Implement cancellable runnables.

Additionally, make image load events cancellable. Resolves #7731.

components/script/script_task.rs

@@ -95,6 +95,7 @@ use std::option::Option;
 use std::ptr;
 use std::rc::Rc;
 use std::result::Result;
+use std::sync::atomic::{Ordering, AtomicBool};
 use std::sync::mpsc::{Receiver, Select, Sender, channel};
 use std::sync::{Arc, Mutex};
 use string_cache::Atom;
@@ -158,7 +159,38 @@ impl InProgressLoad {
     }
 }
 
+/// Encapsulated state required to create cancellable runnables from non-script threads.
+pub struct RunnableWrapper {
+    pub cancelled: Arc<AtomicBool>,
+}
+
+impl RunnableWrapper {
+    pub fn wrap_runnable<T: Runnable + Send + 'static>(&self, runnable: T) -> Box<Runnable + Send> {
+        box CancellableRunnable {
+            cancelled: self.cancelled.clone(),
+            inner: box runnable,
+        }
+    }
+}
+
+/// A runnable that can be discarded by toggling a shared flag.
+pub struct CancellableRunnable<T: Runnable + Send> {
+    cancelled: Arc<AtomicBool>,
+    inner: Box<T>,
+}
+
+impl<T: Runnable + Send> Runnable for CancellableRunnable<T> {
+    fn is_cancelled(&self) -> bool {
+        self.cancelled.load(Ordering::Relaxed)
+    }
+
+    fn handler(self: Box<CancellableRunnable<T>>) {
+        self.inner.handler()
+    }
+}
+
 pub trait Runnable {
+    fn is_cancelled(&self) -> bool { false }
     fn handler(self: Box<Self>);
 }
 
@@ -990,10 +1022,13 @@ impl ScriptTask {
                 runnable.handler(self),
             MainThreadScriptMsg::DocumentLoadsComplete(id) =>
                 self.handle_loads_complete(id),
-            MainThreadScriptMsg::Common(CommonScriptMsg::RunnableMsg(_, runnable)) =>
+            MainThreadScriptMsg::Common(CommonScriptMsg::RunnableMsg(_, runnable)) => {
                 // The category of the runnable is ignored by the pattern, however
                 // it is still respected by profiling (see categorize_msg).
-                runnable.handler(),
+                if !runnable.is_cancelled() {
+                    runnable.handler()
+                }
+            }
             MainThreadScriptMsg::Common(CommonScriptMsg::RefcountCleanup(addr)) =>
                 LiveDOMReferences::cleanup(addr),
             MainThreadScriptMsg::Common(CommonScriptMsg::CollectReports(reports_chan)) =>