Courier generates new UIDVALIDITY

[Timo Sirainen]

On 01/15/2016 03:34 PM, Peer Heinlein wrote:
> 
> We just detected, that under some circumstances Courier-IMAP will
> generate a new UIDVALIDITY for every access to an IMAP-folder, if the
> folder had been created by a simple "mkdir" and not with maildirmake.
> Courier doesn't save his uidvalidity-file in this case.
> 
> Courier DOES generate a sticky UIDVALIDITY after the first "real" access
> by a User through the IMAP-SELECT-command. That's why this problem isn't
> a "real" problem in the world. Courier does NOT create this
> IMAP-UIDVALIDITY if the user just uses POP3-access, because there the
> UIDVALIDITY isn't used.
> 
> We have a migration scenario where we have many users with POP3-only
> access (so that they DO have many mails in der INBOX) and an incremental
> migration trough doveadm/dsync/imapc fails (caused by consequently
> changing UIDVALIDITYs).
> 
> doveadm just uses the IMAP EXAMINE command to open the source
> IMAP-folder on the Courier's side readonly. Courier DOES handle this
> access "readonly" and doesn't consequently doesn't generate a
> UIDVALIDITY-file. Instead Courier uses random UIDVALIDITYS by every
> access and doveadm always has to copy all mails (and duplicates them if
> "-1 -R" is used!).
> 
> To fix this problem and to make a migration and/or doveadm backup/sync
> much more reliable Dovecot should open the source folder with a SELECT
> command instead.
> 
> (Maybe one SELECT and then change to EXAMINE, if you think EXAMINE would
> be more safe.)
> 
> Maybe this is also relevant for a real permanent imapc-situation, where
> Dovecot/imapc is used to fix broken IMAP-implementations from an older
> backend.

SELECT drops the \Recent flags, so I don't think this should ever be
done by default. But this should help:
https://github.com/dovecot/core/commit/df596e34b604e6ac873de9ca92fb5df2a5fed45f