Robert J. Cups writes:
I realize that if I had X EXISTS cached somewhere I could do a fetch with that, but I didn't have convenient access to that value and capturing it would require additional processing, the overhead of which I wished to avoid.
All IMAP clients must keep track of the number of messages in the mailbox, if they want to perform any kind of a useful task. This is one of the basic IMAP tenets.
I am aware that I could request STATUS <mailboxname> (UIDNEXT), but this is not guaranteed to be fast. Courier also does not seem to send an untagged UIDNEXT on SELECT either. If you have any suggestions on a better way to get the highest (or next highest) UID that is reasonably efficient on Courier as well as the other major IMAP servers, I would be all ears. I'm not trying to split hairs, I'm honestly trying to accomplish something.
First of all, if the highest existing UID is 5, you cannot assume that the next message you see will have UID of 6. Nope, nope. If you thought so, think again.
For starters, by the time you get around to checking for new messages, another message could've been added to the mailbox, got a UID of 6, then someone else deleted and expunged it, so the next message you get will have a UID of 7.
Yes, that means that UIDNEXT is completely and utterly meaningless as well, for precisely the same exact reasons. If you rely on what UIDNEXT tells you, you're going to have a spectacular blowup the next time someone manages to sneak in and quickly delete a message, under your nose. UIDNEXT is fundamentally broken. It is completely meaningles. Well, it's there, but only because it's supposed to be there.
The closest thing to what you want is: <tag> FETCH * UIDshould get you the UID of the last message in a nonempty mailbox. For empty mailboxes you may get an error, or no results, depending on the server.
But I still think that whatever you're trying to do, your approach is not the right one. I wrote an fully-functional IMAP client. It does everything with an IMAP client that any IMAP client might want to do, including IDLE, and the whole jazz, while side-stepping all that's broken with IMAP. And I never needed to twist myself into such a pretzel, not even close.
As an IMAP client, if you do it right you will always know exactly what's in the folder. You get the message count when you SELECT it, and by paying attention to EXISTS, FLAGS, and EXPUNGED messages you know exactly what's in it, at any time. When you initially SELECT the mailbox you do, sadly, need to do a "FETCH 1:* UID FLAGS", to capture the mailbox's state (which I think is a design flaw), but from that point on you're in the driver's seat and you will always know exactly what's in there, simply by paying attention to three messages when you receive them.
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