rsync to sync time without attempting to modify the content

[Brice Rebsamen]

Hello

I am setting up a central data repository for my team (several thousands 
of files, totaling about 4TB). There are multiple sources that I need to 
consolidate: a source may have a fraction of the total number of file, 
and there can be conflicts between different sources (that should be 
very occasional though). I want to detect those conflicts and manually 
merge them. Also, the timestamps of the different copies of the files 
can be different as some users have copied the same files to different 
locations and changed their mtime (because by default cp does not 
preserve timestamps...).

So what I want, is to be able to compare files this way:

if the file does not exist at the destination, then transfer it with 
timestamps (rsync -t)
otherwise:
     if timestamps and sizes are different:
         if md5sums match
             if the source time stamp is earlier than the destination 
timestamp
                 update the timestamp of the destination
         otherwise report (in a log file or something so that I can come 
back to those later)

I tried writing a script to do it, but it turned out to be tedious and I 
have the feeling that I am reinventing the wheel here because I spent a 
lot of time just parsing the paths on the command line the way rsync 
does it. I have the feeling that rsync could do this for me, but I could 
not find, in the man page, the option to prevent modifying the content 
of the remote file, but allow to update the timestamps. I was thinking 
that using rsync in several passes could do the job: a pass in dry run 
to compare files, put the files with differences in several lists 
according to whether their md5sum matches or based on timestamp 
differences, then process those files again with rsync with appropriate 
options. But how to get rsync to give me info why files are different?

If that turns out to be too tough, I think I can do it with Unison, it's 
just going to take a lot of time to compute all the md5sums and review 
all the files for changes...

Hope it makes sense
Brice