[BuildStream] bst master no longer downloads artifact 'build trees' by default

[Tom Pollard <tom pollard codethink co uk>]

Hi all,

I just wanted to point out a default behavioral change that has recently
landed in BuildStream master.

Taking the entry from NEWS:

"Due to the element `build tree` being cached in the respective artifact
their size in some cases has significantly increased. In *most* cases
the build trees are not utilised when building targets, as such by
default bst 'pull' & 'build' will not fetch build trees from remotes.
This behavior can be overridden with the cli main option
'--pull-buildtrees', or the user configuration cache group option
'pull-buildtrees = True'. The override will also add the build tree to
already cached artifacts. When attempting to populate an artifactcache
server with cached artifacts, only 'complete' elements can be pushed. If
the element is expected to have a populated build tree then it must be
cached before pushing."

Following this landing there will be follow up issues to finish closing
the issue which raised it [2] along with those to track the work needed
in other areas of the codebase which assume or require a complete
artifact or buildtree dir to be present.

Cheers,

Tom

[1]
https://gitlab.com/BuildStream/buildstream/commit/0d309478f16e5c8f53c2e066b078dfd1587801aa
[2] https://gitlab.com/BuildStream/buildstream/issues/494
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