Do V8 GC ASAP if system memory is pressured

base/memory/memory_pressure_listener.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // MemoryPressure provides static APIs for handling memory pressure on
 // platforms that have such signals, such as Android and ChromeOS.
 // The app will try to discard buffers that aren't deemed essential (individual
 // modules will implement their own policy).
 
 #ifndef BASE_MEMORY_MEMORY_PRESSURE_LISTENER_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_LISTENER_H_
 
 #include "base/base_export.h"
 #include "base/callback.h"
+#include "base/lazy_instance.h"
 #include "base/macros.h"
+#include "base/observer_list_threadsafe.h"
 
 namespace base {
 
 // To start listening, create a new instance, passing a callback to a
 // function that takes a MemoryPressureLevel parameter. To stop listening,
 // simply delete the listener object. The implementation guarantees
 // that the callback will always be called on the thread that created
 // the listener.
 // Note that even on the same thread, the callback is not guaranteed to be
 // called synchronously within the system memory pressure broadcast.
@@ skipping 18 matching lines @@
 //    delete my_listener;
 //
 class BASE_EXPORT MemoryPressureListener {
  public:
   // A Java counterpart will be generated for this enum.
   // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.base
   enum MemoryPressureLevel {
     // No problems, there is enough memory to use. This event is not sent via
     // callback, but the enum is used in other places to find out the current
     // state of the system.
-    MEMORY_PRESSURE_LEVEL_NONE = -1,
+    MEMORY_PRESSURE_LEVEL_NONE,
 
     // Modules are advised to free buffers that are cheap to re-allocate and not
     // immediately needed.
-    MEMORY_PRESSURE_LEVEL_MODERATE = 0,
+    MEMORY_PRESSURE_LEVEL_MODERATE,
 
     // At this level, modules are advised to free all possible memory.  The
     // alternative is to be killed by the system, which means all memory will
     // have to be re-created, plus the cost of a cold start.
-    MEMORY_PRESSURE_LEVEL_CRITICAL = 2,
+    MEMORY_PRESSURE_LEVEL_CRITICAL,
   };
 
   typedef base::Callback<void(MemoryPressureLevel)> MemoryPressureCallback;
+  typedef base::Callback<void(MemoryPressureLevel)> SyncMemoryPressureCallback;
 
   explicit MemoryPressureListener(
       const MemoryPressureCallback& memory_pressure_callback);
+  explicit MemoryPressureListener(

[kinuko, 2016/04/11 09:30:40]

nit: no need of explicit here

[hong.zheng, 2016/04/15 06:05:57]

Done.

+      const MemoryPressureCallback& memory_pressure_callback,
+      const SyncMemoryPressureCallback& sync_memory_pressure_callback);
+
   ~MemoryPressureListener();
 
   // Intended for use by the platform specific implementation.
   static void NotifyMemoryPressure(MemoryPressureLevel memory_pressure_level);
 
   // These methods should not be used anywhere else but in memory measurement
   // code, where they are intended to maintain stable conditions across
   // measurements.
   static bool AreNotificationsSuppressed();
   static void SetNotificationsSuppressed(bool suppressed);
   static void SimulatePressureNotification(
       MemoryPressureLevel memory_pressure_level);
 
  private:
+  // ObserverListThreadSafe is RefCountedThreadSafe, this traits is needed
+  // to ensure the LazyInstance will hold a reference to it.
+  struct LeakyLazyObserverListTraits :
+      base::internal::LeakyLazyInstanceTraits<
+          ObserverListThreadSafe<MemoryPressureListener> > {
+    static ObserverListThreadSafe<MemoryPressureListener>*
+        New(void* instance) {
+      ObserverListThreadSafe<MemoryPressureListener>* ret =
+          base::internal::LeakyLazyInstanceTraits<
+              ObserverListThreadSafe<MemoryPressureListener>>::New(instance);
+      // Leaky.
+      ret->AddRef();
+      return ret;
+    }
+  };
+
+  MemoryPressureListener();
+
   void Notify(MemoryPressureLevel memory_pressure_level);
+  void SyncNotify(MemoryPressureLevel memory_pressure_level);
 
   static void DoNotifyMemoryPressure(MemoryPressureLevel memory_pressure_level);
 
   MemoryPressureCallback callback_;
+  SyncMemoryPressureCallback sync_memory_pressure_callback_;
+
+  LazyInstance<ObserverListThreadSafe<MemoryPressureListener>,
+      LeakyLazyObserverListTraits> observers_;
+  LazyInstance<ObserverList<MemoryPressureListener>> sync_observers_;
+  base::LazyInstance<base::Lock>::Leaky sync_observers_lock_;
+
+  static MemoryPressureListener* g_listener;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureListener);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_LISTENER_H_