diff -r 000000000000 -r 6474c204b198 toolkit/components/finalizationwitness/FinalizationWitnessService.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/finalizationwitness/FinalizationWitnessService.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,213 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FinalizationWitnessService.h"
+
+#include "nsString.h"
+#include "jsapi.h"
+#include "js/CallNonGenericMethod.h"
+#include "mozJSComponentLoader.h"
+#include "nsZipArchive.h"
+
+#include "mozilla/Scoped.h"
+#include "mozilla/Services.h"
+#include "mozilla/NullPtr.h"
+#include "nsIObserverService.h"
+#include "nsThreadUtils.h"
+
+
+// Implementation of nsIFinalizationWitnessService
+
+namespace mozilla {
+
+namespace {
+
+/**
+ * An event meant to be dispatched to the main thread upon finalization
+ * of a FinalizationWitness, unless method |forget()| has been called.
+ *
+ * Held as private data by each instance of FinalizationWitness.
+ * Important note: we maintain the invariant that these private data
+ * slots are already addrefed.
+ */
+class FinalizationEvent MOZ_FINAL: public nsRunnable
+{
+public:
+  FinalizationEvent(const char* aTopic,
+                  const jschar* aValue)
+    : mTopic(aTopic)
+    , mValue(aValue)
+  { }
+
+  NS_METHOD Run() {
+    nsCOMPtr<nsIObserverService> observerService =
+      mozilla::services::GetObserverService();
+    if (!observerService) {
+      // This is either too early or, more likely, too late for notifications.
+      // Bail out.
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    (void)observerService->
+      NotifyObservers(nullptr, mTopic.get(), mValue.get());
+    return NS_OK;
+  }
+private:
+  /**
+   * The topic on which to broadcast the notification of finalization.
+   *
+   * Deallocated on the main thread.
+   */
+  const nsCString mTopic;
+
+  /**
+   * The result of converting the exception to a string.
+   *
+   * Deallocated on the main thread.
+   */
+  const nsString mValue;
+};
+
+enum {
+  WITNESS_SLOT_EVENT,
+  WITNESS_INSTANCES_SLOTS
+};
+
+/**
+ * Extract the FinalizationEvent from an instance of FinalizationWitness
+ * and clear the slot containing the FinalizationEvent.
+ */
+already_AddRefed<FinalizationEvent>
+ExtractFinalizationEvent(JSObject *objSelf)
+{
+  JS::Value slotEvent = JS_GetReservedSlot(objSelf, WITNESS_SLOT_EVENT);
+  if (slotEvent.isUndefined()) {
+    // Forget() has been called
+    return nullptr;
+  }
+
+  JS_SetReservedSlot(objSelf, WITNESS_SLOT_EVENT, JS::UndefinedValue());
+
+  return dont_AddRef(static_cast<FinalizationEvent*>(slotEvent.toPrivate()));
+}
+
+/**
+ * Finalizer for instances of FinalizationWitness.
+ *
+ * Unless method Forget() has been called, the finalizer displays an error
+ * message.
+ */
+void Finalize(JSFreeOp *fop, JSObject *objSelf)
+{
+  nsRefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
+  if (event == nullptr) {
+    // Forget() has been called
+    return;
+  }
+
+  // Notify observers. Since we are executed during garbage-collection,
+  // we need to dispatch the notification to the main thread.
+  (void)NS_DispatchToMainThread(event);
+  // We may fail at dispatching to the main thread if we arrive too late
+  // during shutdown. In that case, there is not much we can do.
+}
+
+static const JSClass sWitnessClass = {
+  "FinalizationWitness",
+  JSCLASS_HAS_RESERVED_SLOTS(WITNESS_INSTANCES_SLOTS),
+  JS_PropertyStub /* addProperty */,
+  JS_DeletePropertyStub /* delProperty */,
+  JS_PropertyStub /* getProperty */,
+  JS_StrictPropertyStub /* setProperty */,
+  JS_EnumerateStub /* enumerate */,
+  JS_ResolveStub /* resolve */,
+  JS_ConvertStub /* convert */,
+  Finalize /* finalize */
+};
+
+bool IsWitness(JS::Handle<JS::Value> v)
+{
+  return v.isObject() && JS_GetClass(&v.toObject()) == &sWitnessClass;
+}
+
+
+/**
+ * JS method |forget()|
+ *
+ * === JS documentation
+ *
+ *  Neutralize the witness. Once this method is called, the witness will
+ *  never report any error.
+ */
+bool ForgetImpl(JSContext* cx, JS::CallArgs args)
+{
+  if (args.length() != 0) {
+    JS_ReportError(cx, "forget() takes no arguments");
+    return false;
+  }
+  JS::Rooted<JS::Value> valSelf(cx, args.thisv());
+  JS::Rooted<JSObject*> objSelf(cx, &valSelf.toObject());
+
+  nsRefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
+  if (event == nullptr) {
+    JS_ReportError(cx, "forget() called twice");
+    return false;
+  }
+
+  args.rval().setUndefined();
+  return true;
+}
+
+bool Forget(JSContext *cx, unsigned argc, JS::Value *vp)
+{
+  JS::CallArgs args = CallArgsFromVp(argc, vp);
+  return JS::CallNonGenericMethod<IsWitness, ForgetImpl>(cx, args);
+}
+
+static const JSFunctionSpec sWitnessClassFunctions[] = {
+  JS_FN("forget", Forget, 0, JSPROP_READONLY | JSPROP_PERMANENT),
+  JS_FS_END
+};
+
+}
+
+NS_IMPL_ISUPPORTS(FinalizationWitnessService, nsIFinalizationWitnessService)
+
+/**
+ * Create a new Finalization Witness.
+ *
+ * A finalization witness is an object whose sole role is to notify
+ * observers when it is gc-ed. Once the witness is created, call its
+ * method |forget()| to prevent the observers from being notified.
+ *
+ * @param aTopic The notification topic.
+ * @param aValue The notification value. Converted to a string.
+ *
+ * @constructor
+ */
+NS_IMETHODIMP
+FinalizationWitnessService::Make(const char* aTopic,
+                                 const char16_t* aValue,
+                                 JSContext* aCx,
+                                 JS::MutableHandle<JS::Value> aRetval)
+{
+  JS::Rooted<JSObject*> objResult(aCx, JS_NewObject(aCx, &sWitnessClass, JS::NullPtr(),
+                                                    JS::NullPtr()));
+  if (!objResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  if (!JS_DefineFunctions(aCx, objResult, sWitnessClassFunctions)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsRefPtr<FinalizationEvent> event = new FinalizationEvent(aTopic, aValue);
+
+  // Transfer ownership of the addrefed |event| to |objResult|.
+  JS_SetReservedSlot(objResult, WITNESS_SLOT_EVENT,
+                     JS::PrivateValue(event.forget().take()));
+
+  aRetval.setObject(*objResult);
+  return NS_OK;
+}
+
+} // namespace mozilla