BuildStream tries to push incomplete artifacts and fails

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1239)
In GitLab by [[Gitlab user @BenjaminSchubert]](https://gitlab.com/BenjaminSchubert) on Dec 12, 2019, 16:25

## Summary

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

We have a scenario where we do:

```bash
bst --pushers 0 build my-stack.bst
bst push --deps all my-stack.bst
```

The second command sometimes fails pushing because it is missing some files in the stack.
The last time we investigated, it turned out that the build logs of the stack were missing.

We had however more than 500GB space free for the CAS.

## Steps to reproduce

[//]: # (How one can reproduce the issue - this is very important)

Build a big project then push it? We don't know more than that.

## What is the current bug behavior?

[//]: # (What actually happens)

The push will fail to push the stack (all other items are fine), as it is missing some files in CAS.

## What is the expected correct behavior?

[//]: # (What you should see instead)

We should either:
1) Not push the stack since it should not be 'cached'
2) Not fail and push without the logs?

----

Start by reproducing the two commands shown with a large project and inspect the stack's CAS contents, especially its build logs. Compare the push behavior for the stack with the expected handling of incomplete artifacts; the issue needs a confirmed reproduction and a decided expected outcome before implementation can be considered done.

Written by the indexing model from the issue text.