Switch MessagePumpForIO to use completion ports on Windows....

base/message_pump_win.cc

Index: base/message_pump_win.cc
===================================================================
--- base/message_pump_win.cc	(revision 4870)
+++ base/message_pump_win.cc	(working copy)
@@ -11,41 +11,6 @@
 
 using base::Time;
 
-namespace {
-
-class HandlerData : public base::MessagePumpForIO::Watcher {
- public:
-  typedef base::MessagePumpForIO::IOHandler IOHandler;
-  HandlerData(OVERLAPPED* context, IOHandler* handler)
-      : context_(context), handler_(handler) {}
-  ~HandlerData() {}
-
-  virtual void OnObjectSignaled(HANDLE object);
-
- private:
-  OVERLAPPED* context_;
-  IOHandler* handler_;
-  
-  DISALLOW_COPY_AND_ASSIGN(HandlerData);
-};
-
-void HandlerData::OnObjectSignaled(HANDLE object) {
-  DCHECK(object == context_->hEvent);
-  DWORD transfered;
-  DWORD error = ERROR_SUCCESS;
-  BOOL ret = GetOverlappedResult(NULL, context_, &transfered, FALSE);
-  if (!ret) {
-    error = GetLastError();
-    DCHECK(ERROR_HANDLE_EOF == error || ERROR_BROKEN_PIPE == error);
-    transfered = 0;
-  }
-
-  ResetEvent(context_->hEvent);
-  handler_->OnIOCompleted(context_, transfered, error);
-}
-
-}  // namespace
-
 namespace base {
 
 static const wchar_t kWndClass[] = L"Chrome_MessagePumpWindow";
@@ -54,24 +19,9 @@
 // task (a series of such messages creates a continuous task pump).
 static const int kMsgHaveWork = WM_USER + 1;
 
-#ifndef NDEBUG
-// Force exercise of polling model.
-static const int kMaxWaitObjects = 8;
-#else
-static const int kMaxWaitObjects = MAXIMUM_WAIT_OBJECTS;
-#endif
-
 //-----------------------------------------------------------------------------
 // MessagePumpWin public:
 
-MessagePumpWin::MessagePumpWin() : have_work_(0), state_(NULL) {
-  InitMessageWnd();
-}
-
-MessagePumpWin::~MessagePumpWin() {
-  DestroyWindow(message_hwnd_);
-}
-
 void MessagePumpWin::AddObserver(Observer* observer) {
   observers_.AddObserver(observer);
 }
@@ -88,36 +38,6 @@
   FOR_EACH_OBSERVER(Observer, observers_, DidProcessMessage(msg));
 }
 
-void MessagePumpWin::PumpOutPendingPaintMessages() {
-  // If we are being called outside of the context of Run, then don't try to do
-  // any work.
-  if (!state_)
-    return;
-
-  // Create a mini-message-pump to force immediate processing of only Windows
-  // WM_PAINT messages.  Don't provide an infinite loop, but do enough peeking
-  // to get the job done.  Actual common max is 4 peeks, but we'll be a little
-  // safe here.
-  const int kMaxPeekCount = 20;
-  bool win2k = win_util::GetWinVersion() <= win_util::WINVERSION_2000;
-  int peek_count;
-  for (peek_count = 0; peek_count < kMaxPeekCount; ++peek_count) {
-    MSG msg;
-    if (win2k) {
-      if (!PeekMessage(&msg, NULL, WM_PAINT, WM_PAINT, PM_REMOVE))
-        break;
-    } else {
-      if (!PeekMessage(&msg, NULL, 0, 0, PM_REMOVE | PM_QS_PAINT))
-        break;
-    }
-    ProcessMessageHelper(msg);
-    if (state_->should_quit)  // Handle WM_QUIT.
-      break;
-  }
-  // Histogram what was really being used, to help to adjust kMaxPeekCount.
-  DHISTOGRAM_COUNTS(L"Loop.PumpOutPendingPaintMessages Peeks", peek_count);
-}
-
 void MessagePumpWin::RunWithDispatcher(
     Delegate* delegate, Dispatcher* dispatcher) {
   RunState s;
@@ -139,7 +59,38 @@
   state_->should_quit = true;
 }
 
