[Dovecot] Bug in HIGHESTMODSEQ when MODSEQ tracking is not enabled for version 2.0.18.

[Glenn Wurster]

Greetings,

I'm new to the list.  I have encountered what I believe to be a bug in the return of HIGHESTMODSEQ when talking to a IMAP client.  HIGHESTMODSEQ does not appear to be incrementing when a new message arrives.  The exact steps I took are:

1) Create a new mail account
2) Send a mail to the account, check that you can receive the email.  Send a second mail to the account.
3) perform a SELECT on the mailbox from a new session.

PMLK4 SELECT INBOX
* FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
* OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)] Flags permitted.
* 2 EXISTS
* 1 RECENT
* OK [UNSEEN 1] First unseen.
* OK [UIDVALIDITY 1334074920] UIDs valid
* OK [UIDNEXT 3] Predicted next UID
* OK [HIGHESTMODSEQ 1] Highest
PMLK4 OK [READ-WRITE] Select completed.

4) Retrieve the mail.  Then perform another SELECT on the mailbox from yet another session.

LLEP4 SELECT INBOX
* FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
* OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)] Flags permitted.
* 2 EXISTS
* 0 RECENT
* OK [UNSEEN 1] First unseen.
* OK [UIDVALIDITY 1334074920] UIDs valid
* OK [UIDNEXT 3] Predicted next UID
* OK [HIGHESTMODSEQ 1] Highest
LLEP4 OK [READ-WRITE] Select completed.

5) Send another mail to the account, and check SELECT again.

FKDA4 SELECT INBOX
* FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
* OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)] Flags permitted.
* 3 EXISTS
* 1 RECENT
* OK [UNSEEN 1] First unseen.
* OK [UIDVALIDITY 1334074920] UIDs valid
* OK [UIDNEXT 4] Predicted next UID
* OK [HIGHESTMODSEQ 1] Highest
FKDA4 OK [READ-WRITE] Select completed.

It appears as if HIGHESTMODSEQ is not being updated.  I can get HIGHESTMODSEQ to start updating correctly if I send a "UID FETCH 1 MODSEQ" or similar command, which appears to enable MODSEQ tracking at the server (according to the comment around line 173 in file src/lib-index/mail-index-modseq.c), but until that command is sent, MODSEQ tracking is not enabled and hence HIGHESTMODSEQ is always going to return 1.

According to RFC4551, the combination of HIGHESTMODSEQ and UIDVALIDITY should be sufficient to determine if the metadata associated with the mailbox has changed, but in this case looking at only those two parameters does not yield sufficient information about changes in the mailbox.  The mail client I'm using relies on the combination of HIGHESTMODSEQ and UIDVALIDITY to determine if there are changes in the mailbox, and hence does not see new mail come in.

It seems that Dovecot should not be returning HIGHESTMODSEQ in response to a command if MODSEQ tracking is not enabled, but I could be wrong.  I've attached my configuration (it's Dovecot 2.0.18 running on Debian Stable).

Let me know if you need more information.

Glenn.

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