Fix a rare crash in some X11 implementations (details below).

In some X11 implementations, calling XCloseDisplay from any thread
other than the main thread causes heap corruption and an eventual
crash.

Previously, the window proxy struct held a Weak<> ptr to the display,
which was upgraded to a strong arc only during the send of the X
message.

However, if the main thread closes the window *after* the thread
upgrades the weak ref, and this was the last strong reference,
then the only remaining strong reference is in the window proxy
thread, so the XCloseDisplay is executed on the thread.

Instead, use a simpler solution where the display reference used
by the window proxy threads is cleared by the main thread when
the window is closed, and the XCloseDisplay can only be executed
by the main thread.

This has the added benefit of not relying on the unstable Weak<>
library feature, so we will be able to get this upstreamed.

src/lib.rs

@@ -23,8 +23,6 @@
 //!
 //! By default only `window` is enabled.
 
-#![feature(arc_weak)]
-
 #[macro_use]
 extern crate lazy_static;
 

src/x11/window/mod.rs

@@ -7,7 +7,7 @@ use std::cell::Cell;
 use std::sync::atomic::AtomicBool;
 use std::collections::VecDeque;
 use super::ffi;
-use std::sync::{Arc, Mutex, Once, ONCE_INIT, Weak};
+use std::sync::{Arc, Mutex, Once, ONCE_INIT};
 
 use Api;
 use CursorState;
@@ -46,6 +46,13 @@ fn with_c_str<F, T>(s: &str, f: F) -> T where F: FnOnce(*const libc::c_char) ->
     f(c_str.as_ptr())
 }
 
+struct WindowProxyData {
+    display: *mut ffi::Display,
+    window: ffi::Window,
+}
+
+unsafe impl Send for WindowProxyData {}
+
 struct XWindow {
     display: *mut ffi::Display,
     window: ffi::Window,
@@ -56,6 +63,7 @@ struct XWindow {
     ic: ffi::XIC,
     im: ffi::XIM,
     colormap: ffi::Colormap,
+    window_proxy_data: Arc<Mutex<Option<WindowProxyData>>>,
 }
 
 unsafe impl Send for XWindow {}
@@ -67,6 +75,10 @@ unsafe impl Sync for Window {}
 impl Drop for XWindow {
     fn drop(&mut self) {
         unsafe {
+            // Clear out the window proxy data arc, so that any window proxy objects
+            // are no longer able to send messages to this window.
+            *self.window_proxy_data.lock().unwrap() = None;
+
             // we don't call MakeCurrent(0, 0) because we are not sure that the context
             // is still the current one
             ffi::glx::DestroyContext(self.display as *mut _, self.context);
@@ -87,26 +99,28 @@ impl Drop for XWindow {
 
 #[derive(Clone)]
 pub struct WindowProxy {
-    x: Weak<XWindow>,
+    data: Arc<Mutex<Option<WindowProxyData>>>,
 }
 
 impl WindowProxy {
     pub fn wakeup_event_loop(&self) {
-        if let Some(x) = self.x.upgrade() {
+        let window_proxy_data = self.data.lock().unwrap();
+
+        if let Some(ref data) = *window_proxy_data {
             let mut xev = ffi::XClientMessageEvent {
                 type_: ffi::ClientMessage,
-                window: x.window,
+                window: data.window,
                 format: 32,
                 message_type: 0,
                 serial: 0,
                 send_event: 0,
-                display: x.display,
+                display: data.display,
                 data: unsafe { mem::zeroed() },
             };
 
             unsafe {
-                ffi::XSendEvent(x.display, x.window, 0, 0, mem::transmute(&mut xev));
-                ffi::XFlush(x.display);
+                ffi::XSendEvent(data.display, data.window, 0, 0, mem::transmute(&mut xev));
+                ffi::XFlush(data.display);
             }
         }
     }
@@ -643,6 +657,12 @@ impl Window {
         }
 
         // creating the window object
+        let window_proxy_data = WindowProxyData {
+            display: display,
+            window: window,
+        };
+        let window_proxy_data = Arc::new(Mutex::new(Some(window_proxy_data)));
+
         let window = Window {
             x: Arc::new(XWindow {
                 display: display,
@@ -654,6 +674,7 @@ impl Window {
                 is_fullscreen: builder.monitor.is_some(),
                 xf86_desk_mode: xf86_desk_mode,
                 colormap: cmap,
+                window_proxy_data: window_proxy_data,
             }),
             is_closed: AtomicBool::new(false),
             wm_delete_window: wm_delete_window,
@@ -738,7 +759,7 @@ impl Window {
 
     pub fn create_window_proxy(&self) -> WindowProxy {
         WindowProxy {
-            x: self.x.downgrade()
+            data: self.x.window_proxy_data.clone()
         }
     }