Staging stat'ed an artifact's path, read the path, and stat'ed it again.
When the path was a symbolic link switched to another file of the same
size during the read and back before the second stat, both identities
were those of the verified file, so the bytes of the other file were
staged without hashing. Staging now opens the artifact once and takes
both identities from the open file with fstat, so they describe the
file the bytes come from.
A test switches the artifact's link to a same-sized file during the read
and checks that the staged copy has the verified bytes.
Chris Tate
1d579ea5a2