fix multithreading error in GeneratorDataset

mindspore/dataset/engine/datasets.py

@@ -3174,7 +3174,7 @@ class SamplerFn:
         self.workers = []
         self.num_worker = num_worker
         self.multi_process = multi_process
-        self.joined = False
+        self.need_join = False
         self.ppid = os.getpid()
         self.pid = []
         # Event for end of epoch
@@ -3192,6 +3192,7 @@ class SamplerFn:
                 # In this phase, the main process is not locked.
                 worker.start()
                 self.pid.append(worker.pid)
+                self.need_join = True
             else:
                 worker = _GeneratorWorkerMt(dataset, self.eof)
                 worker.daemon = True
@@ -3237,9 +3238,9 @@ class SamplerFn:
 
     def _stop_subprocess(self):
         # Only the main process can call join
-        if self.joined is False and self.ppid == os.getpid():
+        if self.need_join is True and self.ppid == os.getpid():
             self.eof.set()
-            self.joined = True
+            self.need_join = False
             for w in self.workers:
                 w.join()