[X11] Grab server while focusing window.

[lihop]

Fixes #65425

More detail in #65425 (comment).

When handling the ConfigureNotify event, we get window attributes using XGetWindowAttributes and only call XSetInputFocus if the map_state attribute is IsViewable.

Occasionally, the map_state attribute was IsViewable but would become IsUnmapped by the time the XSetInputFocus request was received due to a request from another X server connection (e.g. the window manager). This would result in a BadMatch error and cause Godot to crash.

By calling XGrabServer before XGetWindowAttributes and XSetInputFocus we ensure that the X server will only receive requests from this connection and therefore the map_state attribute will not change until XUngrabServer is called.

[lihop]

I have converted this PR to draft as I am a little concerned about the wider impacts it may have.

The Xlib Programming Manual says in the XGrabServer section:

You should not grab the X server any more than is absolutely necessary.

And in the XUngrabServer section:

You should avoid grabbing the X server as much as possible.

I have just noticed that ConfigureNotify is called a lot of times (when dragging around a window for example):

So this patch would violate those suggestions as we only really care about calling XGrabServer in the specific case when the window is about to become unviewable.

Something I'm wondering is if we really need to call XSetInputFocus every time we handle ConfigureNotify with a viewable window, or if we could be a bit more selective about when we actually call it. If so, we might be able to avoid calling it in the case of the associated issue.

[mxnemu]

Hey, thanks for locating the bug and testing it more. For some reason this PR still crashes for me. Strangely XGrabServer doesn't seem to help at all, when it should. I still get the same error.

Commenting out XSetInputFocus makes the crash go away, so it's definitely at that location.

[lihop]

Closing as this PR does not appear to fix the issue and @mxnemu has provided a suitable alternative patch in #65425 (comment).