PyPy CI: allmydata.test.test_system.SystemTest.test_filesystem sometimes fails

[sajith]

There are these intermittent failures on pypy2.7-buster on CircleCI (​example), which goes away when the task is restarted:

Traceback (most recent call last):
testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/tmp/tahoe-lafs.tox/pypy27-coverage/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/tmp/tahoe-lafs.tox/pypy27-coverage/site-packages/allmydata/test/test_system.py", line 2414, in <lambda>
    d.addCallback(lambda out_err: self.failUnlessEqual(out_err[0], datas[0]))
  File "/tmp/tahoe-lafs.tox/pypy27-coverage/site-packages/twisted/trial/_synctest.py", line 434, in assertEqual
    super(_Assertions, self).assertEqual(first, second, msg)
  File "/opt/pypy/lib-python/2.7/unittest/case.py", line 513, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/opt/pypy/lib-python/2.7/unittest/case.py", line 506, in _baseAssertEqual
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: '' != 'data to be uploaded: file0\n'

Possibly related to #3299.

[exarkun]

Another example - ​https://app.circleci.com/pipelines/github/tahoe-lafs/tahoe-lafs/901/workflows/b7919505-f9da-465c-93fc-3e72ebf2ae7a/jobs/29878

[exarkun]

test_filesystem is too big to make any sense of. I filed ​https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3438 for breaking one piece off of it.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/875

[exarkun]

​https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3572 was a duplicate of this