Linux 3.11 introduced O_TMPFILE as a flag to open() sys call. This would
enable users to get a fd to an unnamed temporary file. As it's unnamed,
it does not require the caller to devise unique names. It is also not
accessible through any path. Hence, file creation is race-free.
This file is initially unreachable. It is then populated with data(write),
metadata(fsetxattr) and fsync'd before being atomically linked into the
filesystem in a fully formed state using linkat() sys call. Only after a
successful linkat() will the object file will be available for reference.
Caveats
* Unlike os.rename(), linkat() cannot overwrite destination path if it
already exists. If path exists, we unlink and try again.
* XFS support for O_TMPFILE was only added in Linux 3.15.
* If client disconnects during object upload, although there is no
incomplete/stale file on disk, the object directory would persist
and is not cleaned up immediately.
Change-Id: I8402439fab3aba5d7af449b5e465f89332f606ec
Signed-off-by: Prashanth Pai <ppai@redhat.com>
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