Issue 3594 - remove circular references JsEngine-JsValue-JsEngine

src/JsValue.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #include <vector>
 #include <AdblockPlus.h>
 
 #include "JsContext.h"
 #include "JsError.h"
 #include "Utils.h"
 
-AdblockPlus::JsValue::JsValue(AdblockPlus::JsEnginePtr jsEngine,
-      v8::Handle<v8::Value> value)
-    : jsEngine(jsEngine),
-      value(new v8::Persistent<v8::Value>(jsEngine->GetIsolate(), value))
+AdblockPlus::JsValue::JsValue(const std::weak_ptr<AdblockPlus::JsEngine>& jsEngine,
+  v8::Handle<v8::Value> value)
+  : jsEngine(jsEngine)
+  , value(new v8::Persistent<v8::Value>(Utils::lockJsEngine(jsEngine)->GetIsolate(), value))

[Eric, 2016/11/28 14:21:13]

This use of 'lockJsEngine' has a race condition. The lock keeping the engine
object in existence expires when 'lockJsEngine' returns and is no longer in
place for the dereference operator ->. This is a tiny window for failure, but
it's not thread-safe and may occasionally generate null pointer violations.

[sergei, 2016/11/29 10:45:18]

lockJsEngine returns an object not a reference or pointer, according to the
standard the temporary here will be alive until the finishing of execution of
the whole expression 'value(new
v8::Persistent<v8::Value>(Utils::lockJsEngine(jsEngine)->GetIsolate(), value))'.
So, there is no race condition.

 {
 }
 
 AdblockPlus::JsValue::JsValue(AdblockPlus::JsValue&& src)
-    : jsEngine(src.jsEngine),
+    : jsEngine(move(src.jsEngine)),
       value(std::move(src.value))

[Eric, 2016/11/28 14:21:13]

For consistency, this should either be (1) `std::move` in both places or (2) a
declaration `using std::move;` and `move` in both places. Personally, `move` is
so fundamental to rvalue semantics that constantly putting `std::` in front of
it is distracting.

This compiles, though, so somehow the compiler is allowing it. A future
compiler, however, might not.

[sergei, 2016/11/29 10:45:19]

I have added std:: rather for aesthetic view than for consistency.
Here it works without std:: because of ADL, it's pretty stable and widely used
feature of C++.

 {
 }
 
 AdblockPlus::JsValue::~JsValue()
 {
-  if (value)
+  try
   {
-    value->Dispose();
-    value.reset();
+    JsContext context(jsEngine);
+    if (value)
+    {
+      value->Dispose();
+      value.reset();
+    }
+  }
+  catch (const JsEngine::JsEngineNotAvailableException&)
+  {
   }
 }

[Eric, 2016/11/28 14:21:14]

It's excessive to instantiate a `JsContext` object to ensure the existence of
`jsEngine` for the duration of the destructor. Calling `weak_ptr::lock()` is
enough.

Incidentally, there's also no need to call `value.reset()`, since the destructor
for `value` will take care of that, but that was already there.

AdblockPlus::JsValue::~JsValue()
{
  if (value)
  {
    auto engineGuard = jsEngine.lock();
    if (engineGuard)
    {
      value->Dispose();
    }
  }
}

[sergei, 2016/11/29 10:45:18]

JsContext is not only to ensure that JsEngine is still alive, it also saves from
data race (thanks to JsContext::locker), which happens when `value->Dispose();`
is called from different threads for different values.
Actually there should not be a call of Dispose. That call will be deleted here
after updating of v8. The destructor of v8::UniquePersistent should make that
call, however in the currently used version of v8 it's commented.

 
 bool AdblockPlus::JsValue::IsUndefined() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IsUndefined();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IsUndefined();
 }
 
 bool AdblockPlus::JsValue::IsNull() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IsNull();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IsNull();
 }
 
 bool AdblockPlus::JsValue::IsString() const
 {
-  const JsContext context(jsEngine);
-  v8::Local<v8::Value> value = UnwrapValue();
+  JsContext context(jsEngine);
+  v8::Local<v8::Value> value = UnwrapValue(context.GetJsEngine());
   return value->IsString() || value->IsStringObject();
 }
 
 bool AdblockPlus::JsValue::IsNumber() const
 {
-  const JsContext context(jsEngine);
-  v8::Local<v8::Value> value = UnwrapValue();
+  JsContext context(jsEngine);
+  v8::Local<v8::Value> value = UnwrapValue(context.GetJsEngine());
   return value->IsNumber() || value->IsNumberObject();
 }
 
