Proper fix for skipped file downloads due to race condition

Properly fixes the skipped file downloads due to race condition caused by trying create same directory from multiple threads at the same time.
4c480465b3 fixed the issue by having additional check to see if the directory exists after boost::filesystem::create_directories() returned false.
This fix uses mutex to make sure only one thread at a time can check if the directory exists and create it

src/downloader.cpp

@@ -34,6 +34,7 @@ std::vector<DownloadInfo> vDownloadInfo;
 ThreadSafeQueue<gameFile> dlQueue;
 ThreadSafeQueue<Message> msgQueue;
 ThreadSafeQueue<gameFile> createXMLQueue;
+std::mutex mtx_create_directories; // Mutex for creating directories in Downloader::processDownloadQueue
 
 Downloader::Downloader(Config &conf)
 {
@@ -2956,26 +2957,31 @@ void Downloader::processDownloadQueue(Config conf, const unsigned int& tid)
         msgQueue.push(Message("Begin download: " + filepath.filename().string(), MSGTYPE_INFO, msg_prefix));
 
         // Check that directory exists and create subdirectories
+        mtx_create_directories.lock(); // Use mutex to avoid possible race conditions
         if (boost::filesystem::exists(directory))
         {
             if (!boost::filesystem::is_directory(directory))
             {
+                mtx_create_directories.unlock();
                 msgQueue.push(Message(directory.string() + " is not directory, skipping file (" + filepath.filename().string() + ")", MSGTYPE_WARNING, msg_prefix));
                 continue;
             }
+            else
+            {
+                mtx_create_directories.unlock();
+            }
         }
         else
         {
             if (!boost::filesystem::create_directories(directory))
             {
-                /* Multiple threads could have tried to create the directory at the same time because they detected that the directory didn't exist at the time
+                mtx_create_directories.unlock();
-                 * boost::filesystem::create_directories() could have returned false because of this race condition so we have to check again if directory exists */
-                if (!boost::filesystem::exists(directory))
-                {
-                    // Failed to create directory and another thread hasn't created it either. Print error and skip the file download.
                 msgQueue.push(Message("Failed to create directory (" + directory.string() + "), skipping file (" + filepath.filename().string() + ")", MSGTYPE_ERROR, msg_prefix));
                 continue;
             }
+            else
+            {
+                mtx_create_directories.unlock();
             }
         }
 
@@ -3042,6 +3048,7 @@ void Downloader::processDownloadQueue(Config conf, const unsigned int& tid)
             {
                 // Check that directory exists and create subdirectories
                 boost::filesystem::path path = xml_directory;
+                mtx_create_directories.lock(); // Use mutex to avoid race conditions
                 if (boost::filesystem::exists(path))
                 {
                     if (!boost::filesystem::is_directory(path))
@@ -3056,6 +3063,7 @@ void Downloader::processDownloadQueue(Config conf, const unsigned int& tid)
                         msgQueue.push(Message("Failed to create directory: " + path.string(), MSGTYPE_ERROR, msg_prefix));
                     }
                 }
+                mtx_create_directories.unlock();
                 std::ofstream ofs(local_xml_file.string().c_str());
                 if (ofs)
                 {