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

[Eli Zaretskii <eliz@gnu.org>]

> From: Bob Rogers <rogers@rgrjr.com>
> Date: Thu, 24 Feb 2022 12:30:53 -0500
> Cc: 54133@debbugs.gnu.org
> 
>    From: Eli Zaretskii <eliz@gnu.org>
>    Date: Thu, 24 Feb 2022 17:15:54 +0200
> 
>    > From: Bob Rogers <rogers@rgrjr.com>
>    > Date: Wed, 23 Feb 2022 16:37:11 -0500
>    >
>    > . . .
>    >
>    >    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?
> 
> This does ineed work, and I had considered something similar, but was
> concerned that it would not be as robust.  etags-verify-tags-table is
> called from etags-recognize-tags-table, which is only a part of the
> tags-table-format-functions extension mechanism used by
> initialize-new-tags-table.  So to avoid breaking this mechanism I would
> either have had to call initialize-new-tags-table and have it possibly
> set unwanted local variables, or delve more deeply into the internals by
> running tags-table-format-functions myself.

Maybe we should have a more thorough implementation in
etags-verify-tags-table, then.  But that function's purpose is to do
what we need here.

>    In short, I thought checking the major mode was the better choice,
> since the file was already present in a buffer.

But what if the file is already in a buffer, but not under the right
major-mode?  E.g., what if the file was visited literally?