bug#67628: Emacs 29.1: (x-open-connection (getenv "DISPLAY")) gets "Display :0.0 does not exist" error

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 5 Dec 2023 00:44:05 +0200
> From: Herman <mahlamytsike@gmail.com>
> Cc: 67628@debbugs.gnu.org
> 
> On Mon, 04 Dec 2023 22:11:43 +0200
> Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > > Date: Mon, 4 Dec 2023 19:31:38 +0200
> > > From: Herman <mahlamytsike@gmail.com>
> > > 
> > > 'x-create-frame' succeeds, but 'x-open-connection' fails with same
> > > display argument.
> > > 
> > > This error "Display :0.0 does not exist" happens only, when
> > > variable 'debug-on-error' is non-nil. When 'debug-on-error' is nil,
> > > the error is not present. I.e. it works on the principle - you get
> > > what you are looking for.  
> > 
> > It could be that the code catches the error and proceeds, but your
> > setting debug-on-error non-nil prevents that.
> > 
> > Why do you need to set debug-on-error in that case?
> 
> At first I also thought that it is actually normal to catch an error when debug is enabled. I had debug turned on to look for other errors.
> 
> One argument could be that in version 28.2 debug did not catch such an error when debug is enabled. Version 29.1 behaves differently.
> 
> Another argument is that in the Elisp manual, in the Multiple terminals chapter, it is written that: "Normally, Emacs Lisp programs need not call 'x-open-connection' function, as 'make-frame-on-display' calls it automatically. The only reason for calling it is to check whether communication can be established with a given X display." 
> If in 29.1 now 
> $ emacs --daemon -Q
> $ emacsclient -e "(setq debug-on-error t)"
> then 
> $ emacsclient -e "(make-frame-on-display (getenv \"DISPLAY\"))"
> does not give an error, i.e. the display is found
> 
> but if
> $ emacs --daemon -Q
> $ emacsclient -e "(setq debug-on-error t)"
> then
> $ emacsclient -e "(x-open-connection (getenv \"DISPLAY\"))"
> gives an error, i.e. the display does not exist.

Po Lu, any ideas or suggestions?