Issue #3593, #1197- fix isolate management

src/JsEngine.cpp

Index: src/JsEngine.cpp
diff --git a/src/JsEngine.cpp b/src/JsEngine.cpp
index 98e0e1b0eadf1929f5d536dd400f9f34a4e30633..232cb398541f5cde25eeebedb648bd1b69150f97 100644
--- a/src/JsEngine.cpp
+++ b/src/JsEngine.cpp
@@ -65,8 +65,18 @@ namespace
   };
 }
 
+AdblockPlus::ScopedV8Isolate::ScopedV8Isolate()
+  : isolate(v8::Isolate::New())

[Eric, 2015/08/05 22:29:16]

This is changing the behavior non-trivially, since initializing v8 creates an
isolate by default. Calling 'New()' here would (I think) abandon the default
instance.

Regardless, initialization policy seems like a different issue from getting
order of initialization right.

[sergei, 2015/11/16 16:52:09]

There is no any default isolate. It `v8::Isolate::GetCurrent` returns the
isolate, pointer on which is stored thread local storage.
Sorry, I don't understand about which issues you are talking here. The changes
here are part of updating of v8. In newer v8 one may not use
`v8::Isolate::GetCurrent()` when isolate is not initialized yet, so we need to
create it.

[Eric, 2015/11/17 21:27:43]

Sure there is. See isolate.cc:340
    Isolate* Isolate::default_isolate_ = NULL;
And also see isolate.c:396
    void Isolate::EnsureDefaultIsolate() {

In the current v8 code, each thread has a default isolate. When you call New()
instead of GetCurrent(), you're changing behavior by discarding the default
isolate that's initialized on the thread.

After quite a bit of research, it seems that there's no longer a default
isolate:
    https://code.google.com/p/chromium/issues/detail?id=359977

I'd suggest that a line of documentation is in order here.

+{
+}
+
+AdblockPlus::ScopedV8Isolate::~ScopedV8Isolate()
+{
+  isolate->Dispose();

[Eric, 2015/08/05 22:29:16]

If we initialize with 'New()' rather than 'GetCurrent()', then we need to call
'Dispose()'. Otherwise we don't.

Even if we do need it, we don't need it in a destructor of a separate class. v8,
as a Google creature, doesn't do RAII or throw its own exceptions.

[sergei, 2015/11/16 16:52:09]

Do you mean we should create a typedef on `std::shared_ptr` for example?
Google may be does not do RAII in v8 but it's a reason for us to avoid RAII.

[Eric, 2015/11/17 21:27:43]

No.
And I'm not arguing against RAII.

I am arguing that we don't need a separate class for a destructor to call
Dispose() in. Such a destructor can be called in the destructor for JsEngine. 

My point about Google's practice is that Isolate::New() does not throw, so we
can just initialize it in the constructor for JsEngine without any trouble.

+  isolate = nullptr;
+}
+
 AdblockPlus::JsEngine::JsEngine()
-  : isolate(v8::Isolate::GetCurrent())
 {
 }
 
@@ -76,7 +86,8 @@ AdblockPlus::JsEnginePtr AdblockPlus::JsEngine::New(const AppInfo& appInfo)
   JsEnginePtr result(new JsEngine());
 
   const v8::Locker locker(result->isolate);
-  const v8::HandleScope handleScope;
+  const v8::Isolate::Scope isolateScope(result->isolate);
+  const v8::HandleScope handleScope(result->isolate);
 
   result->context.reset(result->isolate, v8::Context::New(result->isolate));
   v8::Local<v8::Object> globalContext = v8::Local<v8::Context>::New(