etags-regen: Don't try to generate tags during completion, by default

* etc/NEWS: Describe the change.

* lisp/progmodes/etags-regen.el
(etags-regen-create-on-completion): New option.
(etags-regen-create-on-completion--set): New function.
(etags-regen-mode): Use it.  Discussed in
https://lists.gnu.org/archive/html/emacs-devel/2026-03/msg00405.html

etc/NEWS

@@ -3763,6 +3763,14 @@ the mode line without polling for changes every
 means the mode line will update only when the battery power state,
 percentage, or presence in the bay changes.
 
+** Etags Regen mode
+
+*** Tags table is not created during completion anymore.
+Previously, when there was no tags table loaded and the default
+completion function was called, 'etags-regen-mode' ensured that tags
+were created.  This has been disabled, and the new user option
+'etags-regen-create-on-completion' can be used to enable it again.
+
 
 * New Modes and Packages in Emacs 31.1
 

lisp/progmodes/etags-regen.el

@@ -142,6 +142,21 @@ File extensions to generate the tags for."
           (string :tag "Glob to ignore"))
   :version "30.1")
 
+(defun etags-regen-create-on-completion--set (symbol value)
+  (when (and etags-regen-mode value)
+    (advice-add 'tags-completion-at-point-function :before
+                #'etags-regen--maybe-generate))
+  (set-default-toplevel-value symbol value))
+
+(defcustom etags-regen-create-on-completion nil
+  "Non-nil to ensure tags are created before completion.
+
+This applies to the function `tags-completion-at-point-function', which
+is used in buffers that have no alternative completion configured."
+  :type 'boolean
+  :set #'etags-regen-create-on-completion--set
+  :version "31.1")
+
 ;;;###autoload
 (put 'etags-regen-ignores 'safe-local-variable
      (lambda (value) (and (listp value) (seq-every-p #'stringp value))))
@@ -471,8 +486,9 @@ to countermand the effect of a previous \\[visit-tags-table]."
       (progn
         (advice-add 'etags--xref-backend :before
                     #'etags-regen--maybe-generate)
-        (advice-add 'tags-completion-at-point-function :before
-                    #'etags-regen--maybe-generate))
+        (when etags-regen-create-on-completion
+          (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)))