[OpenJDK 2D-Dev] RFR: 8239894: Xserver crashes when the wrong high refresh rate is used

[Alexander Zuev]

On Tue, 19 Jan 2021 00:57:31 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

> We use two different approaches to listing the display modes on the system:
>  - For single-screen configuration use version 1.1 protocol XRRGetScreenInfo/XRRConfigRates/XRRConfigSizes
>  - For multi-screen configuration use version 1.2 protocol: XRRGetScreenResources/XRRGetOutputInfo/XRRModeInfo
> 
> Unfortunately, the current code does not work for some specific double scan modes, it doubles the real refresh rate. If this refresh rate will be passed to the Xserver later it could hang/crash.
> 
> The same bug existed in xrandr utility before version 1.4:
> https://bugs.freedesktop.org/show_bug.cgi?id=37043
> https://gitlab.freedesktop.org/xorg/app/xrandr/commit/7fd4f18b649f22fad4dbf9fc64b69b3e7f172207
> 
> As suggested in the bug report I have tried to unify the code for single and multiscreen systems and use updated code currently used on the multiscreen.
> 
> But unfortunately, I have found that the multi-screen code is even more broken. The problematic code is this:
> 
>     int scr = 0, out = 0;
>     if (usingXinerama && XScreenCount(awt_display) > 0) {
>         out = screen;
>     } else {
>         scr = screen;
>     }
> The idea of the code is: if the passed screen is Xinerama index used by the java2d, then the "scr" will be zero, and "out" will be the "virtual" index for that screen. If the screen is X11 screen then "src" will be the correct screen and "out" will be zero.
> 
> The problem is that the code assumes that xrandr expect the "virtual" index as the "out" parameter, but this is not. This parameter is the index of the "output" such as DP1, DP2, other ports/etc.
> 
> So sometimes the code works when the first screen is connected to the first port on the video card and the second screen to the second port, otherwise, we just report data for the wrong screen or report nothing.
> 
> I did not found a reliable way to map Xinerama srceens to the xrandr. It is better to rework the java2d to use the xrandr instead of xinerama.
> 
> As a fix, I suggest dropping the code which uses the incorrect mapping screen->output. This includes the code related to the display modes added in the JDK-8167486 + JDK-8022810, and also the code related to the calculation of scale factor from the "ubuntu property" which does not work on the recent ubuntu version after migration to gnome3 added in the JDK-8149115
> 
> I will file a separate two bugs to reimplement this functionality, most probably the "display modes" will be implementing when we fully migrate to the xrandr, and calculation of the scale factor will be implemented via the gtk library.

I guess not having this functionality at all is better than having a code that hangs. Can we have these new bugs about re-implementing this functionality linked in the initial bug so they can be tracked easier?

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Marked as reviewed by kizune (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2136