bug#43086: [PATCH] Allow tags backend to not query for TAGS file

[Dmitry Gutov <dgutov@yandex.ru>]

Hi!

On 28.08.2020 15:50, Philip K. wrote:

> the xref backend for etags can be annoying at times, especially in
> combination with other backends. This patch should improve the
> situation, by allowing the user to configure how and when the etags
> backend is activated. The new user option etags-query-file would allow
> the backend to never query a TAGS file, or conditionally, depending on
> the existence of a TAGS file (in which case it can also be automatically
> loaded).

This is a interesting patch, but it calls for some discussion:

- The possible values all look pretty clever, but there are a lot of 
them! Do we expect them all to be in demand? Ideally, I'd only leave 2-3 
of them, to reduce the number of workflows we need to care about. The 
rest could probably be set up in individual user configurations in 
find-file-hook (like Projectile does).

- The variable name implies it affects how etags.el works globally, but 
the actual effect seems limited to the xref backend function. We should 
either rename it to something like etags-xref-query-file, or consider 
having it affect tags-completion-at-point-function as well. Maybe 
find-tag too. But given that tags-completion-at-point-function has for a 
long time behaved in the "never query" fashion, perhaps the easiest and 
most backward-compatible option is the former.

- One current persistent annoyance is that currently 
xref-find-references doesn't work well in many files where the xref 
backend is the default one (etags) when ido-mode or icomplete-mode are 
enabled because it prompts for the tags file to do identifier 
completion. I wonder if the "no query" option will help with this, too.

> I could imagine this might be extended to allow an auto-generate option,
> but that feature seems out of scope of this patch, and probably would
> require some interoperation with project.el.

Indeed. Actually, I have an old, WIP patch for tag file auto-generation 
which, yes, uses project.el. I can post it again if you're curious.