Introduce safe checking whether DOMRefCell<T>.borrow_for_layout() is …

…called from layout task.

This also introduces that checking DOMRefCell<T>'s borrow methods are NOT
called from layout task.

components/layout/layout_task.rs

@@ -39,6 +39,7 @@ use script::layout_interface::{ContentBoxResponse, HitTestResponse, MouseOverRes
 use script::layout_interface::{LayoutChan, Msg, PrepareToExitMsg};
 use script::layout_interface::{GetRPCMsg, LayoutRPC, ReapLayoutDataMsg, Reflow};
 use script::layout_interface::{ReflowForDisplay, ReflowMsg};
+use script::layout_interface::LayoutBorrowTLS;
 use script_traits::{SendEventMsg, ReflowEvent, ReflowCompleteMsg, OpaqueScriptLayoutChannel};
 use script_traits::{ScriptControlChan, UntrustedNodeAddress};
 use servo_msg::compositor_msg::Scrollable;
@@ -286,6 +287,8 @@ impl LayoutTask {
 
     /// Starts listening on the port.
     fn start(self) {
+        let _layout_borrow_tls = LayoutBorrowTLS::new();
+
         let mut possibly_locked_rw_data = Some(self.rw_data.lock());
         while self.handle_request(&mut possibly_locked_rw_data) {
             // Loop indefinitely.

components/script/dom/bindings/cell.rs

@@ -5,6 +5,9 @@
 use dom::bindings::trace::JSTraceable;
 use js::jsapi::{JSTracer};
 
+#[cfg(debug)]
+use layout_interface::LayoutBorrowMarker;
+
 use std::cell;
 use std::cell::RefCell;
 use std::mem;
@@ -36,6 +39,7 @@ impl<T> DOMRefCell<T> {
 
     #[inline(always)]
     pub fn try_borrow<'a>(&'a self) -> Option<Ref<'a, T>> {
+        check_not_called_from_layout();
         self.base.try_borrow()
     }
 
@@ -46,6 +50,7 @@ impl<T> DOMRefCell<T> {
 
     #[inline(always)]
     pub fn try_borrow_mut<'a>(&'a self) -> Option<RefMut<'a, T>> {
+        check_not_called_from_layout();
         self.base.try_borrow_mut()
     }
 
@@ -56,6 +61,8 @@ impl<T> DOMRefCell<T> {
 
     /// This returns the pointer which refers T in `RefCell<T>` directly.
     pub unsafe fn borrow_for_layout<'a>(&'a self) -> &'a T {
+        check_called_in_layout();
+
         let val = mem::transmute::<&RefCell<T>, &T>(&self.base);
         val
     }
@@ -66,3 +73,25 @@ impl<T: JSTraceable> JSTraceable for DOMRefCell<T> {
         (*self).base.borrow().trace(trc)
     }
 }
+
+#[cfg(debug)]
+fn check_called_in_layout() {
+    if LayoutBorrowMarker.get().is_none() {
+        fail!("you must not call this method in non layout task.");
+    }
+}
+
+#[cfg(not(debug))]
+fn check_called_in_layout() {
+}
+
+#[cfg(debug)]
+fn check_not_called_from_layout() {
+    if LayoutBorrowMarker.get().is_some() {
+        fail!("you must not call this method in layout task.");
+    }
+}
+
+#[cfg(not(debug))]
+fn check_not_called_from_layout() {
+}

components/script/layout_interface.rs

@@ -137,3 +137,31 @@ impl ScriptLayoutChan for OpaqueScriptLayoutChannel {
         *receiver.downcast::<Receiver<Msg>>().unwrap()
     }
 }
+
+/// A task local data to check whether a function (e.g. DOMRefCell<T>.borrow_for_layout)
+/// is called from layout task or not.
+/// This checker is enabled on debug build only by performance reason.
+#[cfg(debug)]
+local_data_key!(pub LayoutBorrowMarker: ())
+
+pub struct LayoutBorrowTLS;
+
+impl LayoutBorrowTLS {
+    #[cfg(debug)]
+    pub fn new() -> LayoutBorrowTLS {
+        LayoutBorrowMarker.replace(Some(()));
+        LayoutBorrowTLS
+    }
+
+    #[cfg(not(debug))]
+    pub fn new() -> LayoutBorrowTLS {
+        LayoutBorrowTLS
+    }
+}
+
+#[cfg(debug)]
+impl Drop for LayoutBorrowTLS {
+    fn drop(&mut self) {
+        let _ = LayoutBorrowMarker.replace(None);
+    }
+}