Synchronise trees

[manchmal]

It would be nice if rsync has the abiltiy to sync a directory full of
files with another directory full of files - but only to update the
local directory once the sync had finished.

Example:
Say you run a mirror of the fedora rawhide rpm repository, and you run
an rsync with the master during the time someone is downloading the
rpms.
You want to make sure that if someone runs an update from you, they
don't end up with half the rpms pre-rsync, and half from post-rsync.

Perhaps rsync has this already, in which case - sorry about that!
I realise this could be done with a clever shell script, but it's the
built in ability to do this that would make it good :)