download_store: ensure that session files land on the disk

Currently, save() stores session to the disk by:

- serialize to bencode and write to files with .new ext
- validate the new files
- rename new files, removing .new ext

Technically, this is done to make sure that session files on the
disk are always valid/uncorrupted. However, if catastrophic events
happen during save(), such as system crash / power outage, the actual
content of session files may not have been synced to the disk.

To make things much worse, rename() is atomic. That means we discarded
the previous session files without actually landing the new session files
on the disk, and we got a bunch of empty session files to deal with.

This commit adds a fsync step to ensure that session files land on the
disk.

src/core/download_store.cc

@@ -4,6 +4,7 @@
 // DownloadStore handles the saving and listing of session torrents.
 
 #include <cstdio>
+#include <fcntl.h>
 #include <fstream>
 #include <unistd.h>
 
@@ -84,6 +85,7 @@ DownloadStore::write_bencode(const std::string&     filename,
                              uint32_t               skip_mask) {
   torrent::Object tmp;
   std::fstream    output(filename.c_str(), std::ios::out | std::ios::trunc);
+  int fd = -1;
 
   if (!output.is_open())
     goto download_store_save_error;
@@ -103,6 +105,15 @@ DownloadStore::write_bencode(const std::string&     filename,
     goto download_store_save_error;
 
   output.close();
+
+  // Ensure that the new file is actually written to the disk
+  fd = ::open(filename.c_str(), O_WRONLY);
+  if (fd < 0)
+    goto download_store_save_error;
+
+  fsync(fd);
+  ::close(fd);
+
   return true;
 
 download_store_save_error: