bug#54133: 29.0.50; Buffer-menu-visit-tags-table disrupts non-tags buffers

[Eli Zaretskii]

> From: Bob Rogers <rogers@rgrjr.com>
> Date: Wed, 23 Feb 2022 16:37:11 -0500
> 
>    1.  "emacs -Q &"
> 
>    2.  Visit pretty much any random file that is NOT a tags file; for
> this purpose, lisp/abbrev.el from the source tree will do nicely.
> 
>    3.  Type "C-x C-b" to get the "*Buffer List*" window to appear.
> 
>    4.  Type "C-x o" to move the to the buffer list, then move to the
> abbrev.el line, and type "t" to invoke Buffer-menu-visit-tags-table on
> it (which I seem to do accidentally more often than I care to admit).
> This produces the error message:
> 
>       user-error: File /scratch/rogers/emacs/lisp/abbrev.el is not a valid 
> tags table
> 
> but the abbrev.el buffer is left in tags-table-mode, and any undo
> information is thrown away.
> 
>    The attached patch addresses the problem in a straightforward way by
> making Buffer-menu-visit-tags-table prompt the user for buffers not
> already in tags-table-mode.
> 
>    The real problem is that visit-tags-table assumes that the user knows
> what they're doing, and makes these irreversible changes before being
> sure of having a valid tags table.  That would be a harder thing to fix,
> though -- and might not deal as well with my buffer-menu typos.  ;-}

Does the alternative patch below give good results?

diff --git a/lisp/buff-menu.el b/lisp/buff-menu.el
index 50c2c15..179cc54 100644
--- a/lisp/buff-menu.el
+++ b/lisp/buff-menu.el
@@ -527,13 +527,18 @@ Buffer-menu-multi-occur
   (multi-occur (Buffer-menu-marked-buffers) regexp nlines))
 
 
+(autoload 'etags-verify-tags-table "etags")
 (defun Buffer-menu-visit-tags-table ()
   "Visit the tags table in the buffer on this line.  See `visit-tags-table'."
   (interactive nil Buffer-menu-mode)
-  (let ((file (buffer-file-name (Buffer-menu-buffer t))))
-    (if file
-       (visit-tags-table file)
-      (error "Specified buffer has no file"))))
+  (let* ((buf (Buffer-menu-buffer t))
+         (file (buffer-file-name buf)))
+    (cond
+     ((not file) (error "Specified buffer has no file"))
+     ((and buf (with-current-buffer buf
+                 (etags-verify-tags-table)))
+      (visit-tags-table file))
+     (t (error "Specified buffer is not a tags-table")))))
 
 (defun Buffer-menu-1-window ()
   "Select this line's buffer, alone, in full frame."