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

[Alexander Adolf <alexander.adolf@condition-alpha.com>]

Hello Eric,

Apologies for chiming late into this.

Eric Abrahamsen <eric@ericabrahamsen.net> writes:

> On 11/16/22 14:18 PM, Thomas Fitzsimmons wrote:
>> [...]
>>> 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.
>> [...]
>> (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.

That the behaviour changes depending on the CAPF front-end (corfu in
this case) could be indicative of the completion styles [1] kicking in.
I've seen cases where my completion function did return results, but
nothing was shown in the CAPF UI because the completion styles'
"filtering" wouldn't leave any alternatives to show. Not to suggest that
this is what's actually happening, but it's one possibility.

[1] https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Styles.html

Other than that, I would be interested in reproducing this at my end -
if possible - to see whether there's any corner case I missed. I seems
that the change of eudc-server-hotlist from `nil' to `(("localhost" .
ecomplete) ("localhost" . mailabbrev))` triggered this behaviour for
you. With that value, no database files for ecomplete or mailabbrev, and
all Elisp settings for ecomplete and mailabbrev at their default values,
`eudc-capf-complete` does return nil for me. What else should I be doing
to reproduce the issue?


Many thanks and cheers,

  --alexander