[Dovecot] dsync deleted my mailbox - what did I do wrong?

[Jiri Bourek]

>> The last sentence describes what happened to you: all new mail on the
>> new machine is a "change" and was discarded (by deleting new mail.) If
>> I'm not mistaken, this is correct behaviour for backup mode - you get
>> exact copy of the source side (maildir:/tmp/mail_backup) on
>> destination side (dar at darklajid.de)
>
> That would be sort of okay. Except that isn't what happened:
>
> - The target mailbox was killed completely
> - Nothing was restored
>
> If what you're suggesting here is true I'd expect a clean copy of my
> source - even if it destroys all other changes. That did NOT happen
> though. It nuked the target and didn't restore a thing.

True - if we move from "problem is dsync deleted new mail" to "problem 
is dsync was unable to restore the backup", the described behaviour 
looks like a bug to me too. It may have something to do with the maildir 
format, I recall some discussion regarding folder INBOX, which needs 
special handling (because it's physically stored in maildir root, 
whereas every other folder is stored in folder-named subdirectory)

That said, I tried something along what you did and it failed for me 
too. So I deleted the mailbox completely, recreated it, tried again and 
this time the restore succeeded. It seems the easiest possible way to 
reproduce the faulty behaviour is:

1. create mailbox for testing, here test at example.com
2. create IMAP folder under INBOX
( namespace inbox { separator = / } )

# doveadm mailbox create -u test at example.com INBOX/test

3. attempt to restore from backup

# doveadm backup -u test at example.com -R maildir:/mnt/mail-backups/test/

which yields (on Dovecot 2.2.12)

dsync(test at example.com): Error: Mailbox INBOX sync: mailbox_delete 
failed: INBOX can't be deleted.

Another try shows that IMAP folder created somewhere else (not under 
INBOX) isn't a problem:

# doveadm mailbox create -u test at example.com testtest
# doveadm backup -u test at example.com -R maildir:/mnt/mail-backups/test/

This yields no output, folder testtest is deleted (as expected), INBOX 
is populated from backup.

Another try, this time I used mbox instead of maildir by specifying

-o mail_location=mbox:/path/test/mail

to doveadm. Worked without error even with INBOX/test folder (which got 
deleted during restore)

No idea if this can be considered as a bug, or the test does something 
that is not supposed to be done in the first place (Although different 
results with different storage format suggests a bug to me.)

>
> Plus, dsync mirror did exactly the same: Nuked the (live) mailbox once
> more, same error message, not a single message restored (but also no
> modification to the source).
>

I was doing some trial and error testing with doveadm sync (should be 
the same as dsync mirror.) If used on a mailbox which has seen some 
changes, this sync's behaviour is just strange.

Or - to be more precise - it seems strange on first sight, but when you 
think about it, it does what is supposed to do. The sync mode is (AFAIK) 
designed to keep single mailbox synchronized on two hosts. If you 
created new mailbox on the new host, then had some mail delivered to it 
and after some time decided to add mail from old host, then you don't 
have single mailbox - you have two mailboxes with the same name.

In other words this scenario is probably something dsync wasn't designed 
to be used for and there's no surprise mirror mode can't handle it.

And again - I'm no expert, so it's entirely possible everything I wrote 
here is complete and utter nonsense Let's hope someone more 
knowledgeable corrects me if that is the case.