This causes an extra HEAD request for each remote file, which greatly
slows down execution, and increases monetary cost $(0.01/10000
requests), but guarantees files whose mtime has changed will get
This is necessary for yum repositories, where repodata/* files may be
updated but not change size. It also correctly handles large files
whose md5 values as returned by S3 are incorrect having their content
(and thus mtime) changed, perhaps by RPM signing.