fix: calling start multiple times should no-op (#901)

* fix: calling start multiple times should no-op

* give test more time after start

* add a mutex

scheduler.go

@@ -824,6 +824,11 @@ func (s *scheduler) RemoveJob(id uuid.UUID) error {
 }
 
 func (s *scheduler) Start() {
+	if s.started.Load() {
+		s.logger.Warn("gocron: scheduler already started")
+		return
+	}
+
 	select {
 	case <-s.shutdownCtx.Done():
 	case s.startCh <- struct{}{}:

scheduler_test.go

@@ -578,6 +578,41 @@ func TestScheduler_Shutdown(t *testing.T) {
 	})
 }
 
+func TestScheduler_Start(t *testing.T) {
+	defer verifyNoGoroutineLeaks(t)
+
+	t.Run("calling start multiple times is a no-op", func(t *testing.T) {
+		s := newTestScheduler(t)
+
+		var counter int
+		var mu sync.Mutex
+
+		_, err := s.NewJob(
+			DurationJob(
+				100*time.Millisecond,
+			),
+			NewTask(
+				func() {
+					mu.Lock()
+					counter++
+					mu.Unlock()
+				},
+			),
+		)
+		require.NoError(t, err)
+
+		s.Start()
+		s.Start()
+		s.Start()
+
+		time.Sleep(1000 * time.Millisecond)
+
+		require.NoError(t, s.Shutdown())
+
+		assert.Contains(t, []int{9, 10}, counter)
+	})
+}
+
 func TestScheduler_NewJob(t *testing.T) {
 	defer verifyNoGoroutineLeaks(t)
 	tests := []struct {