Tom Pollard authored Mar 21, 2019 and bst-marge-bot committed Mar 29, 2019

By default the reachable set() includes all file digests from a
given reachable directory/tree digest, however this does not
guarantee the file objects are actually reachable.

testutils/artifactshare.py has_artifact() ascertained if a given
artifact is available by ensuring any 'reachable' digests are
actually on the filesystem, this is made redudant with the optional
check_exists objpath check during the reachable set creation.