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

[Roland Winkler <winkler@gnu.org>]

On Sat, Mar 19 2022, Andrew Cohen wrote:
> I think the problem is that the code in mail-source-fetch-imap doesn't
> handle a list of mailboxes properly (I don't know why it would have
> worked in emacs-27 though). Adam added a loop over a list of mailboxes
> way back in 2015, but it seems to me that some of the variables bound
> outside the loop should be bound inside (imap usually works on a per
> mailbox basis)
>
> So I think this might fix it (although I don't have a setup available
> to test it).

Thank you!  I can confirm: I identified one scenario with emails I send
to myself such that they go into different folders on the IMAP server.
Then downloading these messages with Gnus from Emacs 28.0.91 gives me
reproducibly the duplicate mail downloads as described in the original
post.  When I apply Andrew's patch to `mail-source-fetch-imap' the
downloads happen as expected, properly deleting the messages on the
server after download, as confirmed also with imap-log bound to to.
So for this one test case the patch appears to solve the problem.

The patch may require more testing and also checking the code by someone
who knows what is supposed to happen in such an imap session.  I am
surprised that `mail-source-fetch-imap' in Gnus from Emacs 27.2 and from
Emacs 28.0.91 are essentially the same (from all I can tell).  So I am
wondering why `mail-source-fetch-imap' in Gnus from Emacs 27.2 was
behaving as expected; but the bug showed up only in Emacs 28.  Can this
be in subtle ways related to the fact that mail-source.el in Emacs 27
used dynamic binding, whereas in Emacs 28 it uses lexical binding, as
speculated previously by Eric?