Fix heap package usage

Adds tests to ensure order of submitted
tasks with the same priority.

pool.go

@@ -1,6 +1,7 @@
 package priopool
 
 import (
+	"container/heap"
 	"errors"
 	"fmt"
 	"sync"
@@ -48,9 +49,9 @@ func New(poolCapacity, queueCapacity int) (*PriorityPool, error) {
 
 	switch {
 	case queueCapacity >= 0:
-		queue = make(priorityQueue, 0, queueCapacity)
+		queue.tasks = make([]*priorityQueueTask, 0, queueCapacity)
 	case queueCapacity < 0:
-		queue = make(priorityQueue, 0, defaultQueueCapacity)
+		queue.tasks = make([]*priorityQueueTask, 0, defaultQueueCapacity)
 	}
 
 	return &PriorityPool{
@@ -81,7 +82,7 @@ func (p *PriorityPool) Submit(priority uint32, task func()) error {
 				p.mu.Unlock()
 				return
 			}
-			queueF := p.queue.Pop()
+			queueF := heap.Pop(&p.queue)
 			p.mu.Unlock()
 
 			queueF.(*priorityQueueTask).value()
@@ -95,15 +96,13 @@ func (p *PriorityPool) Submit(priority uint32, task func()) error {
 		return fmt.Errorf("pool submit: %w", err)
 	}
 
-	ln := p.queue.Len()
-	if p.limit >= 0 && ln >= p.limit {
+	if p.limit >= 0 && p.queue.Len() >= p.limit {
 		return ErrQueueOverload
 	}
 
-	p.queue.Push(&priorityQueueTask{
+	heap.Push(&p.queue, &priorityQueueTask{
 		value:    task,
 		priority: int(priority),
-		index:    ln,
 	})
 
 	return nil

pool_test.go

@@ -1,6 +1,7 @@
 package priopool_test
 
 import (
+	"fmt"
 	"sync"
 	"testing"
 	"time"
@@ -122,6 +123,30 @@ func TestPriorityPool_Submit(t *testing.T) {
 
 		wg.Wait()
 	})
+
+	t.Run("non-priority order", func(t *testing.T) {
+		const n = 5
+
+		p, err := priopool.New(1, -1)
+		require.NoError(t, err)
+
+		wg := new(sync.WaitGroup)
+		wg.Add(n)
+		result := new(syncList)
+
+		for i := 0; i < n; i++ {
+			id := i
+			err = p.Submit(lowPriority, taskGenerator(id, result, wg))
+			require.NoError(t, err)
+		}
+
+		wg.Wait()
+
+		fmt.Println(result.list)
+		for i := 0; i < n; i++ {
+			require.Equal(t, i, result.list[i])
+		}
+	})
 }
 
 func taskGenerator(ind int, output *syncList, wg *sync.WaitGroup) func() {

queue.go

@@ -4,49 +4,41 @@ package priopool
 // Priority queue itself is not thread safe.
 // See https://cs.opensource.google/go/go/+/refs/tags/go1.17.2:src/container/heap/example_pq_test.go
 
-import (
-	"container/heap"
-)
-
 type priorityQueueTask struct {
 	value    func()
 	priority int
-	index    int // the index is needed by update and is maintained by the heap.Interface methods
+	index    uint64 // monotonusly increasing index to sort values with same priority
 }
 
-type priorityQueue []*priorityQueueTask
+type priorityQueue struct {
+	nextIndex uint64
+	tasks     []*priorityQueueTask
+}
 
-func (pq priorityQueue) Len() int { return len(pq) }
+func (pq priorityQueue) Len() int { return len(pq.tasks) }
 
 func (pq priorityQueue) Less(i, j int) bool {
-	return pq[i].priority > pq[j].priority
+	if pq.tasks[i].priority == pq.tasks[j].priority {
+		return pq.tasks[i].index < pq.tasks[j].index
+	}
+	return pq.tasks[i].priority > pq.tasks[j].priority
 }
 
 func (pq priorityQueue) Swap(i, j int) {
-	pq[i], pq[j] = pq[j], pq[i]
-	pq[i].index = i
-	pq[j].index = j
+	pq.tasks[i], pq.tasks[j] = pq.tasks[j], pq.tasks[i]
 }
 
 func (pq *priorityQueue) Push(x interface{}) {
-	n := len(*pq)
 	item := x.(*priorityQueueTask)
-	item.index = n
-	*pq = append(*pq, item)
+	item.index = pq.nextIndex
+	pq.nextIndex++
+	pq.tasks = append(pq.tasks, item)
 }
 
 func (pq *priorityQueue) Pop() interface{} {
-	old := *pq
-	n := len(old)
-	item := old[n-1]
-	old[n-1] = nil
-	item.index = -1
-	*pq = old[0 : n-1]
+	n := len(pq.tasks)
+	item := pq.tasks[n-1]
+	pq.tasks[n-1] = nil
+	pq.tasks = pq.tasks[0 : n-1]
 	return item
 }
-
-func (pq *priorityQueue) update(item *priorityQueueTask, value func(), priority int) {
-	item.value = value
-	item.priority = priority
-	heap.Fix(pq, item.index)
-}