Add a downloader tools to download files.

The downloader is using libaria2.

For now, only one download can be run a the time.
A download will start only if (and as soon as) no download is running.

include/downloader.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2018 Matthieu Gautier <mgautier@kymeria.fr>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU  General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+ * MA 02110-1301, USA.
+ */
+
+#ifndef KIWIX_DOWNLOADER_H
+#define KIWIX_DOWNLOADER_H
+
+#include <string>
+#include <aria2/aria2.h>
+#include <pthread.h>
+
+namespace kiwix
+{
+
+
+struct DownloadedFile {
+  DownloadedFile()
+   : success(false) {}
+  bool success;
+  std::string path;
+};
+
+/**
+ * A tool to download things.
+ *
+ */
+class Downloader
+{
+ public:
+  Downloader();
+  ~Downloader();
+
+  /**
+   * Download a content.
+   *
+   * @param url the url to download
+   * @return the content downloaded.
+   */
+  DownloadedFile download(const std::string& url);
+
+ private:
+  static pthread_mutex_t globalLock;
+
+  aria2::Session* session;
+  DownloadedFile* fileHandle;
+  std::string tmpDir;
+
+  static int downloadEventCallback(aria2::Session* session,
+                                   aria2::DownloadEvent event,
+                                   aria2::A2Gid gid,
+                                   void* userData);
+};
+}
+
+#endif

include/meson.build

@@ -2,6 +2,7 @@ headers = [
   'library.h',
   'manager.h',
   'opds_dumper.h',
+  'downloader.h',
   'reader.h',
   'searcher.h'
 ]