heap in opposite orders in a go routine.
+	go func() {
+		l := []interface{}{}
+		for i := amount; i > 0; i-- {
+			l = append(l, mkHeapObj(string([]rune{'a', rune(i)}), i))
+		}
+		h.BulkAdd(l)
+	}()
+	prevNum := -1
+	for i := 0; i < amount; i++ {
+		obj, err := h.Pop()
+		num := obj.(testHeapObject).val.(int)
+		// All the items must be sorted.
+		if err != nil || prevNum >= num {
+			t.Errorf("got %v out of order, last was %v", obj, prevNum)
+		}
+		prevNum = num
+	}
+}
+
+// TestHeapEmptyPop tests that pop returns properly after heap is closed.
+func TestHeapEmptyPop(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	go func() {
+		time.Sleep(1 * time.Second)
+		h.Close()
+	}()
+	_, err := h.Pop()
+	if err == nil || err.Error() != closedMsg {
+		t.Errorf("pop should have returned heap closed error: %v", err)
+	}
+}
+
+// TestHeap_AddIfNotPresent tests Heap.AddIfNotPresent and ensures that heap
+// invariant is preserved after adding items.
+func TestHeap_AddIfNotPresent(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.AddIfNotPresent(mkHeapObj("foo", 10))
+	h.AddIfNotPresent(mkHeapObj("bar", 1))
+	h.AddIfNotPresent(mkHeapObj("baz", 11))
+	h.AddIfNotPresent(mkHeapObj("zab", 30))
+	h.AddIfNotPresent(mkHeapObj("foo", 13)) // This is not added.
+
+	if len := len(h.data.items); len != 4 {
+		t.Errorf("unexpected number of items: %d", len)
+	}
+	if val := h.data.items["foo"].obj.(testHeapObject).val; val != 10 {
+		t.Errorf("unexpected value: %d", val)
+	}
+	item, err := h.Pop()
+	if e, a := 1, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	item, err = h.Pop()
+	if e, a := 10, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	// bar is already popped. Let's add another one.
+	h.AddIfNotPresent(mkHeapObj("bar", 14))
+	item, err = h.Pop()
+	if e, a := 11, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	item, err = h.Pop()
+	if e, a := 14, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+}
+
+// TestHeap_Delete tests Heap.Delete and ensures that heap invariant is
+// preserved after deleting items.
+func TestHeap_Delete(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+	h.Add(mkHeapObj("bal", 31))
+	h.Add(mkHeapObj("baz", 11))
+
+	// Delete head. Delete should work with "key" and doesn't care about the value.
+	if err := h.Delete(mkHeapObj("bar", 200)); err != nil {
+		t.Fatalf("Failed to delete head.")
+	}
+	item, err := h.Pop()
+	if e, a := 10, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	h.Add(mkHeapObj("zab", 30))
+	h.Add(mkHeapObj("faz", 30))
+	len := h.data.Len()
+	// Delete non-existing item.
+	if err = h.Delete(mkHeapObj("non-existent", 10)); err == nil || len != h.data.Len() {
+		t.Fatalf("Didn't expect any item removal")
+	}
+	// Delete tail.
+	if err = h.Delete(mkHeapObj("bal", 31)); err != nil {
+		t.Fatalf("Failed to delete tail.")
+	}
+	// Delete one of the items with value 30.
+	if err = h.Delete(mkHeapObj("zab", 30)); err != nil {
+		t.Fatalf("Failed to delete item.")
+	}
+	item, err = h.Pop()
+	if e, a := 11, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	item, err = h.Pop()
+	if e, a := 30, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	if h.data.Len() != 0 {
+		t.Fatalf("expected an empty heap.")
+	}
+}
+
+// TestHeap_Update tests Heap.Update and ensures that heap invariant is
+// preserved after adding items.
+func TestHeap_Update(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+	h.Add(mkHeapObj("bal", 31))
+	h.Add(mkHeapObj("baz", 11))
+
+	// Update an item to a value that should push it to the head.
+	h.Update(mkHeapObj("baz", 0))
+	if h.data.queue[0] != "baz" || h.data.items["baz"].index != 0 {
+		t.Fatalf("expected baz to be at the head")
+	}
+	item, err := h.Pop()
+	if e, a := 0, item.(testHeapObject).val; err != nil || a != e {
+		t.Fatalf("expected %d, got %d", e, a)
+	}
+	// Update bar to push it farther back in the queue.
