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

[Philip K.]

Hi,

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).

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.

-- 
        Philip K.

>From 6b141be5123d4cf37d743a9a12818442333658ed Mon Sep 17 00:00:00 2001
From: Philip K <philipk@posteo.net>
Date: Fri, 28 Aug 2020 14:20:56 +0200
Subject: [PATCH] Allow tags backend to not query for TAGS file

* lisp/progmodes/etags.el (etags-query-file): Add variable
(etags--xref-backend): Respect etags-query-file
---
 lisp/progmodes/etags.el | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/etags.el b/lisp/progmodes/etags.el
index 2c5c36504a..60b162f19e 100644
--- a/lisp/progmodes/etags.el
+++ b/lisp/progmodes/etags.el
@@ -2069,8 +2069,39 @@ etags-xref-find-definitions-tag-order
 If you want `xref-find-definitions' to find the tagged files by their
 file name, add `tag-partial-file-name-match-p' to the list value.")
 
+(defcustom etags-query-file t
+  "Specify how and when to query TAGS file.
+If t, always query if no tags file has been loaded.
+If `check', only query if a TAGS file exists.
+If `check-and-set', automatically set TAGS file if exists, and don't
+query otherwise.
+If `set-or-check', set TAGS file if exists, or query user if not.
+If nil, a new table can be loaded using `visit-tags-table'."
+  :type '(choice (const :tag "Always query" t)
+                 (const :tag "Never query" nil)
+                 (const :tag "Query if exists" check)
+                 (const :tag "Set if exists" check-and-set)
+                 (const :tag "Query if not exists" set-or-check))
+  :version "28.1")
+
 ;;;###autoload
-(defun etags--xref-backend () 'etags)
+(defun etags--xref-backend ()
+  (and (cond ((or (null etags-query-file)
+                  tags-table-computed-list))
+             ((eq etags-query-file 'check)
+              (locate-dominating-file default-directory "TAGS"))
+             ((eq etags-query-file 'check-and-set)
+              (let ((dir (locate-dominating-file default-directory "TAGS")))
+                (when dir
+                  (visit-tags-table dir)
+                  t)))
+             ((eq etags-query-file 'set-or-check)
+              (let ((dir (locate-dominating-file default-directory "TAGS")))
+                (when dir
+                  (visit-tags-table dir))
+                t))
+             (etags-query-file))
+       'etags))
 
 (cl-defmethod xref-backend-identifier-at-point ((_backend (eql etags)))
   (find-tag--default))
-- 
2.26.2