bug#69762: X11 versions of Emacs 29 on sparc fail at startup

[Ali Bahrami]

On 3/16/24 12:28 AM, Po Lu wrote:
> Ali Bahrami <ali_gnu2@emvision.com> writes:
> > With that in place, Lucid emacs starts, and runs normally.
> > This is clearly not a proper fix, but it is an effective
> > workaround, and presumably, no worse that using emacs 28.2,
> > which completely lacks sync fence support.
> >
> > I wonder if we might be looking at a problem with the
> > sync fence extension on sparc?
>
> That's more than likely, yes, though one wonders why Emacs is the first
> program to call attention to this problem.  The only role of the
> drawable parameter to SyncCreateFence is as a reference to its screen,
> which is completely defeated if not even the screen's root window is
> deemed valid.

It's a good question, and I don't know the answer.
However, it's worth noting that it's been 20 years
since sparc machines came in desktop form, with a
display, so the number of people running X11 clients
on them likely isn't large. I myself usually use the
plain tty version in those contexts.

> > Although I now have usable way around the issue, I'm willing
> > to continue with any experiments you want to try. Let me
> > know...
>
> I don't think such a drastic measure is necessary under the
> circumstances.  We should (please test) put this down as a bug in the
> X.Org server and install an error trap around SyncCreateFence requests,
> thus:

> Scratch that,
>
> diff --git a/src/xterm.c b/src/xterm.c
> index c8a43785564..2358918ac5b 100644
> --- a/src/xterm.c
> +++ b/src/xterm.c
> @@ -7292,6 +7292,7 @@ x_sync_init_fences (struct frame *f)
>      && dpyinfo->xsync_minor < 1))
>       return;
>
> +  x_ignore_errors_for_next_request (dpyinfo, 0);
>     output->sync_fences[0]
>       = XSyncCreateFence (FRAME_X_DISPLAY (f),
>                    /* The drawable given below is only used to
> @@ -7303,6 +7304,7 @@ x_sync_init_fences (struct frame *f)
>       = XSyncCreateFence (FRAME_X_DISPLAY (f),
>                    FRAME_X_WINDOW (f),
>                    False);
> +  x_stop_ignoring_errors (dpyinfo);
>
>     XChangeProperty (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f),
>               dpyinfo->Xatom_net_wm_sync_fences, XA_CARDINAL,

The x_ignore_errors_for_next_request() in 29.2 only
takes the dpyinfo argument. I dropped the second argument
from this patch, applied it, and it works. We could go with this,
rather than my

    #ifdef __sparc
        return
    #endif

construct. It probably amounts to the same thing,
except that should a fixed XSyncCreateFence() come
along, this version would start using it.

Thanks!

- Ali