add NewWithContext

Before this patch, forwarding context cancellation to the workerpool
tasks was difficult and required to spawn a goroutine to call wp.Close
when the parent context was done.

This patch introduce NewWithContext allowing callers to provide a parent
context from which is derived the tasks context, removing the need to
manually forward context cancellation.

Signed-off-by: Alexandre Perrin <alex@isovalent.com>

workerpool.go

@@ -50,14 +50,20 @@ type WorkerPool struct {
 // New creates a new pool of workers where at most n workers process submitted
 // tasks concurrently. New panics if n ≤ 0.
 func New(n int) *WorkerPool {
+	return NewWithContext(context.Background(), n)
+}
+
+// NewWithContext creates a new pool of workers where at most n workers process submitted
+// tasks concurrently. New panics if n ≤ 0. The context is used as the parent context to the context of the task func passed to Submit.
+func NewWithContext(ctx context.Context, n int) *WorkerPool {
 	if n <= 0 {
 		panic(fmt.Sprintf("workerpool.New: n must be > 0, got %d", n))
 	}
 	wp := &WorkerPool{
 		workers: make(chan struct{}, n),
 		tasks:   make(chan *task),
 	}
-	ctx, cancel := context.WithCancel(context.Background())
+	ctx, cancel := context.WithCancel(ctx)
 	wp.cancel = cancel
 	go wp.run(ctx)
 	return wp

workerpool_test.go

@@ -402,3 +402,60 @@ func TestWorkerPoolClose(t *testing.T) {
 	}
 	wg.Wait() // all routines should have returned
 }
+
+func TestWorkerPoolNewWithContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	n := runtime.NumCPU()
+	wp := workerpool.NewWithContext(ctx, n)
+
+	// working is written to by each task as soon as possible.
+	working := make(chan struct{})
+	var wg sync.WaitGroup
+	// Create n tasks waiting on the context to be cancelled.
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		id := fmt.Sprintf("task #%2d", i)
+		err := wp.Submit(id, func(ctx context.Context) error {
+			working <- struct{}{}
+			<-ctx.Done()
+			wg.Done()
+			return ctx.Err()
+		})
+		if err != nil {
+			t.Errorf("failed to submit task '%s': %v", id, err)
+		}
+	}
+
+	// ensure n workers are busy
+	for i := 0; i < n; i++ {
+		<-working
+	}
+
+	// cancel the parent context, which should complete all tasks.
+	cancel()
+	wg.Wait()
+
+	// Submitting a task once the parent context has been cancelled should
+	// succeed and give a cancelled context to the task. This is not ideal and
+	// might change in the future.
+	wg.Add(1)
+	id := "last"
+	err := wp.Submit(id, func(ctx context.Context) error {
+		defer wg.Done()
+		select {
+		case <-ctx.Done():
+		default:
+			t.Errorf("last task expected context to be cancelled")
+		}
+		return nil
+	})
+	if err != nil {
+		t.Errorf("failed to submit task '%s': %v", id, err)
+	}
+
+	wg.Wait()
+
+	if err := wp.Close(); err != nil {
+		t.Errorf("close: got '%v', want no error", err)
+	}
+}