Write downloads to disk and enable download resume

This enables the download method in Fetcher to resume in case a partial
download is already saved to disk.
For full implementation of download resumes, more changes are required,
because the current application will automatically clean up files that
don't match the expected size at startup.

src/fetcher.cpp

@@ -360,17 +360,44 @@ QNetworkReply *Fetcher::download(const QString &url, const QString &filePath) co
 {
     QNetworkRequest request((QUrl(url)));
     request.setTransferTimeout();
+
+    QFile *file = new QFile(filePath);
+    if (file->exists() && file->size() > 0) {
+        // try to resume download
+        qDebug() << "Resuming download at" << file->size() << "bytes";
+        QByteArray rangeHeaderValue = QByteArray("bytes=") + QByteArray::number(file->size()) + QByteArray("-");
+        request.setRawHeader(QByteArray("Range"), rangeHeaderValue);
+        file->open(QIODevice::WriteOnly | QIODevice::Append);
+    } else {
+        qDebug() << "Starting new download";
+        file->open(QIODevice::WriteOnly);
+    }
+
     QNetworkReply *reply = get(request);
-    connect(reply, &QNetworkReply::finished, this, [=]() {
-        if (reply->isOpen()) {
+
+    connect(reply, &QNetworkReply::readyRead, this, [=]() {
+        if (reply->isOpen() && file) {
             QByteArray data = reply->readAll();
-            QFile file(filePath);
-            file.open(QIODevice::WriteOnly);
-            file.write(data);
-            file.close();
+            file->write(data);
+        }
+    });
+
+    connect(reply, &QNetworkReply::finished, this, [=]() {
+        if (reply->isOpen() && file) {
+            QByteArray data = reply->readAll();
+            file->write(data);
+            file->close();
 
             Q_EMIT downloadFinished(url);
         }
+
+        // clean up; close file if still open in case something has gone wrong
+        if (file) {
+            if (file->isOpen()) {
+                file->close();
+            }
+            delete file;
+        }
         reply->deleteLater();
     });
 

src/fetcher.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <QFile>
 #include <QNetworkAccessManager>
 #include <QNetworkReply>
 #include <QObject>