-void MessagePumpWin::ScheduleWork() {
+//-----------------------------------------------------------------------------
+// MessagePumpWin protected:
+
+int MessagePumpWin::GetCurrentDelay() const {
+  if (delayed_work_time_.is_null())
+    return -1;
+
+  // Be careful here.  TimeDelta has a precision of microseconds, but we want a
+  // value in milliseconds.  If there are 5.5ms left, should the delay be 5 or
+  // 6?  It should be 6 to avoid executing delayed work too early.
+  double timeout = ceil((delayed_work_time_ - Time::Now()).InMillisecondsF());
+
+  // If this value is negative, then we need to run delayed work soon.
+  int delay = static_cast<int>(timeout);
+  if (delay < 0)
+    delay = 0;
+
+  return delay;
+}
+
+//-----------------------------------------------------------------------------
+// MessagePumpForUI public:
+
+MessagePumpForUI::MessagePumpForUI() {
+  InitMessageWnd();
+}
+
+MessagePumpForUI::~MessagePumpForUI() {
+  DestroyWindow(message_hwnd_);
+}
+
+void MessagePumpForUI::ScheduleWork() {
   if (InterlockedExchange(&have_work_, 1))
     return;  // Someone else continued the pumping.
 
@@ -147,7 +98,7 @@
   PostMessage(message_hwnd_, kMsgHaveWork, reinterpret_cast<WPARAM>(this), 0);
 }
 
-void MessagePumpWin::ScheduleDelayedWork(const Time& delayed_work_time) {
+void MessagePumpForUI::ScheduleDelayedWork(const Time& delayed_work_time) {
   //
   // We would *like* to provide high resolution timers.  Windows timers using
   // SetTimer() have a 10ms granularity.  We have to use WM_TIMER as a wakeup
@@ -180,24 +131,110 @@
   SetTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this), delay_msec, NULL);
 }
 
+void MessagePumpForUI::PumpOutPendingPaintMessages() {
+  // If we are being called outside of the context of Run, then don't try to do
+  // any work.
+  if (!state_)
+    return;
+
+  // Create a mini-message-pump to force immediate processing of only Windows
+  // WM_PAINT messages.  Don't provide an infinite loop, but do enough peeking
+  // to get the job done.  Actual common max is 4 peeks, but we'll be a little
+  // safe here.
+  const int kMaxPeekCount = 20;
+  bool win2k = win_util::GetWinVersion() <= win_util::WINVERSION_2000;
+  int peek_count;
+  for (peek_count = 0; peek_count < kMaxPeekCount; ++peek_count) {
+    MSG msg;
+    if (win2k) {
+      if (!PeekMessage(&msg, NULL, WM_PAINT, WM_PAINT, PM_REMOVE))
+        break;
+    } else {
+      if (!PeekMessage(&msg, NULL, 0, 0, PM_REMOVE | PM_QS_PAINT))
+        break;
+    }
+    ProcessMessageHelper(msg);
+    if (state_->should_quit)  // Handle WM_QUIT.
+      break;
+  }
+  // Histogram what was really being used, to help to adjust kMaxPeekCount.
+  DHISTOGRAM_COUNTS(L"Loop.PumpOutPendingPaintMessages Peeks", peek_count);
+}
+
 //-----------------------------------------------------------------------------
-// MessagePumpWin protected:
+// MessagePumpForUI private:
 
 // static
-LRESULT CALLBACK MessagePumpWin::WndProcThunk(
+LRESULT CALLBACK MessagePumpForUI::WndProcThunk(
     HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) {
   switch (message) {
     case kMsgHaveWork:
-      reinterpret_cast<MessagePumpWin*>(wparam)->HandleWorkMessage();
+      reinterpret_cast<MessagePumpForUI*>(wparam)->HandleWorkMessage();
       break;
     case WM_TIMER:
-      reinterpret_cast<MessagePumpWin*>(wparam)->HandleTimerMessage();
+      reinterpret_cast<MessagePumpForUI*>(wparam)->HandleTimerMessage();
       break;
   }
   return DefWindowProc(hwnd, message, wparam, lparam);
 }
 
