Provide configuration for the optional creation of buildtreestpollard/896

Artifacts can be cached explicitly with an empty `build tree` when
built via the cli main options or user config for all or only
successful build artifacts. Default behaviour is to still create
and cache all expected buildtrees.

element.py: _cache_artifact() Check if context for cache_buildtrees
has been set to always or failure with a corresponding build
result, if not skip attempting to export the build-root. Element
types without a build-root are cached with an empty buildtree
regardless. Update _stage_sources_at() to warn the user that the
buildtree import is empty.

tests/integration: Add test to artifact.py for the optional caching
of buildtree content from bst build. Rename build-tree.py to
shellbuildtrees.py to reflect included test cases, add test for
empty buildtree warning and failure option.

NEWS: Add entry for new option.

1 files changed, 8 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 50d136886..9876210c3 100644
--- a/NEWS
+++ b/NEWS
@@ -126,6 +126,14 @@ buildstream 1.3.1
     Providing a remote will limit build's pull/push remote actions to the given
     remote specifically, ignoring those defined via user or project configuration.
 
+  o Artifacts can now be cached explicitly with an empty `build tree` when built.
+    Element types without a build-root were already cached with an empty build tree
+    directory, this can now be extended to all or successful artifacts to save on cache
+    overheads. The cli main option '--cache-buildtrees' or the user configuration cache
+    group option 'cache-buildtrees' can be set as 'always', 'failure' or 'never', with
+    the default being always. Note, as the cache-key for the artifact is independant of
+    the cached build tree input it will remain unaltered, however the availbility of the
+    build tree content may differ.
 
 =================
 buildstream 1.1.5