On X11, fix use after free during xinput2 processing

Fixes #3536.

src/platform_impl/linux/x11/util/cookie.rs

@@ -0,0 +1,55 @@
+use std::ffi::c_int;
+use std::sync::Arc;
+
+use x11_dl::xlib::{self, XEvent, XGenericEventCookie};
+
+use crate::platform_impl::x11::XConnection;
+
+/// XEvents of type GenericEvent store their actual data in an XGenericEventCookie data structure.
+/// This is a wrapper to extract the cookie from a GenericEvent XEvent and release the cookie data
+/// once it has been processed
+pub struct GenericEventCookie {
+    cookie: XGenericEventCookie,
+    xconn: Arc<XConnection>,
+}
+
+impl GenericEventCookie {
+    pub fn from_event(xconn: Arc<XConnection>, event: XEvent) -> Option<GenericEventCookie> {
+        unsafe {
+            let mut cookie: XGenericEventCookie = From::from(event);
+            if (xconn.xlib.XGetEventData)(xconn.display, &mut cookie) == xlib::True {
+                Some(GenericEventCookie { cookie, xconn })
+            } else {
+                None
+            }
+        }
+    }
+
+    #[inline]
+    pub fn extension(&self) -> u8 {
+        self.cookie.extension as u8
+    }
+
+    #[inline]
+    pub fn evtype(&self) -> c_int {
+        self.cookie.evtype
+    }
+
+    /// Borrow inner event data as `&T`.
+    ///
+    /// ## SAFETY
+    ///
+    /// The caller must ensure that the event has the `T` inside of it.
+    #[inline]
+    pub unsafe fn as_event<T>(&self) -> &T {
+        unsafe { &*(self.cookie.data as *const _) }
+    }
+}
+
+impl Drop for GenericEventCookie {
+    fn drop(&mut self) {
+        unsafe {
+            (self.xconn.xlib.XFreeEventData)(self.xconn.display, &mut self.cookie);
+        }
+    }
+}

src/platform_impl/linux/x11/util/mod.rs

@@ -8,6 +8,7 @@ use std::{
 };
 
 mod client_msg;
+pub mod cookie;
 mod cursor;
 mod geometry;
 mod hint;