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It seems that attempting to ungrab an Xinput slave device while a different VT is active causes X to crash.

Compiling and running test.c and then switching VTs before it terminates causes X to crash with the following backtrace on my machine.

X.Org X Server 1.20.4

...

[1076359.609] (EE) Backtrace:
[1076359.610] (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x139) [0x55555a6932c9]
[1076359.610] (EE) 1: /lib/x86_64-linux-gnu/libpthread.so.0 (funlockfile+0x50) [0x7f5231d3077f]
[1076359.610] (EE) 2: /usr/lib/xorg/Xorg (InitCallbackManager+0x1bb8) [0x55555a53ce18]
[1076359.610] (EE) 3: /usr/lib/xorg/Xorg (DeliverEvents+0x102d) [0x55555a54609d]
[1076359.610] (EE) 4: /usr/lib/xorg/Xorg (AssignTypeAndName+0x6e7c) [0x55555a62c74c]
[1076359.610] (EE) 5: /usr/lib/xorg/Xorg (SendErrorToClient+0x35e) [0x55555a5349ce]
[1076359.610] (EE) 6: /usr/lib/xorg/Xorg (InitFon[test.c](/uploads/189b2af7dea4b8d947e3d323993d612e/test.c)ts+0x3b6) [0x55555a538956]
[1076359.611] (EE) 7: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xeb) 
[1076359.611] (EE) 8: /usr/lib/xorg/Xorg (_start+0x2a) [0x55555a52267a]

test.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/time.h>
#include <ctype.h>
#include <X11/Xlib.h>
#include <X11/keysym.h>
#include <X11/extensions/XTest.h>
#include <X11/extensions/XInput2.h>
#include <assert.h>

Display *dpy;

void grab(int devid)
{
	size_t i;
	XIEventMask mask;

	mask.deviceid = XIAllDevices;
	mask.mask_len = XIMaskLen(XI_LASTEVENT);
	mask.mask = calloc(mask.mask_len, sizeof(char));
	XISetMask(mask.mask, XI_KeyPress);
	XISetMask(mask.mask, XI_KeyRelease);

	if(XIGrabDevice(dpy, devid,
			DefaultRootWindow(dpy),
			CurrentTime,
			None,
			GrabModeAsync,
			GrabModeAsync,
			False, &mask)) {
		fprintf(stderr, "FATAL: Failed to grab keyboard\n");
		exit(-1);
	}

	XSync(dpy, False);
}

static int get_dev_id()
{
	int i, n;
	XIDeviceInfo* devs = XIQueryDevice(dpy, XIAllDevices, &n);

	for (i = 0; i < n; i++) {
		if (devs[i].use == XISlaveKeyboard &&
		    !strstr(devs[i].name, "XTEST")) {
			printf("Grabbing %s\n", devs[i].name);
			return devs[i].deviceid;
		}
	}
}

int main()
{
	int i;
	dpy = XOpenDisplay(NULL);
	int id = get_dev_id();
	grab(id);

	sleep(3); //Switch vts here, observe crash

	XIUngrabDevice(dpy, id, CurrentTime);
	XSync(dpy, False);
}