bug#59314: 29.0.50; EUDC and message-mode header completion

[Thomas Fitzsimmons]

Hi Eric,

Thanks for filing this.

Eric Abrahamsen <eric@ericabrahamsen.net> writes:

> Address completion in message-mode has stopped working in master,
> possibly as a result of 0e25a39e69acca0324c326ea8e46b1725594bff5. This
> has been reported for several contact-management backends that expect to
> have their completions available with <TAB>.
>
> `completion-at-point-functions' contains '(eudc-capf-complete
> message-completion-function t) at this point -- `eudc-capf-complete'
> returns no matches, and no other functions in the list are consulted.

I just checked and I didn't think the recent patch I pushed,
0e25a39e6..., should have affected completion-at-point-functions.  It
did change the default of eudc-server-hotlist from `nil' to
`(("localhost" . ecomplete) ("localhost" . mailabbrev))".  I thought
that should only affect EUDC users who have not customized
eudc-server-hotlist.

`eudc-capf-complete' was added to `message-mode' in commit
620ac6735...  I'm pretty sure that commenting out this line in
message.el will restore prior behaviour, but I don't yet know what prior
behaviour should be (see below).

(add-hook 'completion-at-point-functions #'message-completion-function nil t)

> On gnus.general, someone using BBDB and corfu reported that this recipe
> fixed the problem:
>
>   (setq eudc-server-hotlist '(("localhost" . bbdb)))
>
>   (add-hook 'message-mode-hook
>             (lambda ()
>               (setq-local completion-at-point-functions
>                           (delq 'message-completion-function
>                                 completion-at-point-functions))))
>
> Someone else *not* using corfu reported that that didn't work for them.
> Dunno.

I'm not sure what the out-of-the-box behaviour here is meant to be.  Can
you make a recipe starting from "emacs -Q" (including adding dummy email
addresses somewhere) that makes completion work how you want it to?  For
me:

emacs -Q
C-x m TAB

inserts four spaces and prints in *Messages*:

Loading eudcb-ecomplete...done
Loading eudcb-mailabbrev...done

(Those are new, due to 0e25a39e6... but I thought should be harmless.)

Thanks,
Thomas