bug#65332: 29.1; browse-url calls pgtk-backend-display-class, which errors when window-system is nil

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

"Trent W. Buck" <trentbuck@gmail.com> writes:

> Minimum recipe to reproduce:
>
>   1. Get emacs with pgtk, start daemon, connect to it from a tty frame:
>
>         twb@hera[Desktop]$ emacs-pgtk --version
>         GNU Emacs 29.1
>         Copyright (C) 2023 Free Software Foundation, Inc.
>         GNU Emacs comes with ABSOLUTELY NO WARRANTY.
>         You may redistribute copies of GNU Emacs
>         under the terms of the GNU General Public License.
>         For more information about these matters, see the file named COPYING.
>
>         twb@hera[Desktop]$ emacs-pgtk --quick --daemon
>         Due to a limitation in GTK 3, Emacs built with PGTK will simply exit when a
>         display connection is closed.  The problem is especially difficult to fix,
>         such that Emacs on Wayland with multiple displays is unlikely ever to be able
>         to survive disconnects.
>         Starting Emacs daemon.
>
>         twb@hera[Desktop]$ emacsclient --create-frame --tty
>
>   2. M-x browse url RET https://example.com RET
>
>      Get this error:
>
>          Frames are not in use or not initialized
>
>   3. Explicitly setting browse-url-browser-function to eww-browse-url does not help.
>
> With toggle-debug-on-error, the backtrace I got during normal usage was:
>
>     Debugger entered--Lisp error: (error "Frames are not in use or not initialized")
>       browse-url("http://ix.io/4DCN")
>       find-file-at-point()
>       ido-file-internal(selected-window)
>       ido-find-file()
>       funcall-interactively(ido-find-file)
>       command-execute(ido-find-file)
>
> The problem is it's calling (pgtk-backend-display-class) even when the current frame isn't a GTK frame, I think?
>
>     ELISP> (browse-url "https://example.com")
>     *** Eval error ***  Frames are not in use or not initialized
>     ELISP> (w3m-browse-url "https://example.com")
>     nil
>     ELISP> browse-url-browser-function
>     w3m-browse-url
>     ELISP> (pgtk-backend-display-class)
>     *** Eval error ***  Frames are not in use or not initialized
>     ELISP> window-system
>     nil
>     ELISP> (frame-parameter nil 'display)
>     ":0"
>     ELISP> (getenv "DISPLAY")
>     ":0"
>     ELISP> (getenv "WAYLAND_DISPLAY")
>     "wayland-0"

Your analysis of the problem is correct.

But to correct it, we need to know what display our users expect the
browser window to open in under such a situation?