bug#62392: frame-monitor-attributes fails to calculate mm-size for the monitor

[tomasralph2000]

Whenever I try to run tetris, or snake, or pong, I get "arithmetic overflow error". After digging a bit, I found where the issue lies. This function:

(defun gamegrid-calculate-glyph-size ()
"Calculate appropriate glyph size in pixels based on display resolution.
Return a multiple of 8 no less than 16."
(let ((atts (frame-monitor-attributes))
y-pitch)
(setq y-pitch (cond
(atts
(/ (nth 4 (assq 'geometry atts))
(nth 2 (assq 'mm-size atts))
(or (cdr (assq 'scale-factor atts)) 1.0)))
((and (display-pixel-height) (display-mm-height))
(/ (display-pixel-height) (float (display-mm-height))))))
(if y-pitch
(let* ((pixels (* y-pitch gamegrid-glyph-height-mm))
(rounded (* (floor (/ (+ pixels 4) 8)) 8)))
(max 16 rounded))
16)))

Is the one causing the error. Where in particular? This section:

(/ (nth 4 (assq 'geometry atts))
(nth 2 (assq 'mm-size atts))

What's the issue here? `(assq 'mm-size atts)` returns 0. For some reason, emacs can't seem to get my monitor size in milimeters. I have tried this on two different laptops, they both return proper values. This is happening on my desktop, where it fails.

I am using Wayland (with Hyprland) and the PGTK version of emacs, which likely has something to do with this. This is what `frame-monitor-attributes` returns for me:

((name . "HDMI") (geometry 0 0 1920 1080) (workarea 0 0 1920 1080) (mm-size 0 0) (scale-factor . 1.0) (frames #<frame *scratch* - GNU Emacs at Rainbow 0x55799b79aa48> #<frame 0x55799d4561e0>) (source . "Gdk"))

As you can see, the `mm-size` is 0, so emacs divides by zero. Amazingly, this is not what causes the error, the variable simply gets set to infinity. The error comes later, when it tries to multiply by infinity. Oh lord.

This is what emacs outputs on my laptop for the same `frame-monitor-attributes`. Note that my laptop runs the exact same setup, it also runs Hyprland, it also runs emacs with PGTK, it uses the same config for everything since I have the same dotifles repo cloned in both systems:

((name . "0x0335") (geometry 0 0 1366 768) (workarea 0 0 1366 768) (mm-size 310 170) (scale-factor . 1.0) (frames #<frame *scratch* - GNU Emacs at T430 0x55c4c1cdad20> #<frame 0x55c4c356b7e0>) (source . "Gdk"))

This time, the `mm-size` gets properly calculated.

So something is funky with `frame-monitor-attributes`. Any direction on
how I can further debug this would be appreciated.



In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.37, cairo version 1.17.8) of 2023-03-21 built on Rainbow
Repository revision: 42fba8f36b19536964d6deb6a34f3fd1c02b43dd
Repository branch: makepkg
System Description: Arch Linux

Configured using:
'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
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