DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Header

Mercurial (409f3966645a)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 "ThrottledEventQueue.h"

#include "mozilla/Atomics.h"
#include "mozilla/ClearOnShutdown.h"
#include "mozilla/EventQueue.h"
#include "mozilla/Mutex.h"
#include "mozilla/Services.h"
#include "mozilla/Unused.h"
#include "nsThreadUtils.h"

namespace mozilla {

namespace {

} // anonymous namespace

// The ThrottledEventQueue is designed with inner and outer objects:
//
//       XPCOM code     base event target
//            |               |
//            v               v
//        +-------+       +--------+
//        | Outer |   +-->|executor|
//        +-------+   |   +--------+
//            |       |       |
//            |   +-------+   |
//            +-->| Inner |<--+
//                +-------+
//
// Client code references the outer nsIEventTarget which in turn references
// an inner object, which actually holds the queue of runnables.
//
// Whenever the queue is non-empty (and not paused), it keeps an "executor"
// runnable dispatched to the base event target. Each time the executor is run,
// it draws the next event from Inner's queue and runs it. If that queue has
// more events, the executor is dispatched to the base again.
//
// The executor holds a strong reference to the Inner object. This means that if
// the outer object is dereferenced and destroyed, the Inner object will remain
// live for as long as the executor exists - that is, until the Inner's queue is
// empty.
//
// The xpcom shutdown process drains the main thread's event queue several
// times, so if a ThrottledEventQueue is being driven by the main thread, it
// should get emptied out by the time we reach the "eventq shutdown" phase.
class ThrottledEventQueue::Inner final : public nsISupports
{
  // The runnable which is dispatched to the underlying base target.  Since
  // we only execute one event at a time we just re-use a single instance
  // of this class while there are events left in the queue.
  class Executor final : public Runnable
  {
    // The Inner whose runnables we execute. mInner->mExecutor points
    // to this executor, forming a reference loop.
    RefPtr<Inner> mInner;

  public:
    explicit Executor(Inner* aInner)
      : Runnable("ThrottledEventQueue::Inner::Executor")
      , mInner(aInner)
    { }

    NS_IMETHODIMP
    Run() override
    {
      mInner->ExecuteRunnable();
      return NS_OK;
    }

#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
    NS_IMETHODIMP
    GetName(nsACString& aName) override
    {
      return mInner->CurrentName(aName);
    }
#endif
  };

  mutable Mutex mMutex;
  mutable CondVar mIdleCondVar;

  // As-of-yet unexecuted runnables queued on this ThrottledEventQueue.
  // (Used from any thread, protected by mMutex.)
  EventQueue mEventQueue;

  // The event target we dispatch our events (actually, just our Executor) to.
  // (Written during construction on main thread; read by any thread.)
  nsCOMPtr<nsISerialEventTarget> mBaseTarget;

  // The Executor that we dispatch to mBaseTarget to draw runnables from our
  // queue. mExecutor->mInner points to this Inner, forming a reference loop.
  // (Used from any thread, protected by mMutex.)
  nsCOMPtr<nsIRunnable> mExecutor;

  explicit Inner(nsISerialEventTarget* aBaseTarget)
    : mMutex("ThrottledEventQueue")
    , mIdleCondVar(mMutex, "ThrottledEventQueue:Idle")
    , mBaseTarget(aBaseTarget)
  {
  }

  ~Inner()
  {
#ifdef DEBUG
    MutexAutoLock lock(mMutex);
    MOZ_ASSERT(!mExecutor);
    MOZ_ASSERT(mEventQueue.IsEmpty(lock));
#endif
  }

  nsresult
  CurrentName(nsACString& aName)
  {
    nsCOMPtr<nsIRunnable> event;

#ifdef DEBUG
    bool currentThread = false;
    mBaseTarget->IsOnCurrentThread(&currentThread);
    MOZ_ASSERT(currentThread);
#endif

    {
      MutexAutoLock lock(mMutex);

      // We only check the name of an executor runnable when we know there is something
      // in the queue, so this should never fail.
      event = mEventQueue.PeekEvent(lock);
      MOZ_ALWAYS_TRUE(event);
    }

    if (nsCOMPtr<nsINamed> named = do_QueryInterface(event)) {
      nsresult rv = named->GetName(aName);
      return rv;
    }

    aName.AssignLiteral("non-nsINamed ThrottledEventQueue runnable");
    return NS_OK;
  }

  void
  ExecuteRunnable()
  {
    // Any thread
    nsCOMPtr<nsIRunnable> event;

#ifdef DEBUG
    bool currentThread = false;
    mBaseTarget->IsOnCurrentThread(&currentThread);
    MOZ_ASSERT(currentThread);
#endif

    {
      MutexAutoLock lock(mMutex);

      // We only dispatch an executor runnable when we know there is something
      // in the queue, so this should never fail.
      event = mEventQueue.GetEvent(nullptr, lock);
      MOZ_ASSERT(event);

      // If there are more events in the queue, then dispatch the next
      // executor.  We do this now, before running the event, because
      // the event might spin the event loop and we don't want to stall
      // the queue.
      if (mEventQueue.HasReadyEvent(lock)) {
        // Dispatch the next base target runnable to attempt to execute
        // the next throttled event.  We must do this before executing
        // the event in case the event spins the event loop.
        MOZ_ALWAYS_SUCCEEDS(
          mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL));
      }