 bool AdblockPlus::JsValue::IsBool() const
 {
-  const JsContext context(jsEngine);
-  v8::Local<v8::Value> value = UnwrapValue();
+  JsContext context(jsEngine);
+  v8::Local<v8::Value> value = UnwrapValue(context.GetJsEngine());
   return value->IsBoolean() || value->IsBooleanObject();
 }
 
 bool AdblockPlus::JsValue::IsObject() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IsObject();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IsObject();
 }
 
 bool AdblockPlus::JsValue::IsArray() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IsArray();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IsArray();
 }
 
 bool AdblockPlus::JsValue::IsFunction() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IsFunction();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IsFunction();
 }
 
 std::string AdblockPlus::JsValue::AsString() const
 {
-  const JsContext context(jsEngine);
-  return Utils::FromV8String(UnwrapValue());
+  JsContext context(jsEngine);
+  return Utils::FromV8String(UnwrapValue(context.GetJsEngine()));
 }
 
 int64_t AdblockPlus::JsValue::AsInt() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->IntegerValue();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->IntegerValue();
 }
 
 bool AdblockPlus::JsValue::AsBool() const
 {
-  const JsContext context(jsEngine);
-  return UnwrapValue()->BooleanValue();
+  JsContext context(jsEngine);
+  return UnwrapValue(context.GetJsEngine())->BooleanValue();
 }
 
 AdblockPlus::JsValueList AdblockPlus::JsValue::AsList() const
 {
+  JsContext context(jsEngine);
   if (!IsArray())
     throw std::runtime_error("Cannot convert a non-array to list");
 
-  const JsContext context(jsEngine);
   JsValueList result;
-  v8::Local<v8::Array> array = v8::Local<v8::Array>::Cast(UnwrapValue());
+  v8::Local<v8::Array> array = v8::Local<v8::Array>::Cast(UnwrapValue(context.GetJsEngine()));
   uint32_t length = array->Length();
   for (uint32_t i = 0; i < length; i++)
   {
     v8::Local<v8::Value> item = array->Get(i);
     result.push_back(JsValuePtr(new JsValue(jsEngine, item)));
   }
   return result;
 }
 
 std::vector<std::string> AdblockPlus::JsValue::GetOwnPropertyNames() const
 {
+  JsContext context(jsEngine);
   if (!IsObject())
     throw new std::runtime_error("Attempting to get propert list for a non-object");
 
-  const JsContext context(jsEngine);
-  v8::Local<v8::Object> object = v8::Local<v8::Object>::Cast(UnwrapValue());
+  v8::Local<v8::Object> object = v8::Local<v8::Object>::Cast(UnwrapValue(context.GetJsEngine()));
   JsValueList properties = JsValuePtr(new JsValue(jsEngine, object->GetOwnPropertyNames()))->AsList();
   std::vector<std::string> result;
   for (JsValueList::iterator it = properties.begin(); it != properties.end(); ++it)
     result.push_back((*it)->AsString());
   return result;
 }
 
 
 AdblockPlus::JsValuePtr AdblockPlus::JsValue::GetProperty(const std::string& name) const
 {
+  JsContext context(jsEngine);
   if (!IsObject())
     throw new std::runtime_error("Attempting to get property of a non-object");
 
-  const JsContext context(jsEngine);
-  v8::Local<v8::String> property = Utils::ToV8String(jsEngine->GetIsolate(), name);
-  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue());
+  v8::Local<v8::String> property = Utils::ToV8String(context.GetJsEngine().GetIsolate(), name);
+  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue(context.GetJsEngine()));
   return JsValuePtr(new JsValue(jsEngine, obj->Get(property)));
 }
 
 void AdblockPlus::JsValue::SetProperty(const std::string& name, v8::Handle<v8::Value> val)
 {
+  JsContext context(jsEngine);
   if (!IsObject())
     throw new std::runtime_error("Attempting to set property on a non-object");
 
-  v8::Local<v8::String> property = Utils::ToV8String(jsEngine->GetIsolate(), name);
-  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue());
+  v8::Local<v8::String> property = Utils::ToV8String(context.GetJsEngine().GetIsolate(), name);
+  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue(context.GetJsEngine()));
   obj->Set(property, val);
 }
 
