BUG: Message handling out of sync

Summary

Here is an example of error I get in the console while running Buildstream 1.1.5. I got several of those.

Process Process-571:
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python3.6/multiprocessing/process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "/home/valentin/.local/lib/python3.6/site-packages/BuildStream-1.1.5-py3.6.egg/buildstream/_scheduler/jobs/job.py", line 437, in _child_action
    self._queue.put(Envelope('child_data', self.child_process_data()))
  File "/home/valentin/.local/lib/python3.6/site-packages/BuildStream-1.1.5-py3.6.egg/buildstream/_scheduler/jobs/elementjob.py", line 113, in child_process_data
    cache_size = self._element._get_artifact_cache().calculate_cache_size()
  File "/home/valentin/.local/lib/python3.6/site-packages/BuildStream-1.1.5-py3.6.egg/buildstream/_artifactcache/cascache.py", line 463, in calculate_cache_size
    self.cache_size = utils._get_dir_size(self.casdir)
  File "/home/valentin/.local/lib/python3.6/site-packages/BuildStream-1.1.5-py3.6.egg/buildstream/utils.py", line 569, in _get_dir_size
    return get_size(path)
  File "/home/valentin/.local/lib/python3.6/site-packages/BuildStream-1.1.5-py3.6.egg/buildstream/utils.py", line 565, in get_size



BUG: Message handling out of sync, unable to retrieve failure message for element flatpak_image element at platform-arch-libs-debug.bst [line 1 column 0]

I think this happens when I run several builds in the same time on the same machine. Though I am not totally sure.

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