-void MessagePumpWin::InitMessageWnd() {
+void MessagePumpForUI::DoRunLoop() {
+  // IF this was just a simple PeekMessage() loop (servicing all possible work
+  // queues), then Windows would try to achieve the following order according
+  // to MSDN documentation about PeekMessage with no filter):
+  //    * Sent messages
+  //    * Posted messages
+  //    * Sent messages (again)
+  //    * WM_PAINT messages
+  //    * WM_TIMER messages
+  //
+  // Summary: none of the above classes is starved, and sent messages has twice
+  // the chance of being processed (i.e., reduced service time).
+
+  for (;;) {
+    // If we do any work, we may create more messages etc., and more work may
+    // possibly be waiting in another task group.  When we (for example)
+    // ProcessNextWindowsMessage(), there is a good chance there are still more
+    // messages waiting.  On the other hand, when any of these methods return
+    // having done no work, then it is pretty unlikely that calling them again
+    // quickly will find any work to do.  Finally, if they all say they had no
+    // work, then it is a good time to consider sleeping (waiting) for more
+    // work.
+
+    bool more_work_is_plausible = ProcessNextWindowsMessage();
+    if (state_->should_quit)
+      break;
+
+    more_work_is_plausible |= state_->delegate->DoWork();
+    if (state_->should_quit)
+      break;
+
+    more_work_is_plausible |=
+        state_->delegate->DoDelayedWork(&delayed_work_time_);
+    // If we did not process any delayed work, then we can assume that our
+    // existing WM_TIMER if any will fire when delayed work should run.  We
+    // don't want to disturb that timer if it is already in flight.  However,
+    // if we did do all remaining delayed work, then lets kill the WM_TIMER.
+    if (more_work_is_plausible && delayed_work_time_.is_null())
+      KillTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this));
+    if (state_->should_quit)
+      break;
+
+    if (more_work_is_plausible)
+      continue;
+
+    more_work_is_plausible = state_->delegate->DoIdleWork();
+    if (state_->should_quit)
+      break;
+
+    if (more_work_is_plausible)
+      continue;
+
+    WaitForWork();  // Wait (sleep) until we have work to do again.
+  }
+}
+
+void MessagePumpForUI::InitMessageWnd() {
   HINSTANCE hinst = GetModuleHandle(NULL);
 
   WNDCLASSEX wc = {0};
@@ -212,7 +249,41 @@
   DCHECK(message_hwnd_);
 }
 
-void MessagePumpWin::HandleWorkMessage() {
+void MessagePumpForUI::WaitForWork() {
+  // Wait until a message is available, up to the time needed by the timer
+  // manager to fire the next set of timers.
+  int delay = GetCurrentDelay();
+  if (delay < 0)  // Negative value means no timers waiting.
+    delay = INFINITE;
+
+  DWORD result;
+  result = MsgWaitForMultipleObjectsEx(0, NULL, delay, QS_ALLINPUT,
+                                       MWMO_INPUTAVAILABLE);
+
+  if (WAIT_OBJECT_0 == result) {
+    // A WM_* message is available.
+    // If a parent child relationship exists between windows across threads
+    // then their thread inputs are implicitly attached.
+    // This causes the MsgWaitForMultipleObjectsEx API to return indicating
+    // that messages are ready for processing (specifically mouse messages
+    // intended for the child window. Occurs if the child window has capture)
+    // The subsequent PeekMessages call fails to return any messages thus
+    // causing us to enter a tight loop at times.
+    // The WaitMessage call below is a workaround to give the child window
+    // sometime to process its input messages.
+    MSG msg = {0};
+    DWORD queue_status = GetQueueStatus(QS_MOUSE);
+    if (HIWORD(queue_status) & QS_MOUSE &&
+       !PeekMessage(&msg, NULL, WM_MOUSEFIRST, WM_MOUSELAST, PM_NOREMOVE)) {
+      WaitMessage();
+    }
+    return;
+  }
+
+  DCHECK_NE(WAIT_FAILED, result) << GetLastError();
+}
+
+void MessagePumpForUI::HandleWorkMessage() {
   // If we are being called outside of the context of Run, then don't try to do
   // any work.  This could correspond to a MessageBox call or something of that
   // sort.
@@ -233,7 +304,7 @@
     ScheduleWork();
 }
 
-void MessagePumpWin::HandleTimerMessage() {
+void MessagePumpForUI::HandleTimerMessage() {
   KillTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this));
 
