Make TBufferMerger::GetQueueSize thread safe.

This can now be safely use to avoid overloading the queue.

io/io/inc/ROOT/TBufferMerger.hxx

@@ -114,7 +114,7 @@ private:
    size_t fBuffered{0};                                          //< Number of bytes currently buffered
    TFileMerger fMerger{false, false};                            //< TFileMerger used to merge all buffers
    std::mutex fMergeMutex;                                       //< Mutex used to lock fMerger
-   std::mutex fQueueMutex;                                       //< Mutex used to lock fQueue
+   mutable std::mutex fQueueMutex;                               //< Mutex used to lock fQueue
    std::queue<TBufferFile *> fQueue;                             //< Queue to which data is pushed and merged
    std::vector<std::weak_ptr<TBufferMergerFile>> fAttachedFiles; //< Attached files
 };

io/io/src/TBufferMerger.cxx

@@ -62,6 +62,7 @@ std::shared_ptr<TBufferMergerFile> TBufferMerger::GetFile()
 
 size_t TBufferMerger::GetQueueSize() const
 {
+   std::lock_guard<std::mutex> lock(fQueueMutex);
    return fQueue.size();
 }