BuildStream tries to push incomplete artifacts and fails

Summary

We have a scenario where we do:

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

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

What is the current bug behavior?

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?

We should either:

1. Not push the stack since it should not be 'cached'
2. Not fail and push without the logs?

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