   // If we are being called outside of the context of Run, then don't do
@@ -249,7 +320,7 @@
   }
 }
 
-bool MessagePumpWin::ProcessNextWindowsMessage() {
+bool MessagePumpForUI::ProcessNextWindowsMessage() {
   // If there are sent messages in the queue then PeekMessage internally
   // dispatches the message and returns false. We return true in this
   // case to ensure that the message loop peeks again instead of calling
@@ -266,7 +337,7 @@
   return sent_messages_in_queue;
 }
 
-bool MessagePumpWin::ProcessMessageHelper(const MSG& msg) {
+bool MessagePumpForUI::ProcessMessageHelper(const MSG& msg) {
   if (WM_QUIT == msg.message) {
     // Repost the QUIT message so that it will be retrieved by the primary
     // GetMessage() loop.
@@ -293,7 +364,7 @@
   return true;
 }
 
-bool MessagePumpWin::ProcessPumpReplacementMessage() {
+bool MessagePumpForUI::ProcessPumpReplacementMessage() {
   // When we encounter a kMsgHaveWork message, this method is called to peek
   // and process a replacement message, such as a WM_PAINT or WM_TIMER.  The
   // goal is to make the kMsgHaveWork as non-intrusive as possible, even though
@@ -307,7 +378,7 @@
   bool have_message = (0 != PeekMessage(&msg, NULL, 0, 0, PM_REMOVE));
   DCHECK(!have_message || kMsgHaveWork != msg.message ||
          msg.hwnd != message_hwnd_);
-  
+
   // Since we discarded a kMsgHaveWork message, we must update the flag.
   InterlockedExchange(&have_work_, 0);
 
@@ -318,233 +389,63 @@
   return have_message && ProcessMessageHelper(msg);
 }
 
-int MessagePumpWin::GetCurrentDelay() const {
-  if (delayed_work_time_.is_null())
-    return -1;
-
-  // Be careful here.  TimeDelta has a precision of microseconds, but we want a
-  // value in milliseconds.  If there are 5.5ms left, should the delay be 5 or
-  // 6?  It should be 6 to avoid executing delayed work too early.
-  double timeout = ceil((delayed_work_time_ - Time::Now()).InMillisecondsF());
-
-  // If this value is negative, then we need to run delayed work soon.
-  int delay = static_cast<int>(timeout);
-  if (delay < 0)
-    delay = 0;
-
-  return delay;
-}
-
 //-----------------------------------------------------------------------------
-// MessagePumpForUI private:
+// MessagePumpForIO public:
 
-void MessagePumpForUI::DoRunLoop() {
-  // IF this was just a simple PeekMessage() loop (servicing all possible work
-  // queues), then Windows would try to achieve the following order according
-  // to MSDN documentation about PeekMessage with no filter):
-  //    * Sent messages
-  //    * Posted messages
-  //    * Sent messages (again)
-  //    * WM_PAINT messages
-  //    * WM_TIMER messages
-  //
-  // Summary: none of the above classes is starved, and sent messages has twice
-  // the chance of being processed (i.e., reduced service time).
-
-  for (;;) {
-    // If we do any work, we may create more messages etc., and more work may
-    // possibly be waiting in another task group.  When we (for example)
-    // ProcessNextWindowsMessage(), there is a good chance there are still more
-    // messages waiting.  On the other hand, when any of these methods return
-    // having done no work, then it is pretty unlikely that calling them again
-    // quickly will find any work to do.  Finally, if they all say they had no
-    // work, then it is a good time to consider sleeping (waiting) for more
-    // work.
-
-    bool more_work_is_plausible = ProcessNextWindowsMessage();
-    if (state_->should_quit)
-      break;
-
-    more_work_is_plausible |= state_->delegate->DoWork();
-    if (state_->should_quit)
-      break;
-
-    more_work_is_plausible |=
-        state_->delegate->DoDelayedWork(&delayed_work_time_);
-    // If we did not process any delayed work, then we can assume that our
-    // existing WM_TIMER if any will fire when delayed work should run.  We
-    // don't want to disturb that timer if it is already in flight.  However,
-    // if we did do all remaining delayed work, then lets kill the WM_TIMER.
-    if (more_work_is_plausible && delayed_work_time_.is_null())
-      KillTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this));
-    if (state_->should_quit)
-      break;
-
-    if (more_work_is_plausible)
-      continue;
-
-    more_work_is_plausible = state_->delegate->DoIdleWork();
-    if (state_->should_quit)
-      break;
-
-    if (more_work_is_plausible)
-      continue;
-
-    WaitForWork();  // Wait (sleep) until we have work to do again.
-  }
+MessagePumpForIO::MessagePumpForIO() {
+  port_.Set(CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, NULL, 1));
+  DCHECK(port_.IsValid());
 }
 
