New Fedora 22 Change proposal: systemd-sysusers

[Lennart Poettering]

On Fri, 11.07.14 05:41, Simo Sorce (simo at redhat.com) wrote:

> The reason why we *must* use a notification mechanism is that we
> maintain a very fast cache as a mmapped database to avoid roundtrips
> from applications, so we simply *do not* know when someone looks up data
> there. This means we need to proactively monitor /etc/passwd and
> invalidate/change data in the cache when it changes.

Well, if you do this asynchronously, then this will always be racy:
think of an admin who edits /etc/passwd, and immediately checks back if
the entry is there, with "getent passwd" or so. He will race against
your reload logic: sometimes he might be quicker, sometimes
sssd. Sometimes the new entry will exist sometimes it won't.

If you want to fix the race, then your client libs for the mmap
structure should probably have enough logic to compare the mtime of the
files with some timestamp in the mmap, before using the data. And if the
mtime is out of date it needs to ping the server, to sync on. But if you
do that, then there's really no need for inotify, again...

Lennart

-- 
Lennart Poettering, Red Hat