While upgrading my Debian machine from jessie to stretch, offlineimap was upgraded from 6.3.4 to 6.6.1 and it had a unexpected effect in my case.

Since the time of this writing, OfflineIMAP v6.7.0 learned the CLI option --migrate-fmd5-using-nametrans. You should use this feature for your updates!

For versions from 6.3.5 up to 6.7.0, check out if your distribution maintainer has backported this feature for you: offlineimap -h.

Initial configuration and decision to upgrade

I saw the big warning in the Debian NEWS file about the “nametrans” change:

  Reverse nametrans

    Starting with v6.4.0, OfflineImap supports the creation of folders
        on the remote repostory. This change means that people that
        only had a nametrans option on the remote repository
        (everyone) will need to have a nametrans setting on the local
        repository too that will reverse the name transformation.

    For further information, see documentation about this feature at:


But after reading carefully the linked documentation my configuration seemed to be Ok. To be clear my configuration includes the following snippets.

For Local repositories:

    folderfilter = lambda foldername: foldername.startswith("localprefix.")
    nametrans = lambda foldername: trans_local(foldername, "localprefix")

For Remote repositories:

    nametrans = lambda foldername: trans_remote(foldername, "localprefix")

which use those functions

    def trans_local(foldername, prefix):
        if foldername.startswith(prefix):
            return foldername.replace(prefix+".","",1)
            #raise AssertionError("config of nametrans with prefix '"+prefix+"' is not good")
            return "notfound_"+prefix+"_"+foldername

    def trans_remote(foldername, prefix):
           return prefix+"."+foldername

The unexpected effect

So far so good.

I upgrade, modify the completely unrelated ssl parameters, and just wait for my automatic mail process (I have offlineimap in a crontab) to continue business as usual.

And I wait, and I wait, and after some time I just wonder what the hell offlineimap is doing. And I found out that it was completely re-uploading all my mails. I have some accounts with more than 5G of mails and with the crappy connexion I had, it took forever.

I stopped offlineimap, deactivated all my crontabs and investigated the problem.

I think that the problem comes from the use of nametrans in visiblename for Maildir folder (commit Apply nametrans to all Foldertypes) because it leads to change the FMD5 part of the filename calculated by offlineimap.

Offlineimap thus believed that I removed all my mails and added it back. With no way for it to detect it was the same mails. It then reuploaded all the “new” mail before deleting all the “old” mail which is really inefficient.

My problem was exactly the inverse of the one in this blog post.

How I avoided to do some reupload

Once the problem was understood I came up with a workaround which seems to work. The principle is to rename all filenames with the new expected name (with the new calculated md5sum) by offlineimap in concerned Maildirs.

With my setup I used this dirty hack in shell:

    for box in $HOME/Maildir/${MYPREFIX}.* ; do
        cd $box
        oldmd5=$(pwd | sed 's!.*/!!' | tr -d '\n' | md5sum | sed 's/ *-$//')
        newmd5=$(pwd | sed "s!.*/!!;s!^${MYPREFIX}\.!!" | tr -d '\n' | md5sum | sed 's/ *-$//')
        for dir in cur new ; do
            rename -v "s/FMD5=${oldmd5}/FMD5=${newmd5}/" ${dir}/* ;

Note that this only works if done after the last use of offlineimap 6.3.4 and before the first use of offlineimap 6.6.1. If done at other moments I think we risk loss of mails and duplicates of mails.

I opened one Debian bug on this matter and wait for any idea to work up this upgrade cleanly.