-void MessagePumpForUI::WaitForWork() {
-  // Wait until a message is available, up to the time needed by the timer
-  // manager to fire the next set of timers.
-  int delay = GetCurrentDelay();
-  if (delay < 0)  // Negative value means no timers waiting.
-    delay = INFINITE;
+void MessagePumpForIO::ScheduleWork() {
+  if (InterlockedExchange(&have_work_, 1))
+    return;  // Someone else continued the pumping.
 
-  DWORD result;
-  result = MsgWaitForMultipleObjectsEx(0, NULL, delay, QS_ALLINPUT,
-                                       MWMO_INPUTAVAILABLE);
-
-  if (WAIT_OBJECT_0 == result) {
-    // A WM_* message is available.
-    // If a parent child relationship exists between windows across threads
-    // then their thread inputs are implicitly attached.
-    // This causes the MsgWaitForMultipleObjectsEx API to return indicating
-    // that messages are ready for processing (specifically mouse messages
-    // intended for the child window. Occurs if the child window has capture)
-    // The subsequent PeekMessages call fails to return any messages thus
-    // causing us to enter a tight loop at times.
-    // The WaitMessage call below is a workaround to give the child window
-    // sometime to process its input messages.
-    MSG msg = {0};
-    DWORD queue_status = GetQueueStatus(QS_MOUSE);
-    if (HIWORD(queue_status) & QS_MOUSE && 
-       !PeekMessage(&msg, NULL, WM_MOUSEFIRST, WM_MOUSELAST, PM_NOREMOVE)) {
-      WaitMessage();
-    }
-    return;  
-  }
-
-  DCHECK_NE(WAIT_FAILED, result) << GetLastError();
+  // Make sure the MessagePump does some work for us.
+  BOOL ret = PostQueuedCompletionStatus(port_, 0,
+                                        reinterpret_cast<ULONG_PTR>(this),
+                                        reinterpret_cast<OVERLAPPED*>(this));
+  DCHECK(ret);
 }
 
-//-----------------------------------------------------------------------------
-// MessagePumpForIO public:
-
-void MessagePumpForIO::WatchObject(HANDLE object, Watcher* watcher) {
-  DCHECK(object);
-  DCHECK_NE(object, INVALID_HANDLE_VALUE);
-
-  std::vector<HANDLE>::iterator it =
-      find(objects_.begin(), objects_.end(), object);
-  if (watcher) {
-    if (it == objects_.end()) {
-     static size_t warning_multiple = 1;
-     if (objects_.size() >= warning_multiple * MAXIMUM_WAIT_OBJECTS / 2) {
-       LOG(INFO) << "More than " << warning_multiple * MAXIMUM_WAIT_OBJECTS / 2
-           << " objects being watched";
-       // This DCHECK() is an artificial limitation, meant to warn us if we
-       // start creating too many objects.  It can safely be raised to a higher
-       // level, and the program is designed to handle much larger values.
-       // Before raising this limit, make sure that there is a very good reason
-       // (in your debug testing) to be watching this many objects.
-       DCHECK(2 <= warning_multiple);
-       ++warning_multiple;
-      }
-      objects_.push_back(object);
-      watchers_.push_back(watcher);
-    } else {
-      watchers_[it - objects_.begin()] = watcher;
-    }
-  } else if (it != objects_.end()) {
-    std::vector<HANDLE>::difference_type index = it - objects_.begin();
-    objects_.erase(it);
-    watchers_.erase(watchers_.begin() + index);
-  }
+void MessagePumpForIO::ScheduleDelayedWork(const Time& delayed_work_time) {
+  // We know that we can't be blocked right now since this method can only be
+  // called on the same thread as Run, so we only need to update our record of
+  // how long to sleep when we do sleep.
+  delayed_work_time_ = delayed_work_time;
 }
 
 void MessagePumpForIO::RegisterIOHandler(HANDLE file_handle,
                                          IOHandler* handler) {
-#if 0
-  // TODO(rvargas): This is just to give an idea of what this code will look
-  // like when we actually move to completion ports. Of course, we cannot
-  // do this without calling GetQueuedCompletionStatus().
   ULONG_PTR key = reinterpret_cast<ULONG_PTR>(handler);
   HANDLE port = CreateIoCompletionPort(file_handle, port_, key, 1);
-  if (!port_.IsValid())
-    port_.Set(port);
-#endif
+  DCHECK(port == port_.Get());
 }
 
