toolkit/components/finalizationwitness/FinalizationWitnessService.cpp
author Mike Hommey <mh+mozilla@glandium.org>
Sat, 23 Jan 2016 11:40:24 -0800
changeset 281396 1378b39a9085890ce142e6933ebd50b6537f4ec5
parent 268184 e8c7dfe727cd970e2c3294934e2927b14143c205
child 292948 aa88b0d0cd4adf9e2fde0288a0347b91af32c69a
permissions -rw-r--r--
Bug 1242074 - Avoid going back and forth between FasterMake and RecursiveMake; r=gps FasterMake needs some RecursiveMake install manifests to have been processed before doing its work, so we can actually end up processing them twice because of the going back and forth from FasterMake in the hybrid build system. Set the dependency at the RecursiveMake level when doing an hybrid build.

/* 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 "nsIObserverService.h"
#include "nsThreadUtils.h"


// Implementation of nsIFinalizationWitnessService

static bool gShuttingDown = false;

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 final: public nsRunnable
{
public:
  FinalizationEvent(const char* aTopic,
                  const char16_t* 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)
{
  RefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
  if (event == nullptr || gShuttingDown) {
    // NB: event will be null if Forget() has been called
    return;
  }

  // Notify observers. Since we are executed during garbage-collection,
  // we need to dispatch the notification to the main thread.
  nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
  if (mainThread) {
    mainThread->Dispatch(event.forget(), NS_DISPATCH_NORMAL);
  }
  // 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),
  nullptr /* addProperty */,
  nullptr /* delProperty */,
  nullptr /* getProperty */,
  nullptr /* setProperty */,
  nullptr /* enumerate */,
  nullptr /* resolve */,
  nullptr /* mayResolve */,
  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, const 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());

  RefPtr<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
};

} // namespace

NS_IMPL_ISUPPORTS(FinalizationWitnessService, nsIFinalizationWitnessService, nsIObserver)

/**
 * 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));
  if (!objResult) {
    return NS_ERROR_OUT_OF_MEMORY;
  }
  if (!JS_DefineFunctions(aCx, objResult, sWitnessClassFunctions)) {
    return NS_ERROR_FAILURE;
  }

  RefPtr<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;
}

NS_IMETHODIMP
FinalizationWitnessService::Observe(nsISupports* aSubject,
                                    const char* aTopic,
                                    const char16_t* aValue)
{
  MOZ_ASSERT(strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0);
  gShuttingDown = true;
  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
  if (obs) {
    obs->RemoveObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID);
  }

  return NS_OK;
}

nsresult
FinalizationWitnessService::Init()
{
  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
  if (!obs) {
    return NS_ERROR_FAILURE;
  }

  return obs->AddObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID, false);
}

} // namespace mozilla