[Dovecot] Re: Small change to make dovecot pop3uw-imapmigrationfriendly

[Brian Candler]

On Tue, Jun 01, 2004 at 12:06:49AM +0200, Matthias Andree wrote:
> > I don't believe they have any permission to assume this. If the server
> > happens to maintain this ordering, well, the client could attempt some sort
> > of dubious binary-chop to work out where new mail has arrived (since some or
> > all messages may have been deleted)
> 
> Yup, given the lack of a range for UIDL, this is necessary.

I wouldn't say "necessary" - I'd say "a possible optimisation not sanctioned
by the RFC"

But anyway, I'm sure you'd agree that clients are not *required* to perform
this optimisation. So, some (many?) clients do ask for a complete UIDL
listing every time they connect. If there are 1000 old messages in the
mailbox, then you definitely don't want to open and read every one to find
its X-POP3-UID header. Equally, clients may ask for a complete LIST of the
maildrop. Again, you don't want to open and read every message file just to
calculate its size.

courier-imap used to do this, and the load was enormous from clients which
left mail on the server. Once it introduced a cache file
(courierpop3dsizelist), this problem went away. (This file also stores the
UID; previously it had used the Maildir filename as UID, but this turned out
to be too long for some clients)

I'm not trying to be pro-courier-imap here, by the way. I'd be happy to have
a path to bail out; but there are some key features that I need to have
first. One is Maildir++ quotas, and another is efficient POP3 operation in
the presence of clients who leave mail on server, and issue a full maildrop
UIDL and/or LIST each time they connect. A mechanism for transparent
migration of mailbox contents from a remote POP3 server is another. If I can
preserve the original UID of each message when migrating mail, then this
will be a bonus which courier-imap doesn't give me.

Oh, and perhaps most important of all, I need robustness. The pop3/imap
server should not die and drop the connection, say because a message within
the maildrop is mailformed in some way.

Regards,

Brian.