Make ConcurrentQueue::waitAll() const

This member function does not affect the logical state of the class.
Making std::condition_variable and std::mutex data members mutable is
idiomatic as these classes are thread-safe synchronization primitives.

common/concurrent_queue.cpp

@@ -59,7 +59,7 @@ std::size_t ConcurrentQueue::cancelPending()
     return size;
 }
 
-void ConcurrentQueue::waitAll()
+void ConcurrentQueue::waitAll() const
 {
     std::unique_lock<std::mutex> lock(jobsLeftMutex);
     _waitVar.wait(lock, [this] { return jobsLeft == 0; });

common/concurrent_queue.h

@@ -33,7 +33,7 @@ public:
     std::size_t cancelPending();
 
     //! @brief Blocks the current thread until all enqueued jobs are completed.
-    void waitAll();
+    void waitAll() const;
 
 private:
     //! @invariant all worker threads are joinable until the destructor is called.
@@ -42,8 +42,8 @@ private:
     std::size_t jobsLeft = 0; //!< @invariant jobsLeft >= _queue.size()
     bool bailout = false; //!< @invariant is false until the destructor is called.
     std::condition_variable jobAvailableVar;
-    std::condition_variable _waitVar;
-    std::mutex jobsLeftMutex;
+    mutable std::condition_variable _waitVar;
+    mutable std::mutex jobsLeftMutex;
     std::mutex queueMutex;
 
     void nextJob();

tests/concurrent_queue_test/concurrent_queue_test.cpp

@@ -202,7 +202,7 @@ std::size_t cancelAndPrint(ConcurrentQueue &queue, const QueueControlMessagePrin
     return canceledCount;
 }
 
-void waitAndPrint(ConcurrentQueue &queue, const QueueControlMessagePrinter &printer)
+void waitAndPrint(const ConcurrentQueue &queue, const QueueControlMessagePrinter &printer)
 {
     printer.printBeginWaitingMessage();
     queue.waitAll();