is this a bug in gnus v5.13? Re: gnus INBOX housekeeping, kept (copies of) mail lying around

[Gijs Hillenius]

On  7 Jan 2016, Eric Abrahamsen wrote:

> Gijs Hillenius <gijs@hillenius.net> writes:
>
>> [...] hope snipping this much is ok..
>>
>>>>
>>>> If my Gnus (5.13, part of Debian's Emacs) copies messages back into
>>>> the INBOX, does fancy-split set the delete/expunge IMAP flag to
>>>> these?
>>>>
>>>> Because, if it does not flag them for deletion, would that perhaps
>>>> explain why the nightly Cyrus pruning does not do anything? Cyrus
>>>> is set to remove from disk all messages that have been flagged for
>>>> deletion for 3 days.
>>>
>>> No, no flags are set. From IMAP's point of view, splitting is simply
>>> a MOVE operation. Sorry, I misspoke earlier when I said the messages
>>> were *copied* back into the same group. They're actually MOVEd. That
>>> means there's only ever one copy of the message, but it gets a new
>>> UUID number each time.
>>>
>>> No new flags are set at all.
>>
>> So, these message come back int the INBOX, but are not shown anymore
>> by Gnus. They are also not marked as deletable and that would explain
>> why Cyrus does not remove them from disk, yes?
>
> Right. Once they are given the Seen flag, that flag sticks with them
> even if they are moved around (including "moved" into the same group
> they were already in). That Seen flag means Gnus won't show them to
> you again.
>
> I think the distinction here is between message deletion, and adding
> the "Deleted" flag. Deletion occurs as part of the move process (the
> message is in one instant removed from one group and added to
> another). If you're MOVEing to the same group, the message is deleted
> from that group, then added back to that group. But it keeps its old
> flags, and no other flags are put on it (there's nothing in the
> splitting process that would add a Deleted flag), so Cyrus doesn't see
> any reason why it should garbage-collect the message at the end of the
> day.

So, we're saying the same thing, correct?

What I don't understand is - and apologies for being long-winded:

The way I use Gnus: when I start it, or hit g, new messages in the INBOX
are fancy-splitted, and moved to various IMAP folders. So I see several
INBOX.subfolders with a new, unread messages. These copies, that are
kept in the INBOX, as a user I don't know about these, I don't see them
in any IMAP mail client. Is that why 'root' sees 8k messages in the
INBOX when root happens to look at this users' INBOX folder?

How can Cyrus tell these message can safely be trashed? I guess this is
a question for a Cryus mailing list, where it not that this "keeping a
copy" does not happen for mail that is split on the server by a Sieve
script. So, perhaps this is a Gnus fancy-split bug?