Dsync deleting mailboxes due to duplicate UIDs

[Andreas Thienemann]

Hi Sami,

On Mon, 10 Sep 2018, Sami Ketola wrote:

> Currently dovecot does rely on remote to send valid RFC-compliant 
> headers. But you are not alone, we have seen similar problems with mixed 
> header fields on some legacy servers on the migrations we have 
> performed.

Bummer. Would have been _very_ cool if doveadm could just disregard a 
second copy of a header sent.

> I have found dsync_hashed_headers setting as a good workaround for 
> migrating mails from broken imap servers. As per RFC Message-ID should 
> be unique and we have safely used setting 
> dsync_hashed_headers=Message-ID to migrate the problematic users away 
> from the broken servers.

There's a certain irony in hoping that Message-ID headers are RFC 
compliant while we're talking to a non-RFC compliant mailserver. ;-)

That being said, limiting dsync_hashed_headers to only look at the 
Message-ID _does_ seem to do the trick.

But just to clarify: As far as I read the sourecode, the logic is as 
follows:

Fetch UID on remote, hash headers.
Fetch UID on local, hash headers.
Compare that header hashes match _for this one UID_. If yes, good. If not, 
something has changed.

This means there's no comparing the hashed headers to other UIDs going on?
e.g. someone having two copies of the same mail with the same Message-ID 
would _not_ lead to an error, as these are saved under different UIDs.

Is my understanding of the code correct?

cheers,
  Andreas