rsync - Mar 2021 - --compare-dest seems to nullify --update

I'm  using rsync to update my local database snapshot from a remote pooled
mirror server.
The trouble is that the servers in the pool are sometimes out of date, so I only
want to update
when the files are newer than the local copies to avoid unnecessary I/O..


Originally I copied my existing database to a new directory, and updated that:
  cp -r <current> <new>
  rsync --update, <mirror> <new>
  mv <new> <current>
That worked fine, but then I noticed --inplace, and realized my copy to the new
directory should
be redundant.  So I tried:
  rsync --compare-dest=<current> --mkpath --update <mirror>
<new>
  [ -e <new/stuff> ] && mv <new> <current>
Which appeared to work, except it sometimes get backdated!  My current hack is
to change
the second line to
  [ <new/stuff> -nt <current/stuff> ] && mv <new>
<current>
but it's already wasted downloading the old stuff.


Exact reading of the docs says that --update compares source and target dates,
so I guess it
looks in the empty <new> rather than <current>.  I can see it's
justified, but unexpected.


(For the curious, the snapshot is a Gentoo portage tree, about 75 MB in a single
file, so it's
worth trying to avoid unnecessary IO.)
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> I'm  using rsync to update my local database snapshot from a remote
pooled
> mirror server. The trouble is that the servers in the pool are sometimes
> out of date, so I only want to update when the files are newer than the
> local copies to avoid unnecessary I/O..
> ...
I've found a workaround by using hard links.  Instead of:
	cp -r <current> <new>
	rsync --update, <mirror> <new>
	mv <new> <current>
I now:
	mkdir <new>
	ln -f <current>/* <new>
	rsync --update, <mirror> <new>
	if [ <new>.<stuff> -nt <current>.<stuff> ]
	then mv <current> <old> ; mv <new> <current>
	else rm -r <new>
	fi
Since the files in <new> are the same as in <current>, --update does
its job properly.
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