+	h.Update(mkHeapObj("bar", 100))
+	if h.data.queue[0] != "foo" || h.data.items["foo"].index != 0 {
+		t.Fatalf("expected foo to be at the head")
+	}
+}
+
+// TestHeap_Get tests Heap.Get.
+func TestHeap_Get(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+	h.Add(mkHeapObj("bal", 31))
+	h.Add(mkHeapObj("baz", 11))
+
+	// Get works with the key.
+	obj, exists, err := h.Get(mkHeapObj("baz", 0))
+	if err != nil || exists == false || obj.(testHeapObject).val != 11 {
+		t.Fatalf("unexpected error in getting element")
+	}
+	// Get non-existing object.
+	_, exists, err = h.Get(mkHeapObj("non-existing", 0))
+	if err != nil || exists == true {
+		t.Fatalf("didn't expect to get any object")
+	}
+}
+
+// TestHeap_GetByKey tests Heap.GetByKey and is very similar to TestHeap_Get.
+func TestHeap_GetByKey(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+	h.Add(mkHeapObj("bal", 31))
+	h.Add(mkHeapObj("baz", 11))
+
+	obj, exists, err := h.GetByKey("baz")
+	if err != nil || exists == false || obj.(testHeapObject).val != 11 {
+		t.Fatalf("unexpected error in getting element")
+	}
+	// Get non-existing object.
+	_, exists, err = h.GetByKey("non-existing")
+	if err != nil || exists == true {
+		t.Fatalf("didn't expect to get any object")
+	}
+}
+
+// TestHeap_Close tests Heap.Close and Heap.IsClosed functions.
+func TestHeap_Close(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Add(mkHeapObj("foo", 10))
+	h.Add(mkHeapObj("bar", 1))
+
+	if h.IsClosed() {
+		t.Fatalf("didn't expect heap to be closed")
+	}
+	h.Close()
+	if !h.IsClosed() {
+		t.Fatalf("expect heap to be closed")
+	}
+}
+
+// TestHeap_List tests Heap.List function.
+func TestHeap_List(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	list := h.List()
+	if len(list) != 0 {
+		t.Errorf("expected an empty list")
+	}
+
+	items := map[string]int{
+		"foo": 10,
+		"bar": 1,
+		"bal": 30,
+		"baz": 11,
+		"faz": 30,
+	}
+	for k, v := range items {
+		h.Add(mkHeapObj(k, v))
+	}
+	list = h.List()
+	if len(list) != len(items) {
+		t.Errorf("expected %d items, got %d", len(items), len(list))
+	}
+	for _, obj := range list {
+		heapObj := obj.(testHeapObject)
+		v, ok := items[heapObj.name]
+		if !ok || v != heapObj.val {
+			t.Errorf("unexpected item in the list: %v", heapObj)
+		}
+	}
+}
+
+// TestHeap_ListKeys tests Heap.ListKeys function. Scenario is the same as
+// TestHeap_list.
+func TestHeap_ListKeys(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	list := h.ListKeys()
+	if len(list) != 0 {
+		t.Errorf("expected an empty list")
+	}
+
+	items := map[string]int{
+		"foo": 10,
+		"bar": 1,
+		"bal": 30,
+		"baz": 11,
+		"faz": 30,
+	}
+	for k, v := range items {
+		h.Add(mkHeapObj(k, v))
+	}
+	list = h.ListKeys()
+	if len(list) != len(items) {
+		t.Errorf("expected %d items, got %d", len(items), len(list))
+	}
+	for _, key := range list {
+		_, ok := items[key]
+		if !ok {
+			t.Errorf("unexpected item in the list: %v", key)
+		}
+	}
+}
+
+// TestHeapAddAfterClose tests that heap returns an error if anything is added
+// after it is closed.
+func TestHeapAddAfterClose(t *testing.T) {
+	h := NewHeap(testHeapObjectKeyFunc, compareInts)
+	h.Close()
+	if err := h.Add(mkHeapObj("test", 1)); err == nil || err.Error() != closedMsg {
+		t.Errorf("expected heap closed error")
+	}
+	if err := h.AddIfNotPresent(mkHeapObj("test", 1)); err == nil || err.Error() != closedMsg {
+		t.Errorf("expected heap closed error")
+	}
+	if err := h.BulkAdd([]interface{}{mkHeapObj("test", 1)}); err == nil || err.Error() != closedMsg {
+		t.Errorf("expected heap closed error")
+	}
+}