-void MessagePumpForIO::RegisterIOContext(OVERLAPPED* context,
-                                         IOHandler* handler) {
-  DCHECK(context->hEvent);
-  if (handler) {
-    HandlerData* watcher = new HandlerData(context, handler);
-    WatchObject(context->hEvent, watcher);
-  } else {
-    std::vector<HANDLE>::iterator it =
-      find(objects_.begin(), objects_.end(), context->hEvent);
-
-    if (it == objects_.end()) {
-      NOTREACHED();
-      return;
-    }
-
-    std::vector<HANDLE>::difference_type index = it - objects_.begin();
-    objects_.erase(it);
-    delete watchers_[index];
-    watchers_.erase(watchers_.begin() + index);
-  }
-}
-
 //-----------------------------------------------------------------------------
 // MessagePumpForIO private:
 
 void MessagePumpForIO::DoRunLoop() {
-  // IF this was just a simple PeekMessage() loop (servicing all possible work
-  // queues), then Windows would try to achieve the following order according
-  // to MSDN documentation about PeekMessage with no filter):
-  //    * Sent messages
-  //    * Posted messages
-  //    * Sent messages (again)
-  //    * WM_PAINT messages
-  //    * WM_TIMER messages
-  //
-  // Summary: none of the above classes is starved, and sent messages has twice
-  // the chance of being processed (i.e., reduced service time).
-
   for (;;) {
     // If we do any work, we may create more messages etc., and more work may
     // possibly be waiting in another task group.  When we (for example)
-    // ProcessNextWindowsMessage(), there is a good chance there are still more
-    // messages waiting (same thing for ProcessNextObject(), which responds to
-    // only one signaled object; etc.).  On the other hand, when any of these
-    // methods return having done no work, then it is pretty unlikely that
-    // calling them again quickly will find any work to do.  Finally, if they
-    // all say they had no work, then it is a good time to consider sleeping
-    // (waiting) for more work.
+    // WaitForIOCompletion(), there is a good chance there are still more
+    // messages waiting.  On the other hand, when any of these methods return
+    // having done no work, then it is pretty unlikely that calling them
+    // again quickly will find any work to do.  Finally, if they all say they
+    // had no work, then it is a good time to consider sleeping (waiting) for
+    // more work.
 
-    bool more_work_is_plausible = ProcessNextWindowsMessage();
+    bool more_work_is_plausible = state_->delegate->DoWork();
     if (state_->should_quit)
       break;
 
-    more_work_is_plausible |= state_->delegate->DoWork();
+    more_work_is_plausible |= WaitForIOCompletion(0, NULL);
     if (state_->should_quit)
       break;
 
-    more_work_is_plausible |= ProcessNextObject();
-    if (state_->should_quit)
-      break;
-
     more_work_is_plausible |=
         state_->delegate->DoDelayedWork(&delayed_work_time_);
-    // If we did not process any delayed work, then we can assume that our
-    // existing WM_TIMER if any will fire when delayed work should run.  We
-    // don't want to disturb that timer if it is already in flight.  However,
-    // if we did do all remaining delayed work, then lets kill the WM_TIMER.
-    if (more_work_is_plausible && delayed_work_time_.is_null())
-      KillTimer(message_hwnd_, reinterpret_cast<UINT_PTR>(this));
     if (state_->should_quit)
       break;
 
@@ -558,141 +459,92 @@
     if (more_work_is_plausible)
       continue;
 
-    // We service APCs in WaitForWork, without returning.
     WaitForWork();  // Wait (sleep) until we have work to do again.
   }
 }
 
