[libcalamares] Add an async get method

- Mostly a "cheap" wrapper for a half-dozen boilerplate lines
   of Qt NAM code.

src/libcalamares/network/Manager.cpp

@@ -99,6 +99,40 @@ Manager::setCheckHasInternetUrl( const QUrl& url )
     d->m_hasInternetUrl = url;
 }
 
+/** @brief Does a request asynchronously, returns the (pending) reply
+ *
+ * The extra options for the request are taken from @p options,
+ * including the timeout setting. A timeout will cause the reply
+ * to abort.
+ *
+ * On failure, returns nullptr (e.g. bad URL, timeout).
+ */
+static QNetworkReply*
+asynchronousRun( const std::unique_ptr< QNetworkAccessManager >& nam, const QUrl& url, const RequestOptions& options )
+{
+    QNetworkRequest request = QNetworkRequest( url );
+    QNetworkReply* reply = nam->get( request );
+    QTimer* timer = nullptr;
+
+    // Bail out early if the request is bad
+    if ( reply->error() )
+    {
+        reply->deleteLater();
+        return nullptr;
+    }
+
+    options.applyToRequest( &request );
+    if ( options.hasTimeout() )
+    {
+        timer = new QTimer( reply );
+        timer->setSingleShot( true );
+        QObject::connect( timer, &QTimer::timeout, reply, &QNetworkReply::abort );
+        timer->start( options.timeout() );
+    }
+
+    return reply;
+}
+
 /** @brief Does a request synchronously, returns the request itself
  *
  * The extra options for the request are taken from @p options,
@@ -173,5 +207,12 @@ Manager::synchronousGet( const QUrl& url, const RequestOptions& options )
     return reply.first ? reply.second->readAll() : QByteArray();
 }
 
+QNetworkReply*
+Manager::asynchronouseGet( const QUrl& url, const CalamaresUtils::Network::RequestOptions& options )
+{
+    return asynchronousRun( d->m_nam, url, options );
+}
+
+
 }  // namespace Network
 }  // namespace CalamaresUtils

src/libcalamares/network/Manager.h

@@ -28,6 +28,7 @@
 #include <chrono>
 #include <memory>
 
+class QNetworkReply;
 class QNetworkRequest;
 
 namespace CalamaresUtils
@@ -139,6 +140,14 @@ public:
      */
     bool hasInternet();
 
+    /** @brief Do a network request asynchronously.
+     *
+     * Returns a pointer to the reply-from-the-request.
+     * This may be a nullptr if an error occurs immediately.
+     * The caller is responsible for cleaning up the reply (eventually).
+     */
+    QNetworkReply* asynchronouseGet( const QUrl& url, const RequestOptions& options = RequestOptions() );
+
 private:
     struct Private;
     std::unique_ptr< Private > d;