Download updater thread works via a queue

Made the download updater thread process requests from a queue.

Such a scheme paves the path to performing all download actions
asynchronously by placing them onto the queue (which will eliminate
non-responsiveness of the application when those commands hit the
problem with slow responses from aria2c).

src/downloadmanagement.cpp

@@ -79,6 +79,10 @@ DownloadManager::~DownloadManager()
     {
         QThread* t = mp_downloadUpdaterThread;
         mp_downloadUpdaterThread = nullptr; // tell the thread to terminate
+
+        // At this point the thread may be stuck waiting for data.
+        // Let's wake it up.
+        m_requestQueue.enqueue("");
         t->wait();
     }
 }
@@ -88,20 +92,36 @@ bool DownloadManager::downloadingFunctionalityAvailable() const
     return mp_downloader != nullptr;
 }
 
+void DownloadManager::processDownloadActions()
+{
+   while ( mp_downloadUpdaterThread != nullptr ) {
+        const QString bookId = m_requestQueue.dequeue();
+        if ( !bookId.isEmpty() ) {
+            updateDownload(bookId);
+        }
+    }
+}
+
 void DownloadManager::startDownloadUpdaterThread()
 {
     // so that DownloadInfo can be copied across threads
     qRegisterMetaType<DownloadInfo>("DownloadInfo");
 
     mp_downloadUpdaterThread = QThread::create([=]() {
-       while ( mp_downloadUpdaterThread != nullptr ) {
+        processDownloadActions();
+    });
+
+    mp_downloadUpdaterThread->start();
+
+    QTimer *timer = new QTimer(this);
+    connect(timer, &QTimer::timeout, [this]() {
+        if ( m_requestQueue.isEmpty() ) {
             for ( const auto& bookId : m_downloads.keys() ) {
-                updateDownload(bookId);
+                m_requestQueue.enqueue(bookId);
             }
-            QThread::msleep(1000);
         }
     });
-    mp_downloadUpdaterThread->start();
+    timer->start(1000);
 }
 
 void DownloadManager::restoreDownloads()

src/downloadmanagement.h

@@ -5,8 +5,10 @@
 #include <QMap>
 #include <QMutex>
 #include <QMutexLocker>
+#include <QQueue>
 #include <QString>
 #include <QVariant>
+#include <QWaitCondition>
 
 #include <chrono>
 #include <memory>
@@ -17,6 +19,38 @@
 
 typedef QMap<QString, QVariant> DownloadInfo;
 
+template<class T>
+class ThreadSafeQueue
+{
+public:
+    void enqueue(const T& x)
+    {
+        const QMutexLocker threadSafetyGuarantee(&m_mutex);
+        m_queue.enqueue(x);
+        m_queueIsNotEmpty.wakeAll();
+    }
+
+    T dequeue()
+    {
+        const QMutexLocker threadSafetyGuarantee(&m_mutex);
+        if ( m_queue.isEmpty() )
+            m_queueIsNotEmpty.wait(&m_mutex);
+
+        return m_queue.dequeue();
+    }
+
+    bool isEmpty() const
+    {
+        const QMutexLocker threadSafetyGuarantee(&m_mutex);
+        return m_queue.isEmpty();
+    }
+
+private: // data
+    mutable QMutex  m_mutex;
+    QQueue<T>       m_queue;
+    QWaitCondition  m_queueIsNotEmpty;
+};
+
 class DownloadState
 {
 public: // types
@@ -114,7 +148,11 @@ signals:
     void downloadUpdated(QString bookId, const DownloadInfo& );
     void downloadDisappeared(QString bookId);
 
+private: // types
+    typedef ThreadSafeQueue<QString> RequestQueue;
+
 private: // functions
+    void processDownloadActions();
     void updateDownload(QString bookId);
 
 private: // data
@@ -122,6 +160,7 @@ private: // data
     kiwix::Downloader* const mp_downloader;
     Downloads                m_downloads;
     QThread*                 mp_downloadUpdaterThread = nullptr;
+    RequestQueue             m_requestQueue;
 };
 
 #endif // DOWNLOADMANAGEMENT_H