-// If we handle more than the OS limit on the number of objects that can be
-// waited for, we'll need to poll (sequencing through subsets of the objects
-// that can be passed in a single OS wait call).  The following is the polling
-// interval used in that (unusual) case. (I don't have a lot of justifcation
-// for the specific value, but it needed to be short enough that it would not
-// add a lot of latency, and long enough that we wouldn't thrash the CPU for no
-// reason... especially considering the silly user probably has a million tabs
-// open, etc.)
-static const int kMultipleWaitPollingInterval = 20;
-
+// Wait until IO completes, up to the time needed by the timer manager to fire
+// the next set of timers.
 void MessagePumpForIO::WaitForWork() {
-  // Wait until either an object is signaled or a message is available.  Handle
-  // (without returning) any APCs (only the IO thread currently has APCs.)
+  // We do not support nested IO message loops. This is to avoid messy
+  // recursion problems.
+  DCHECK(state_->run_depth == 1) << "Cannot nest an IO message loop!";
 
-  // We do not support nested message loops when we have watched objects.  This
-  // is to avoid messy recursion problems.
-  DCHECK(objects_.empty() || state_->run_depth == 1) <<
-      "Cannot nest a message loop when there are watched objects!";
+  int timeout = GetCurrentDelay();
+  if (timeout < 0)  // Negative value means no timers waiting.
+    timeout = INFINITE;
 
-  int wait_flags = MWMO_ALERTABLE | MWMO_INPUTAVAILABLE;
+  WaitForIOCompletion(timeout, NULL);
+}
 
-  bool use_polling = false;  // Poll if too many objects for one OS Wait call.
-  for (;;) {
-    // Do initialization here, in case APC modifies object list.
-    size_t total_objs = objects_.size();
+bool MessagePumpForIO::WaitForIOCompletion(DWORD timeout, IOHandler* filter) {
+  IOItem item;
+  if (completed_io_.empty() || !MatchCompletedIOItem(filter, &item)) {
+    // We have to ask the system for another IO completion.
+    if (!GetIOItem(timeout, &item))
+      return false;
 
-    int delay;
-    size_t polling_index = 0;  // The first unprocessed object index.
-    do {
-      size_t objs_len =
-          (polling_index < total_objs) ? total_objs - polling_index : 0;
-      if (objs_len >= MAXIMUM_WAIT_OBJECTS) {
-        objs_len = MAXIMUM_WAIT_OBJECTS - 1;
-        use_polling = true;
-      }
-      HANDLE* objs = objs_len ? polling_index + &objects_.front() : NULL;
+    if (ProcessInternalIOItem(item))
+      return true;
+  }
 
-      // Only wait up to the time needed by the timer manager to fire the next
-      // set of timers.
-      delay = GetCurrentDelay();
-      if (use_polling && delay > kMultipleWaitPollingInterval)
-        delay = kMultipleWaitPollingInterval;
-      if (delay < 0)  // Negative value means no timers waiting.
-        delay = INFINITE;
-
-      DWORD result;
-      result = MsgWaitForMultipleObjectsEx(static_cast<DWORD>(objs_len), objs,
-                                           delay, QS_ALLINPUT, wait_flags);
-
-      if (WAIT_IO_COMPLETION == result) {
-        // We'll loop here when we service an APC.  At it currently stands,
-        // *ONLY* the IO thread uses *any* APCs, so this should have no impact
-        // on the UI thread.
-        break;  // Break to outer loop, and waitforwork() again.
-      }
-
-      // Use unsigned type to simplify range detection;
-      size_t signaled_index = result - WAIT_OBJECT_0;
-      if (signaled_index < objs_len) {
-        SignalWatcher(polling_index + signaled_index);
-        return;  // We serviced a signaled object.
-      }
-
-      if (objs_len == signaled_index)
-        return;  // A WM_* message is available.
-
-      DCHECK_NE(WAIT_FAILED, result) << GetLastError();
-
-      DCHECK(!objs || result == WAIT_TIMEOUT);
-      if (!use_polling)
-        return;
-      polling_index += objs_len;
-    } while (polling_index < total_objs);
-    // For compatibility, we didn't return sooner.  This made us do *some* wait
-    // call(s) before returning. This will probably change in next rev.
-    if (!delay || !GetCurrentDelay())
-      return;  // No work done, but timer is ready to fire.
+  if (item.context->handler) {
+    if (filter && item.handler != filter) {
+      // Save this item for later
+      completed_io_.push_back(item);
+    } else {
+      DCHECK(item.context->handler == item.handler);
+      item.handler->OnIOCompleted(item.context, item.bytes_transfered,
+                                  item.error);
+    }
+  } else {
+    // The handler must be gone by now, just cleanup the mess.
+    delete item.context;
   }
+  return true;
 }
 
