Issue 5082 - fix possible race condition in DefaultTimer

src/DefaultTimer.cpp

 /*
 * This file is part of Adblock Plus <https://adblockplus.org/>,
 * Copyright (C) 2006-2017 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 12 matching lines @@
   : shouldThreadStop(false)
 {
   m_thread = std::thread([this]
   {
     ThreadFunc();
   });
 }
 
 DefaultTimer::~DefaultTimer()
 {
-  shouldThreadStop = true;
+  {
+    std::lock_guard<std::mutex> lock(mutex);
+    shouldThreadStop = true;
+  }
   conditionVariable.notify_one();
   if (m_thread.joinable())
     m_thread.join();
 }
 
 void DefaultTimer::SetTimer(const std::chrono::milliseconds& timeout, const TimerCallback& timerCallback)
 {
   if (!timerCallback)
     return;
   std::lock_guard<std::mutex> lock(mutex);
   TimerUnit timer = { std::chrono::steady_clock::now() + timeout, timerCallback };
   timers.push(timer);
   conditionVariable.notify_one();
 }
 
 void DefaultTimer::ThreadFunc()
 {
-  while (!shouldThreadStop)
+  while (true)
   {
     std::unique_lock<std::mutex> lock(mutex);
     if (timers.empty())
     {
       conditionVariable.wait(lock, [this]()->bool
       {
         return shouldThreadStop || !timers.empty();
       });
     }
     else
@@ skipping 10 matching lines @@
       try
       {
         callback();
       }
       catch (...)
       {
         // do nothing, but the thread will be alive.
       }
       lock.lock();
     }
+    if (shouldThreadStop)
+      return;
   }
 }