rsync - Nov 2013 - rsync with (f)locks

From the rsync manual documentation I see that by using the option 
rsync-path, it is possible to specify what program is to be run on the 
remote machine to start up rsync. In particular, the program could be a 
wrapper script which calls the actual rsync command in the middle, but 
which does some actions before and/or after the rsync invocation. One 
possible interesting use would be to acquire/release a lock (e.g., a 
flock), so that the operations of rsync at the remote end could be 
co-ordinated with another process which is contending for write access 
to the same files.

My question is: can you also put a wrapper around rsync when using the 
inetd (or xinetd) daemon approach to running rsync, by adding a line for 
inetd something like the following to /etc/inetd.conf (as per the 
rsyncd.conf man page):

rsync stream tcp nowait root /usr/bin/rsync rsyncd --daemon

but replacing /usr/bin/rsync with the path to your rsync-lookalike 
wrapper, which in this case would be a C/C++ -code program which seizes 
a lock, forks off rsync, waits for rsync to complete, then releases the 
lock.

Thanks

On Sat, Nov 23, 2013 at 11:36 AM, Thomas Jordan <tomjordan766 at
gmail.com>wrote:
> My question is: can you also put a wrapper around rsync when using the
> inetd (or xinetd) daemon approach to running rsync, by adding a line for
> inetd

Yes, that would work fine for an inetd approach.  For a daemon approach,
you could use a type of pid-lock mechanism using the "pre-xfer exec"
and
"post-xfer exec" scripts and the $RSYNC_PID env var.  The only thing
that
is weird about that is that the RSYNC_PID value is the pid of the pre-xfer
exec proc, not the pid of the transfer proc.  If you want to build in
automatic lock busting, you'd need to store both the $RSYNC_PID and the
pre-xfer func's parent-pid inside the lock file so that it can check for a
running process using the parent pid (and the post-xfer script can remove
the lock based on the $RSYNC_PID match).

..wayne..
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