-bool MessagePumpForIO::ProcessNextObject() {
-  size_t total_objs = objects_.size();
-  if (!total_objs) {
-    return false;
+// Asks the OS for another IO completion result.
+bool MessagePumpForIO::GetIOItem(DWORD timeout, IOItem* item) {
+  memset(item, 0, sizeof(*item));
+  ULONG_PTR key = NULL;
+  OVERLAPPED* overlapped = NULL;
+  if (!GetQueuedCompletionStatus(port_.Get(), &item->bytes_transfered, &key,
+                                 &overlapped, timeout)) {
+    if (!overlapped)
+      return false;  // Nothing in the queue.
+    item->error = GetLastError();
+    item->bytes_transfered = 0;
   }
 
-  size_t polling_index = 0;  // The first unprocessed object index.
-  do {
-    DCHECK(polling_index < total_objs);
-    size_t objs_len = total_objs - polling_index;
-    if (objs_len >= kMaxWaitObjects)
-      objs_len = kMaxWaitObjects - 1;
-    HANDLE* objs = polling_index + &objects_.front();
+  item->handler = reinterpret_cast<IOHandler*>(key);
+  item->context = reinterpret_cast<IOContext*>(overlapped);
+  return true;
+}
 
-    // Identify 1 pending object, or allow an IO APC to be completed.
-    DWORD result = WaitForMultipleObjectsEx(static_cast<DWORD>(objs_len), objs,
-                                            FALSE,    // 1 signal is sufficient.
-                                            0,        // Wait 0ms.
-                                            false);  // Not alertable (no APC).
+bool MessagePumpForIO::ProcessInternalIOItem(const IOItem& item) {
+  if (this == reinterpret_cast<MessagePumpForIO*>(item.context) &&
+      this == reinterpret_cast<MessagePumpForIO*>(item.handler)) {
+    // This is our internal completion.
+    DCHECK(!item.bytes_transfered);
+    InterlockedExchange(&have_work_, 0);
+    return true;
+  }
+  return false;
+}
 
-    // Use unsigned type to simplify range detection;
-    size_t signaled_index = result - WAIT_OBJECT_0;
-    if (signaled_index < objs_len) {
-      SignalWatcher(polling_index + signaled_index);
-      return true;  // We serviced a signaled object.
+// Returns a completion item that was previously received.
+bool MessagePumpForIO::MatchCompletedIOItem(IOHandler* filter, IOItem* item) {
+  DCHECK(!completed_io_.empty());
+  for (std::list<IOItem>::iterator it = completed_io_.begin();
+       it != completed_io_.end(); ++it) {
+    if (!filter || it->handler == filter) {
+      *item = *it;
+      completed_io_.erase(it);
+      return true;
     }
-
-    // If an handle is invalid, it will be WAIT_FAILED.
-    DCHECK_EQ(WAIT_TIMEOUT, result) << GetLastError();
-    polling_index += objs_len;
-  } while (polling_index < total_objs);
-  return false;  // We serviced nothing.
+  }
+  return false;
 }
 
-bool MessagePumpForIO::SignalWatcher(size_t object_index) {
-  // Signal the watcher corresponding to the given index.
-
-  DCHECK(objects_.size() > object_index);
-
-  // On reception of OnObjectSignaled() to a Watcher object, it may call
-  // WatchObject(). watchers_ and objects_ will be modified. This is expected,
-  // so don't be afraid if, while tracing a OnObjectSignaled() function, the
-  // corresponding watchers_[result] is non-existant.
-  watchers_[object_index]->OnObjectSignaled(objects_[object_index]);
-
-  // Signaled objects tend to be removed from the watch list, and then added
-  // back (appended).  As a result, they move to the end of the objects_ array,
-  // and this should make their service "fair" (no HANDLEs should be starved).
-
-  return true;
-}
-
 }  // namespace base