BuildStream is now completely broken on MacOS

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1408)
In GitLab by [[Gitlab user @cs-shadow]](https://gitlab.com/cs-shadow) on Nov 5, 2020, 19:55

## Summary

[//]: # (Summarize the bug encountered concisely)

Last time I was using BuildStream on Mac (a good few months ago now) I was able of running most (if not all) non-integration tests locally. And the CLI also used to work fine for the most part. Of course, anything that needed a real sandbox wouldn't work, but `import` elements etc would work fine.

It seems like now, things are completely broken and a `bst build` on our first example project results in a failure with a BUG message.

## Steps to reproduce

pre-requisites:

1. Find a MacOS machine
2. Clone and install BuildStream master

then:

```bash
# this works fine
$ bst -C doc/examples/first-project show
...
buildable b7a8b07d97bafb0ac1029d7f6164a49b636b1dcea34efa89ca5eb951aeeef401 hello.bst

# build fails miserably, pages and pages of the same error is repeated
$ bst -C doc/examples/first-project build
...
[--:--:--][b7a8b07d][ main:hello.bst ] STATUS Fetch terminating
[--:--:--][ ][ main:core activity ] BUG 'NoneType' object has no attribute 'terminate'

Traceback (most recent call last):
File "/usr/local/Cellar/python[[Gitlab user @3]](https://gitlab.com/3).8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/Users/enigma/git/buildstream/buildstream/src/buildstream/_scheduler/scheduler.py", line 496, in _terminate_jobs_real
job.terminate()
File "/Users/enigma/git/buildstream/buildstream/src/buildstream/_scheduler/jobs/job.py", line 216, in terminate
self._process.terminate()
File "/usr/local/Cellar/python[[Gitlab user @3]](https://gitlab.com/3).8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 133, in terminate
self._popen.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'

[--:--:--][b7a8b07d][ main:hello.bst ] STATUS Fetch terminating
[--:--:--][ ][ main:core activity ] BUG 'NoneType' object has no attribute 'terminate'

Traceback (most recent call last):
File "/usr/local/Cellar/python[[Gitlab user @3]](https://gitlab.com/3).8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/Users/enigma/git/buildstream/buildstream/src/buildstream/_scheduler/scheduler.py", line 496, in _terminate_jobs_real
job.terminate()
File "/Users/enigma/git/buildstream/buildstream/src/buildstream/_scheduler/jobs/job.py", line 216, in terminate
self._process.terminate()
File "/usr/local/Cellar/python[[Gitlab user @3]](https://gitlab.com/3).8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/multiprocessing/process.py", line 133, in terminate
self._popen.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'

```

## What is the current bug behavior?

Pages and pages of bug messages repeated over and over again, that is even hard to ctrl-c.

## What is the expected correct behavior?

Ideally, we'd get back to the previous behavior of being able to build elements that don't require a sandbox.

---

I'm not sure when this started happening but is at least an issue since 9f205147b5461cc29486f2c1830e88913a8d098e (which is what I tried today).

Reproduce with `bst -C doc/examples/first-project build` on macOS, then start at `buildstream/src/buildstream/_scheduler/scheduler.py` in `_terminate_jobs_real` and `buildstream/src/buildstream/_scheduler/jobs/job.py` in `terminate`, as shown by the traceback. Done means the first-project build no longer emits repeated `'NoneType' object has no attribute 'terminate'` BUG messages and non-sandbox elements work as expected.

Written by the indexing model from the issue text.