Suppose the following steps:
1. Open speculation switch in the application.
2. Run this app and suppose last task of shuffleMapStage 1 finishes. Let's get the record straight, from the eyes of DAG, this stage really finishes, and from the eyes of TaskSetManager, variable 'isZombie' is set to true, but variable runningTasksSet isn't empty because of speculation.
3. Suddenly, executor 3 is lost. TaskScheduler receiving this signal, invokes all executorLost functions of rootPool's taskSetManagers. DAG receiving this signal, removes all this executor's outputLocs.
4. TaskSetManager adds all this executor's tasks to pendingTasks and tells DAG they will be resubmitted (Attention: possibly not on time).
5. DAG starts to submit a new waitingStage, let's say shuffleMapStage 2, and going to find that shuffleMapStage 1 is its missing parent because some outputLocs are removed due to executor lost. Then DAG submits shuffleMapStage 1 again.
6. DAG still receives Task 'Resubmitted' signal from old taskSetManager, and increases the number of pendingTasks of shuffleMapStage 1 each time. However, old taskSetManager won't resolve new task to submit because its variable 'isZombie' is set to true.
7. Finally shuffleMapStage 1 never finishes in DAG together with all stages depending on it.