Issue 4931 - add possibility to not send data depending on connection properties

src/JsEngine.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
@@ skipping 210 matching lines @@
 }
 
 void AdblockPlus::JsEngine::SetWebRequest(AdblockPlus::WebRequestPtr val)
 {
   if (!val)
     throw std::runtime_error("WebRequest cannot be null");
 
   webRequest = val;
 }
 
+void AdblockPlus::JsEngine::SetIsConnectionAllowedCallback(const IsConnectionAllowedCallback& callback)
+{
+  const JsLocker lock(shared_from_this());
+  isConnectionAllowed = callback;
+}
+
+bool AdblockPlus::JsEngine::IsConnectionAllowed()
+{
+  // The call of isConnectionAllowed can be very expensive and it makes a
+  // little sense to block execution of JavaScript for it. Currently this
+  // method is called from a thread of web request, so let only this thread be
+  // blocked by the call of the callback.
+  IsConnectionAllowedCallback localCopy;
+  {
+    const JsLocker lock(shared_from_this());
+    localCopy = isConnectionAllowed;
+  }
+  return !localCopy || localCopy();
+}
+
 AdblockPlus::LogSystemPtr AdblockPlus::JsEngine::GetLogSystem()
 {
   if (!logSystem)
     logSystem.reset(new DefaultLogSystem());
   return logSystem;
 }
 
 void AdblockPlus::JsEngine::SetLogSystem(AdblockPlus::LogSystemPtr val)
 {
   if (!val)
     throw std::runtime_error("LogSystem cannot be null");
 
   logSystem = val;
 }
 
 
 void AdblockPlus::JsEngine::SetGlobalProperty(const std::string& name, 
                                               AdblockPlus::JsValuePtr value)
 {
   auto global = GetGlobalObject();
   if (!global)
     throw std::runtime_error("Global object cannot be null");
   global->SetProperty(name, value);
 }