bug#54158: 28.0.91; duplicate mail downloads in Gnus (IMAP)

[Roland Winkler <winkler@gnu.org>]

On Fri, Feb 25 2022, Eric Abrahamsen wrote:
> I went down a bit of a rabbit hole here, and my best guess is that
> commit daa4e0120 (which is in emacs-28 but not emacs-27) might have
> resulted in a bug in the dynamic binding of your imap mail-source
> definition data.

...First I thought "let me revert this patch and see what happens", but
that's not a meaningful step.

> Something to try would be to edebug `mail-source-fetch-imap' and make
> sure that the dynamic variables `dontexpunge' and `fetchflag' are what
> you'd expect them to be, ie nil and "\Deleted". And watch what happens
> for both "INBOX" and "JUNK EMAIL"

The values of these variables are always what you said they should be,
nil and "\Deleted".

> (why are you fetching your junk mail, anyway?)

It's the usual problem that the folder "JUNK EMAIL" contains messages
that should not be there.  And I haven't figured out how the server
identifies mail as spam, nor whether it would allow me to fine-tune
this.  I find it easier to do all this on the client side.

> To be honest I don't see why `imap-mailbox-close' would expunge mail
> anyway: its docstring says it does, but no expunge command is given.
> There's a `imap-mailbox-expunge' function, but nothing ever calls it.
> Anyway, that part of the code hasn't changed since emacs-27, so
> <shrug>.

I looked at this code.  Unfortunately, the wrappers it uses make it very
difficult to understand what it is doing, also because I do not know
either what it needs to do to get things right.

> If something funny is happening with the values of fetchflag or
> dontexpunge, we can drag in Stefan M, who changed `mail-source-bind'.

Following your suggestions of monitoring the variables `dontexpunge' and
`fetchflag', I do not see anything unusual in mail-source-fetch-imap.

My understanding is that IMAP servers permit to expunge individual
messages while keeping others, though I do not want to use this feature
(I want to expunge everything after fetching it).  However, I do not see
where mail-source-fetch-imap would implement such fine control.  But my
problem of multiple downloads exists only for *some*, but not all
messages.  So it seems to me we may be looking at the wrong part of the
code altogether.  But that's only a wild guess of someone who does not
know much about all of this.

Any help is appreciated!

Roland