flags not synced correctly with dovecot sync (dsync)

[Dan Christensen]

I'm running dovecot 2.3.4.1 from https://repo.dovecot.org/ on Ubuntu
18.04 on three machines that I'll call server, laptop1 and laptop2.

Both laptop1 and laptop2 run dovecot sync against server to keep local
copies of my imap folders.  Even when I initially had only two machines,
laptop1 and server, I occasionally noticed that flags were lost, usually
custom flags used by Gnus, but I couldn't reliably reproduce the
problem.

Now that I have two laptops syncing against the server, the problem has
gotten worse and I figured out a way to reproduce it:

- on server: create new IMAP folder test, and put two read messages in it
- on laptop1:  doveadm sync -u user -l 10 -m test -f user at server
- on laptop2:  doveadm sync -u user -l 10 -m test -f user at server

At this point, all three machines show the two messages M1 and M2
as being read.

- on laptop1: mark message M1 unread
- on laptop2: mark message M2 unread
- on laptop1:  doveadm sync -u user -l 10 -m test -f user at server
  Both laptop1 and server have M1 unread, M2 read, as expected.
- on laptop2:  doveadm sync -u user -l 10 -m test -f user at server
  Now laptop2 and server have M1 *read*, M2 unread.
- on laptop1:  doveadm sync -u user -l 10 -m test -f user at server
  Now laptop1 and the server have both M1 and M2 *read*.
- on laptop2:  doveadm sync -u user -l 10 -m test -f user at server
  Now laptop2 has both read as well.

The two lines that say "*read*" are wrong in my opinion.  dsync
propagated a read mark to an unread message, even though that message
was marked unread more recently than it was marked read.

I usually use stateful sync, and get many related problems.
I just did a test in which M1 and M2 started out read, and I
started with empty files named dstate.test on laptop1 and laptop2.
Then I did the above procedure, using the command

doveadm sync -u user -l 10 -m test -s "`cat dstate.test`" user at server > dstate.test

At the end, laptop2 and server had both messages unread (which is good),
but laptop1 had only M1 unread, and repeated runs of the sync command
did not correct this.  So the stateful sync failed to detect a change.

Are these bugs in dovecot?  Is there more information that I can
provide?  The output of doveconf -n on one machine is below, and
the others are almost identical.

Thanks for any help!

Dan

# 2.3.4.1 (3c0b8769e): /etc/dovecot/dovecot.conf
# OS: Linux 4.15.0-45-generic x86_64 Ubuntu 18.04.1 LTS 
# Hostname: laptop2
auth_mechanisms = plain login
listen = 127.0.0.1
mail_index_log2_max_age = 10 days
mail_index_log_rotate_min_age = 1 days
mail_index_log_rotate_min_size = 300 k
mail_location = maildir:~/Maildir
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = scheme=CRYPT username_format=%u /etc/dovecot/users
  driver = passwd-file
}
protocols = imap
service imap-login {
  inet_listener imap {
    address = *
    port = 143
  }
  inet_listener imaps {
    address = *
    port = 943
    ssl = yes
  }
}
service imap {
  process_limit = 25
}
ssl_cert = </etc/dovecot/private/dovecot.pem
ssl_client_ca_dir = /etc/ssl/certs
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
  args = username_format=%u /etc/dovecot/users
  driver = passwd-file
}
protocol lda {
  postmaster_address = [elided]
}
protocol imap {
  mail_max_userip_connections = 20
}