      // Otherwise the queue is empty and we can stop dispatching the
      // executor.
      else {
        // Break the Executor::mInner / Inner::mExecutor reference loop.
        mExecutor = nullptr;
        mIdleCondVar.NotifyAll();
      }
    }

    // Execute the event now that we have unlocked.
    Unused << event->Run();
  }

public:
  static already_AddRefed<Inner>
  Create(nsISerialEventTarget* aBaseTarget)
  {
    MOZ_ASSERT(NS_IsMainThread());
    MOZ_ASSERT(ClearOnShutdown_Internal::sCurrentShutdownPhase == ShutdownPhase::NotInShutdown);

    RefPtr<Inner> ref = new Inner(aBaseTarget);
    return ref.forget();
  }

  bool
  IsEmpty() const
  {
    // Any thread
    return Length() == 0;
  }

  uint32_t
  Length() const
  {
    // Any thread
    MutexAutoLock lock(mMutex);
    return mEventQueue.Count(lock);
  }

  void
  AwaitIdle() const
  {
    // Any thread, except the main thread or our base target.  Blocking the
    // main thread is forbidden.  Blocking the base target is guaranteed to
    // produce a deadlock.
    MOZ_ASSERT(!NS_IsMainThread());
#ifdef DEBUG
    bool onBaseTarget = false;
    Unused << mBaseTarget->IsOnCurrentThread(&onBaseTarget);
    MOZ_ASSERT(!onBaseTarget);
#endif

    MutexAutoLock lock(mMutex);
    while (mExecutor) {
      mIdleCondVar.Wait();
    }
  }

  nsresult
  DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
  {
    // Any thread
    nsCOMPtr<nsIRunnable> r = aEvent;
    return Dispatch(r.forget(), aFlags);
  }

  nsresult
  Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
  {
    MOZ_ASSERT(aFlags == NS_DISPATCH_NORMAL ||
               aFlags == NS_DISPATCH_AT_END);

    // Any thread
    MutexAutoLock lock(mMutex);

    // We are not currently processing events, so we must start
    // operating on our base target.  This is fallible, so do
    // it first.  Our lock will prevent the executor from accessing
    // the event queue before we add the event below.
    if (!mExecutor) {
      // Note, this creates a ref cycle keeping the inner alive
      // until the queue is drained.
      mExecutor = new Executor(this);
      nsresult rv = mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL);
      if (NS_WARN_IF(NS_FAILED(rv))) {
        mExecutor = nullptr;
        return rv;
      }
    }

    // Only add the event to the underlying queue if are able to
    // dispatch to our base target.
    mEventQueue.PutEvent(std::move(aEvent), EventPriority::Normal, lock);
    return NS_OK;
  }

  nsresult
  DelayedDispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aDelay)
  {
    // The base target may implement this, but we don't.  Always fail
    // to provide consistent behavior.
    return NS_ERROR_NOT_IMPLEMENTED;
  }

  bool
  IsOnCurrentThread()
  {
    return mBaseTarget->IsOnCurrentThread();
  }

  NS_DECL_THREADSAFE_ISUPPORTS
};

NS_IMPL_ISUPPORTS(ThrottledEventQueue::Inner, nsISupports);

NS_IMPL_ISUPPORTS(ThrottledEventQueue,
                  ThrottledEventQueue,
                  nsIEventTarget,
                  nsISerialEventTarget);

ThrottledEventQueue::ThrottledEventQueue(already_AddRefed<Inner> aInner)
  : mInner(aInner)
{
  MOZ_ASSERT(mInner);
}

already_AddRefed<ThrottledEventQueue>
ThrottledEventQueue::Create(nsISerialEventTarget* aBaseTarget)
{
  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(aBaseTarget);

  RefPtr<Inner> inner = Inner::Create(aBaseTarget);
  if (NS_WARN_IF(!inner)) {
    return nullptr;
  }

  RefPtr<ThrottledEventQueue> ref =
    new ThrottledEventQueue(inner.forget());
  return ref.forget();
}

bool
ThrottledEventQueue::IsEmpty() const
{
  return mInner->IsEmpty();
}

uint32_t
ThrottledEventQueue::Length() const
{
  return mInner->Length();
}

void
ThrottledEventQueue::AwaitIdle() const
{
  return mInner->AwaitIdle();
}

NS_IMETHODIMP
ThrottledEventQueue::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
{
  return mInner->DispatchFromScript(aEvent, aFlags);
}

NS_IMETHODIMP
ThrottledEventQueue::Dispatch(already_AddRefed<nsIRunnable> aEvent,
                                     uint32_t aFlags)
{
  return mInner->Dispatch(std::move(aEvent), aFlags);
}

NS_IMETHODIMP
ThrottledEventQueue::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
                                            uint32_t aFlags)
{
  return mInner->DelayedDispatch(std::move(aEvent), aFlags);
}

NS_IMETHODIMP
ThrottledEventQueue::IsOnCurrentThread(bool* aResult)
{
  *aResult = mInner->IsOnCurrentThread();
  return NS_OK;
}

NS_IMETHODIMP_(bool)
ThrottledEventQueue::IsOnCurrentThreadInfallible()
{
  return mInner->IsOnCurrentThread();
}

} // namespace mozilla