Intermittent magic_folder.test.test_participants.CollectiveParticipantsTests.test_list failure

[exarkun]

On the NixOS CI job:

[FAIL]
Traceback (most recent call last):
Failure: testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/nix/store/jw70h4p4hwk7kz6iaa687v5sn25z6hnf-python2.7-Twisted-18.9.0/lib/python2.7/site-packages/twisted/internet/defer.py", line 151, in maybeDeferred
    result = f(*args, **kw)
  File "/nix/store/i2r7a7slgw3300wcvn79bjdczqw83gnd-python2.7-testtools-2.3.0/lib/python2.7/site-packages/testtools/testcase.py", line 719, in _run_test_method
    return self._get_test_method()()
  File "/nix/store/kaskk8zchplh2vy6ib08clspcb9idlsd-python2.7-magic-folder-2020-02-05/lib/python2.7/site-packages/magic_folder/test/test_participants.py", line 149, in test_list
    dictionaries(
  File "/nix/store/5w1h6gwajic64alfjkxrsfiwz80q62ga-python2.7-hypothesis-4.7.3/lib/python2.7/site-packages/hypothesis/core.py", line 974, in wrapped_test
    state.run()
  File "/nix/store/5w1h6gwajic64alfjkxrsfiwz80q62ga-python2.7-hypothesis-4.7.3/lib/python2.7/site-packages/hypothesis/core.py", line 695, in run
    info.__expected_traceback,
  File "/nix/store/5w1h6gwajic64alfjkxrsfiwz80q62ga-python2.7-hypothesis-4.7.3/lib/python2.7/site-packages/hypothesis/core.py", line 568, in execute
    result = self.test_runner(data, run)
  File "/nix/store/5w1h6gwajic64alfjkxrsfiwz80q62ga-python2.7-hypothesis-4.7.3/lib/python2.7/site-packages/hypothesis/executors.py", line 56, in default_new_style_executor
    return function(data)
  File "/nix/store/5w1h6gwajic64alfjkxrsfiwz80q62ga-python2.7-hypothesis-4.7.3/lib/python2.7/site-packages/hypothesis/core.py", line 566, in run
    return test(*args, **kwargs)
  File "/nix/store/kaskk8zchplh2vy6ib08clspcb9idlsd-python2.7-magic-folder-2020-02-05/lib/python2.7/site-packages/magic_folder/test/test_participants.py", line 216, in test_list
    Equals(int(upload_dircap in collective_contents.values())),
  File "/nix/store/i2r7a7slgw3300wcvn79bjdczqw83gnd-python2.7-testtools-2.3.0/lib/python2.7/site-packages/testtools/testcase.py", line 498, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: Differences: [
2 != 1: after <function <lambda>> on [_CollectiveDirnodeParticipant(name=u'', dirobj=Node(tahoe_client=TahoeClient(node_uri=DecodedURL(url=URL.from_text(u'http://example.invalid./')), agent=<treq.client.HTTPClient object at 0x7f6925e4eed0>), uri=<DirectoryURI aaaaa>), is_self=True), _CollectiveDirnodeParticipant(name=u'0', dirobj=Node(tahoe_client=TahoeClient(node_uri=DecodedURL(url=URL.from_text(u'http://example.invalid./')), agent=<treq.client.HTTPClient object at 0x7f6925e4eed0>), uri=<DirectoryURI aaaaa>), is_self=True)]
]

[exarkun]

I surmise that Hypothesis generated a collective that contained a duplicate DMD which caused the test to find two participants which identify as self but the test was only capable of asserting that the result was either zero or one participants.