Do V8 GC ASAP if system memory is pressured

base/memory/memory_pressure_listener.h

Index: base/memory/memory_pressure_listener.h
diff --git a/base/memory/memory_pressure_listener.h b/base/memory/memory_pressure_listener.h
index a6ce702ccb0f68df89a80b53934a884f4463926f..2c437c8e3bbce9ed7ac5f640d7e634272111ba63 100644
--- a/base/memory/memory_pressure_listener.h
+++ b/base/memory/memory_pressure_listener.h
@@ -12,7 +12,9 @@
 
 #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 {
 
@@ -45,6 +47,29 @@ namespace base {
 //
 class BASE_EXPORT MemoryPressureListener {
  public:
+  // ObserverListThreadSafe is RefCountedThreadSafe, this traits is needed

[esprehn, 2016/03/25 09:20:35]

Why does this all need to be moved into the header?

[hong.zheng, 2016/04/01 09:22:34]

According to Mark's comments

+  // 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;
+    }
+  };
+
+  struct MemoryPressureListenerObservers {
+    LazyInstance<ObserverListThreadSafe<MemoryPressureListener>,
+        LeakyLazyObserverListTraits> observers_;
+    LazyInstance<ObserverList<MemoryPressureListener>> sync_observers_;
+    base::LazyInstance<base::Lock>::Leaky sync_observers_lock_;
+  };
+
   // A Java counterpart will be generated for this enum.
   // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.base
   enum MemoryPressureLevel {
@@ -64,9 +89,14 @@ class BASE_EXPORT MemoryPressureListener {
   };
 
   typedef base::Callback<void(MemoryPressureLevel)> MemoryPressureCallback;
+  typedef base::Callback<void(MemoryPressureLevel)> SyncMemoryPressureCallback;
 
   explicit MemoryPressureListener(
       const MemoryPressureCallback& memory_pressure_callback);
+  explicit MemoryPressureListener(
+      const MemoryPressureCallback& memory_pressure_callback,
+      const SyncMemoryPressureCallback& sync_memory_pressure_callback);
+
   ~MemoryPressureListener();
 
   // Intended for use by the platform specific implementation.
@@ -82,10 +112,14 @@ class BASE_EXPORT MemoryPressureListener {
 
  private:
   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_;
+
+  static MemoryPressureListenerObservers* g_observers;

[esprehn, 2016/03/25 09:20:35]

why does this need to be moved into the header?

[Mark Mentovai, 2016/03/25 16:59:31]

esprehn wrote:
I said in
https://codereview.chromium.org/1749073002/diff/20001/base/memory/memory_pres...
that I thought that it was gross that this singleton object kept some of its
state in member variables and some of its state in globals.

But what was done here is not what I meant at all. This just moved the global to
a static class member, which is the same thing (and private static data usually
doesn’t need to be a class member at all). So no, this is wrong.

I meant that this should stop being static. And when things like this move
around, it’s appropriate to fix the name: the g_ is no longer applicable, but a
trailing underscore is.

[hong.zheng, 2016/04/01 09:22:34]

I am sorry to misunderstand Mark's comments before. I have updated the CL
according to renewed understanding, 

 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureListener);
 };