== Our low-rent approach to verifying that NFS mounts are there

[[Our mail system CurrentMailerConfiguration]] has everyone's inboxes
in an old-fashioned _/var/mail_ style single directory; in fact it
literally is _/var/mail_. This directory is NFS mounted from one
of [[our fileservers ../solaris/ZFSFileserverSetupII]], which raises
a little question: how can we be sure that it's actually there?
Well, there's always going to be a _/var/mail_ directory. But what
we care about is that this directory is the actual NFS mounted
filesystem instead of the directory on the local root filesystem
that is the mount point, because we very much do not want to ever
deliver email to the latter.

(Some people may say that limited directory permissions on the mount
point should make delivery attempts fail. 'Should' is not a word
that I like in this situation, either in 'should fail' or 'that
failure should be retried'.)

There are probably lots of clever solutions to this problem involving
advanced tricks like embedded Perl bits in the mailer that look at
NFS mount state and so on. We opted for a simple and low tech
approach: we have a magic flag file in the NFS version of _/var/mail_,
imaginatively called _.NFS-MOUNTED_. If the flag file is not present,
we assume that the filesystem is not mounted and stall all email
delivery to _/var/mail_.

This scheme is subject to various potential issues (like accidentally
deleting _.NFS-MOUNTED_ some day), but it has the great virtue that
it is simple and relatively bulletproof. It helps that Exim has
robust support for checking whether or not a file exists (although
[[we use a hack EximDeferRouters]] for various reasons). The whole
thing has worked well and basically transparently, and we haven't
removed one those _.NFS-MOUNTED_ files by accident yet.

(We actually use this trick for several NFS-mounted mail related
directories that we need to verify are present before we start
trying to do things involving them, not just _/var/mail_.)

(I mentioned this trick in passing [[here EximDeferRouters]],
but today I feel like writing it up explicitly.)

=== Sidebar: our alternate approach with user home directories

Since user home directories are NFS mounted, you might be wondering
if we also use flag files there to verify that the NFS mounts are
present before checking things like _.forward_ files. Because of
how our NFS mounts are organized, we use an alternate approach
instead. In short, our NFS mounts aren't directly for user home
directories; instead they're for filesystems with user home directories
in them.

(A user has a home directory like _/h/281/cks_, where _/h/281_ is
the actual NFS mounted filesystem.)

In this situation it suffices to just check that the user's home
directory exists. If it does, the NFS filesystem it is in must be
mounted (well, unless someone has done something very perverse).
As a useful side bonus, this guards against various other errors
(eg, 'user home directory was listed wrong in _/etc/passwd_').