Broken uidlist when using NFS on newer kernels

Jeremy Hanmer jhanmer at
Tue Oct 12 21:45:03 EEST 2021

I looked into LMTP, but reconfiguring our 1.5 million mailboxes just to
work around what seems like an obvious bug in the code is a hard sell. I
patched maildir-uidlist.c to strip out the leading null bytes and things
seem to behave just fine, but it feels wrong and I was hoping to get input
from someone more familiar with the codebase.

On Tue, Oct 12, 2021 at 8:39 AM Alessio Cecchi <alessio at> wrote:

> Hi Jeremy,
> I had the same problem as you.
> We run an email hosting service with Maildir on NetApp NFS, Dovecot
> Director and Backend servers for POP/IMAP and messagges deliverd via
> dovecot-lda by MXs. After the upgrade from CentOS 6 to CentOS 7 I found the
> same issue as you (on dovecot-uidlist).
> After many tests we decided to switch from LDA to LMTP, that was already
> in our roadmap, so read and delivery of messagges is always on the same
> backend. And the problem was solved.
> I haven't found any others workarounds.
> Swith from LDA to LMTP was not so simple for us since our MX wasn't able
> to talk LMTP but we have write some custom C++ code and was done. You
> should also consider to add some directors since also incoming emails will
> transit from it.
> If you would like to talk about how we solve on MXs side I will happy to
> talk with you.
> Ciao
> Il 08/10/21 21:01, Jeremy Hanmer ha scritto:
> I know this has been reported in the past, but I think I have some useful
> new information on the problem. After an OS upgrade from Ubuntu Xenial
> (4.4.0 kernel) to Ubuntu Focal (5.4.0 kernel) and corresponding upgrade
> from Dovecot 2.2.27 to, we've started seeing broken uidlist files
> to an extent that's making larger mail boxes nearly unusable because the
> file is constantly being regenerated. I've also used the
> 2.3.16-2+ubuntu20.04 version distributed from and the
> behavior is unchanged. The environment consists of NFS mounts from a NetApp
> device, with a couple dozen MX servers receiving mail and about a hundred
> IMAP/POP servers.
> This is the exact error (note the blank after "invalid data"):
> Error: Mailbox INBOX: Broken file
> /mnt/morty/morty2/gravest/x15775549/Maildir/dovecot-uidlist line 373:
> Invalid data:
> I've been able to trigger the problem rather easily by piping an email to
> dovecot-lda in a loop and reading the resulting dovecot-uidlist file on a
> different server. What it shows is that occasionally we're seeing the last
> line of the file prepended with a number of null bytes equal to the line
> that's being written (for example, if the entry is "35322
> :1633719038.M516419P3623238.pdx1-sub0-mail-mx202,S=2777,W=2832", we'll have
> it prepended by 69 null bytes). This then breaks the IMAP process' ability
> to read the file. My first thought was to extend the retry functionality so
> the imap proces makes more attempts to read the file when it detects a
> problem like this, but would love input from someone more familiar with the
> codebase.
> --
> Alessio Cecchi
> Postmaster @ http://www.qboxmail.it
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