add lock for SynchronizedSeedDataset; add additional os level close s…

…tderr for tests that launch failing process (#4463)

test/test_dataloader.py

@@ -1,5 +1,6 @@
 import math
 import sys
+import os
 import ctypes
 import torch
 import time
@@ -153,15 +154,16 @@ class SynchronizedSeedDataset(Dataset):
 
     def __init__(self, size, num_workers):
         assert size >= num_workers
-        self.count = multiprocessing.Value('i', 0)
+        self.count = multiprocessing.Value('i', 0, lock=True)
         self.barrier = multiprocessing.Semaphore(0)
         self.num_workers = num_workers
         self.size = size
 
     def __getitem__(self, idx):
-        self.count.value += 1
-        if self.count.value == self.num_workers:
-            self.barrier.release()
+        with self.count.get_lock():
+            self.count.value += 1
+            if self.count.value == self.num_workers:
+                self.barrier.release()
         self.barrier.acquire()
         self.barrier.release()
         return torch.initial_seed()
@@ -249,6 +251,7 @@ def test_multiple_dataloaders(self):
     @unittest.skipIf(IS_WINDOWS, "TODO: need to fix this test case for Windows")
     def test_segfault(self):
         def _test_segfault():
+            os.close(sys.stderr.fileno())
             sys.stderr.close()
             dataset = SegfaultDataset(10)
             dataloader = DataLoader(dataset, batch_size=2, num_workers=2)
@@ -266,6 +269,7 @@ def _test_segfault():
     @unittest.skipIf(IS_WINDOWS, "TODO: need to fix this test case for Windows")
     def test_timeout(self):
         def _test_timeout():
+            os.close(sys.stderr.fileno())
             sys.stderr.close()
             dataset = SleepDataset(10, 10)
             dataloader = DataLoader(dataset, batch_size=2, num_workers=2, timeout=1)