workerpool: add Len() to retrieve the count of running workers

workerpool.go

@@ -66,6 +66,11 @@ func (wp *WorkerPool) Cap() int {
 	return cap(wp.workers)
 }
 
+// Len returns the count of concurrent workers currently running.
+func (wp *WorkerPool) Len() int {
+	return len(wp.workers)
+}
+
 // Submit submits f for processing by a worker. The given id is useful for
 // identifying the task once it is completed. The task f must return when the
 // context ctx is cancelled.

workerpool_test.go

@@ -68,6 +68,26 @@ func TestWorkerPoolCap(t *testing.T) {
 	}
 }
 
+func TestWorkerPoolLen(t *testing.T) {
+	wp := New(1)
+	defer wp.Close()
+	if l := wp.Len(); l != 0 {
+		t.Errorf("got %d; want %d", l, 0)
+	}
+
+	err := wp.Submit("", func(ctx context.Context) error {
+		<-ctx.Done()
+		return ctx.Err()
+	})
+	if err != nil {
+		t.Fatalf("failed to submit task: %v", err)
+	}
+
+	if l := wp.Len(); l != 1 {
+		t.Errorf("got %d; want %d", l, 1)
+	}
+}
+
 // TestWorkerPoolConcurrentTasksCount ensure that there is at least, but no
 // more than n workers running in the pool when more than n tasks are
 // submitted.