workerpool: make it so Drain() never returns nil

Before this patch, Drain() would return nil when no tasks were
submitted.

This patch make it so Drain() always returns a non-nil slice for
consistency.

Signed-off-by: Alexandre Perrin <alex@kaworu.ch>

workerpool.go

@@ -56,6 +56,7 @@ func New(n int) *WorkerPool {
 	wp := &WorkerPool{
 		workers: make(chan struct{}, n),
 		tasks:   make(chan *task),
+		results: []Task{},
 	}
 	ctx, cancel := context.WithCancel(context.Background())
 	wp.cancel = cancel
@@ -127,7 +128,7 @@ func (wp *WorkerPool) Drain() ([]Task, error) {
 	// wp.results as no other routine is running at this point besides the
 	// "run" routine which should be waiting on the tasks channel.
 	res := wp.results
-	wp.results = nil
+	wp.results = []Task{}
 
 	wp.mu.Lock()
 	wp.draining = false

workerpool_test.go

@@ -231,6 +231,36 @@ func TestWorkerPool(t *testing.T) {
 	}
 }
 
+func TestDrainWithoutSubmit(t *testing.T) {
+	n := runtime.NumCPU()
+	wp := workerpool.New(n)
+
+	results, err := wp.Drain()
+	switch {
+	case err != nil:
+		t.Errorf("draining failed: %v", err)
+	case results == nil:
+		t.Errorf("unexpected nil Drain() results")
+	case len(results) != 0:
+		t.Errorf("got %d; want 0", len(results))
+	}
+
+	// test a second time as Drain() reset the results.
+	results, err = wp.Drain()
+	switch {
+	case err != nil:
+		t.Errorf("draining failed: %v", err)
+	case results == nil:
+		t.Errorf("unexpected nil Drain() results")
+	case len(results) != 0:
+		t.Errorf("got %d; want 0", len(results))
+	}
+
+	if err := wp.Close(); err != nil {
+		t.Errorf("close: got '%v', want no error", err)
+	}
+}
+
 func TestConcurrentDrain(t *testing.T) {
 	n := runtime.NumCPU()
 	wp := workerpool.New(n)