Crash due to AttributeError when using remote execution

Summary

I experienced a build failure due to and AttributeError, shown below, when using remote execution and having an artifact server defined.

Steps to reproduce

I saw the failure when doing a remote execution build of this element with my project.conf contaning the lines shown below. I only saw this error when the artifact cache was defined, if commented out this didn't occur.

What is the current bug behavior?

Buildstream crashes with an AttributeError as seen below.

What is the expected correct behavior?

Buildstream shouldn't crash.

Relevant logs and/or screenshots

    An unhandled exception occured:

    Traceback (most recent call last):
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_scheduler/jobs/job.py", line 709, in child_action
        result = self.child_process()  # pylint: disable=assignment-from-no-return
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 103, in child_process
        return self._action_cb(self._element)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 93, in _assemble_element
        return element._assemble()
      File "/usr/local/lib64/python3.7/site-packages/buildstream/element.py", line 1572, in _assemble
        collect = self.assemble(sandbox)  # pylint: disable=assignment-from-no-return
      File "/usr/lib64/python3.7/contextlib.py", line 119, in __exit__
        next(self.gen)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/sandbox/sandbox.py", line 328, in batch
        batch.execute()
      File "/usr/local/lib64/python3.7/site-packages/buildstream/sandbox/_sandboxreapi.py", line 236, in execute
        if self.sandbox.run(["sh", "-c", "-e", self.script], self.flags, cwd=first.cwd, env=first.env) != 0:
      File "/usr/local/lib64/python3.7/site-packages/buildstream/sandbox/sandbox.py", line 282, in run
        return self._run(command, flags, cwd=cwd, env=env)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/sandbox/_sandboxreapi.py", line 106, in _run
        action_result = self._execute_action(action, flags)  # pylint: disable=assignment-from-no-return
      File "/usr/local/lib64/python3.7/site-packages/buildstream/sandbox/_sandboxremote.py", line 330, in _execute_action
        artifactcache.fetch_missing_blobs(project, local_missing_blobs)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_artifactcache.py", line 408, in fetch_missing_blobs
        missing_blobs = self.cas.fetch_blobs(remote, missing_blobs, allow_partial=True)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_cas/cascache.py", line 670, in fetch_blobs
        batch.add(digest)
      File "/usr/local/lib64/python3.7/site-packages/buildstream/_cas/casremote.py", line 103, in add
        self._request.instance_name = self._remote.local_cas_instance_name
    AttributeError: 'ArtifactRemote' object has no attribute 'local_cas_instance_name'

artifacts:
  url: https://public.aws.celduin.co.uk

remote-execution:
  execution-service:
    url: https://push.cache.com:30443
    instance-name: remote-execution
    server-cert: server.crt
    client-cert: push.crt
    client-key: push.key
  storage-service:
    url: https://push.cache.com:30443
    instance-name: remote-execution
    server-cert: server.crt
    client-cert: push.crt
    client-key: push.key
  action-cache-service:
    url: https://push.cache.com:30443
    instance-name: remote-execution
    server-cert: server.crt
    client-cert: push.crt
    client-key: push.key

Possible fixes

Other relevant information

• BuildStream version affected: This happened in a container built from the nightly docker image, the latest buildstream commit at the time of building was: 99063c73

/src> bst --version
1.93.3+76.g99063c73

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