Fix AttributeError in handle_missing_dep

[fjetter]

Another minor regression introduced in #4107

Closes #4199

[quasiben]

Thanks for the fixes @fjetter . Do we not test bad_dep in unit tests ?

[fjetter]

At least not explicitly. I was also trying to figure out which tests even trigger this section but couldn't find any, yet (without executing all tests on my machine)
Will have another look :)

[fjetter]

Py3.6 tests fail due to a timeout error in test_gather_then_submit_after_failed_workers but I can't see how this would be connected.

I do see some more logged errors, though

  File "D:\a\distributed\distributed\distributed\worker.py", line 1801, in ensure_communicating
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    assert all(dep.key in self.tasks for dep in deps)
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457
AssertionError

and

Traceback (most recent call last):
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  File "D:\a\distributed\distributed\distributed\worker.py", line 2868, in validate_task
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    self.validate_task_ready(ts)
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  File "D:\a\distributed\distributed\distributed\worker.py", line 2843, in validate_task_ready
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    assert ts.key not in self.data
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581
AssertionError

maybe there is still something else left over :/

[fjetter]

FWIW I can sporadically reproduce the logs locally when running test_worker_who_has_clears_after_failed_connection but I'm still looking for the bug. My suspicion is that we call release_key too often, but I'm still searching

[mrocklin]

cc @gforsyth

[gforsyth]

I'm closing out this PR as 7fa4a32 was merged into master as a part of #4206 -- thanks for the fix, @fjetter !