Arvados

From build 1393:

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ERROR: test_rewrite_on_empty_file (tests.test_arvfile.ArvadosFileWriterTestCase)
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Traceback (most recent call last):
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/tests/test_arvfile.py", line 234, in test_rewrite_on_empty_file
    self.assertEqual("0123456789", writer.readfrom(0, 20))
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/arvfile.py", line 45, in before_close_wrapper
    return orig_func(self, *args, **kwargs)
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/retry.py", line 157, in num_retries_setter
    return orig_func(self, *args, **kwargs)
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/arvfile.py", line 878, in readfrom
    return self.arvadosfile.readfrom(offset, size, num_retries)
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/arvfile.py", line 732, in readfrom
    block = self.parent._my_block_manager().get_block_contents(lr.locator, num_retries=num_retries, cache_only=bool(data))
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/arvfile.py", line 506, in get_block_contents
    return bufferblock.buffer_view[0:bufferblock.write_pointer].tobytes()
TypeError: 'NoneType' object has no attribute '__getitem__'

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