New Fedora 22 Change proposal: systemd-sysusers

[Simo Sorce]

On Fri, 2014-07-11 at 12:52 +0200, Lennart Poettering wrote:
> 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 mtime is out of date it needs to ping the server, to sync on.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...

This means stating for every single getpwnam() or getgrnam(), etc ...
but yes I am aware of the corner cases (I already wrote an email to
sssd-devel@ highlighting another, more important one, this morning), and
we'll experiment a bit to find the best course of action.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York