-v8::Local<v8::Value> AdblockPlus::JsValue::UnwrapValue() const
+v8::Local<v8::Value> AdblockPlus::JsValue::UnwrapValue(JsEngine& jsEngine) const
 {
-  return v8::Local<v8::Value>::New(jsEngine->GetIsolate(), *value);
+  return v8::Local<v8::Value>::New(jsEngine.GetIsolate(), *value);
 }
 
 void AdblockPlus::JsValue::SetProperty(const std::string& name, const std::string& val)
 {
-  const JsContext context(jsEngine);
-  SetProperty(name, Utils::ToV8String(jsEngine->GetIsolate(), val));
+  JsContext context(jsEngine);
+  SetProperty(name, Utils::ToV8String(context.GetJsEngine().GetIsolate(), val));
 }
 
 void AdblockPlus::JsValue::SetProperty(const std::string& name, int64_t val)
 {
-  const JsContext context(jsEngine);
-  SetProperty(name, v8::Number::New(jsEngine->GetIsolate(), val));
+  JsContext context(jsEngine);
+  SetProperty(name, v8::Number::New(context.GetJsEngine().GetIsolate(), val));
 }
 
 void AdblockPlus::JsValue::SetProperty(const std::string& name, const JsValuePtr& val)
 {
-  const JsContext context(jsEngine);
-  SetProperty(name, val->UnwrapValue());
+  JsContext context(jsEngine);
+  SetProperty(name, val->UnwrapValue(context.GetJsEngine()));
 }
 
 void AdblockPlus::JsValue::SetProperty(const std::string& name, bool val)
 {
-  const JsContext context(jsEngine);
+  JsContext context(jsEngine);
   SetProperty(name, v8::Boolean::New(val));
 }
 
 std::string AdblockPlus::JsValue::GetClass() const
 {
+  JsContext context(jsEngine);
   if (!IsObject())
     throw new std::runtime_error("Cannot get constructor of a non-object");
 
-  const JsContext context(jsEngine);
-  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue());
+  v8::Local<v8::Object> obj = v8::Local<v8::Object>::Cast(UnwrapValue(context.GetJsEngine()));
   return Utils::FromV8String(obj->GetConstructorName());
 }
 
 AdblockPlus::JsValuePtr AdblockPlus::JsValue::Call(const JsValueList& params, JsValuePtr thisPtr) const
 {
+  JsContext context(jsEngine);
   if (!IsFunction())
     throw new std::runtime_error("Attempting to call a non-function");
 
-  const JsContext context(jsEngine);
   if (!thisPtr)
   {
     v8::Local<v8::Context> localContext = v8::Local<v8::Context>::New(
-      jsEngine->GetIsolate(), *jsEngine->context);
+      context.GetJsEngine().GetIsolate(), *context.GetJsEngine().context);
     thisPtr = JsValuePtr(new JsValue(jsEngine, localContext->Global()));
   }
   if (!thisPtr->IsObject())
     throw new std::runtime_error("`this` pointer has to be an object");
-  v8::Local<v8::Object> thisObj = v8::Local<v8::Object>::Cast(thisPtr->UnwrapValue());
+  v8::Local<v8::Object> thisObj = v8::Local<v8::Object>::Cast(thisPtr->UnwrapValue(context.GetJsEngine()));
 
   std::vector<v8::Handle<v8::Value>> argv;
   for (JsValueList::const_iterator it = params.begin(); it != params.end(); ++it)
-    argv.push_back((*it)->UnwrapValue());
+    argv.push_back((*it)->UnwrapValue(context.GetJsEngine()));
 
   const v8::TryCatch tryCatch;
-  v8::Local<v8::Function> func = v8::Local<v8::Function>::Cast(UnwrapValue());
+  v8::Local<v8::Function> func = v8::Local<v8::Function>::Cast(UnwrapValue(context.GetJsEngine()));
   v8::Local<v8::Value> result = func->Call(thisObj, argv.size(),
       argv.size() ? &argv.front() : 0);
 
   if (tryCatch.HasCaught())
     throw JsError(tryCatch.Exception(), tryCatch.Message());
 
   return JsValuePtr(new JsValue(jsEngine, result));
 }
 
 AdblockPlus::JsValuePtr AdblockPlus::JsValue::Call(const JsValue& arg) const
 {
-  const JsContext context(jsEngine);
+  JsContext context(jsEngine);
   JsValueList params;
-  params.push_back(JsValuePtr(new JsValue(arg.jsEngine, arg.UnwrapValue())));
+  params.push_back(JsValuePtr(new JsValue(arg.jsEngine, arg.UnwrapValue(context.GetJsEngine()))));
   return Call(params);
 }