Crash on remote failure try

Summary

23:48:13][--:--:--][a78252e5][    main:bootstrap/build/coreutils-manpages.bst] STATUS  Push terminating
[23:48:13][--:--:--][        ][    main:core activity                 ] BUG     'NoneType' object has no attribute 'terminate'

    Traceback (most recent call last):
      File "/usr/lib64/python3.7/asyncio/events.py", line 88, in _run
        self._context.run(self._callback, *self._args)
      File "/home/will/projects/buildstream/buildstream/src/buildstream/_scheduler/scheduler.py", line 562, in _terminate_jobs_real
        job.terminate()
      File "/home/will/projects/buildstream/buildstream/src/buildstream/_scheduler/jobs/job.py", line 217, in terminate
        self._process.terminate()
    AttributeError: 'NoneType' object has no attribute 'terminate'

[23:48:13][--:--:--][a78252e5][    main:bootstrap/build/coreutils-manpages.bst] WARNING Push did not terminate gracefully, killing
[23:48:13][--:--:--][        ][    main:core activity                 ] BUG     'NoneType' object has no attribute 'pid'

    Traceback (most recent call last):
      File "/usr/lib64/python3.7/asyncio/events.py", line 88, in _run
        self._context.run(self._callback, *self._args)
      File "/home/will/projects/buildstream/buildstream/src/buildstream/_scheduler/scheduler.py", line 556, in kill_jobs
        job_.kill()
      File "/home/will/projects/buildstream/buildstream/src/buildstream/_scheduler/jobs/job.py", line 238, in kill
        utils._kill_process_tree(self._process.pid)
    AttributeError: 'NoneType' object has no attribute 'pid'

Steps to reproduce

Cross build freedesktop-sdk, bst -o target_arch aarch64 build bootstrap/bootstrap.bst if it fails to build then pic retry.

What is the current bug behavior?

Crash so that Ctl+C will not stop bst, The hole scroll back is this error over and over again.

What is the expected correct behavior?

tetry to build.

Relevant logs and/or screenshots

running bst branch willsalmon/expandsan

diff --git a/project.conf b/project.conf
index d94b59dc..61c74308 100644
--- a/project.conf
+++ b/project.conf
@@ -74,8 +74,6 @@ plugins:
 options:
   (@): include/_private/options.yml
 
-artifacts:
-  url: https://public.aws.celduin.co.uk
 
 elements:
   cmake:
@@ -92,3 +90,9 @@ elements:
 sources:
   git_tag:
     (@): include/_private/git_tag-conf.yml
+
+
+sandbox:
+  build-arch: '%{arch}'
+
+