Setup process for etags-regen-mode (Emacs 30.0.91 feedback)

[Morgan Willcock <morgan@ice9.digital>]

When testing etags-regen-mode in the Emacs 30 pre-release I've found it
to be working well for my use-case, but one thing which appears to be
missing is an easy way for additional completion functions to opt-in to
the etags-regen-mode setup process.

Currently the completion functions which get advice added to them (to
call etags-regen--maybe-generate) are hard coded in the mode definition:

  (if etags-regen-mode
        (progn
          (advice-add 'etags--xref-backend :before
                      #'etags-regen--maybe-generate )
          (advice-add 'tags-completion-at-point-function :before
                      #'etags-regen--maybe-generate))
      (advice-remove 'etags--xref-backend #'etags-regen--maybe-generate)
      (advice-remove 'tags-completion-at-point-function #'etags-regen--maybe-generate)
      (etags-regen--tags-cleanup))

I was looking for a stable entry point to opt-in to using it, rather
than having to do something like this:

  (advice-add 'my-completion-function-which-does-things-with-tags :before
              #'etags-regen--maybe-generate)

(Where the "--" in the function name is a sign that I probably shouldn't
be doing that.)

I wasn't sure whether it was intentional to hard-code the functions to
advise while etags-regen-mode was still so new, or whether opting in
other functions hadn't been considered.

Would it be possible to store the completion functions which will be
advised as a separate list, so that other packages can add completion
functions to that list and get the TAGS generation to trigger?  Or is
this an intentional boundary?

Thanks,
Morgan

-- 
Morgan Willcock