[libtorrent] Bad sequence of writing operations when resume data used

[Egor Orekhov]

Hi
I have a problem with using resume data.
My lib version is 0.16.19. I have a huge amount of files packed in big compressed pack and implemented custom read/write operations for it. So I want to use them through my own implementation of libtorrent::storage_interface.
After each piece finished I save restore data using same way as described in documentations.
Here is code that write resume data on disk:
if(save_resume_data_alert *srda = alert_cast<save_resume_data_alert>(a))
{
const wg::String tempPath = path + _T(".newresume");

std::ofstream resumeDataStream(tempPath.c_str(), std::ios_base::binary);
resumeDataStream.unsetf(std::ios_base::skipws);
bencode(std::ostream_iterator<char>(resumeDataStream), *srda->resume_data);
resumeDataStream.close();
RenameFile(tempPath.c_str(), path.c_str());
}
and here is some code for reading operaions
std::ifstream resumeDataStream(resumeDataFileName.c_str(), std::ios_base::binary);
const bool haveResumeData = resumeDataStream.good();

if(haveResumeData)
{
resumeDataStream.unsetf(std::ios_base::skipws);
ts.resume_data = new std::vector<char>;
ts.resume_data->insert(
ts.resume_data->begin(),
std::istreambuf_iterator<char>(resumeDataStream),
std::istreambuf_iterator<char>());
}
//ts is add_torrent_params object which then goes to session::add_torrent

If resume data exist I have bad sequence for calls of libtorrent::storage_interface::write function. As I understand this function should be called for each block of piece sequentially. But when resume data exist write calls alternate for different pieces. If there is no resume data all of libtorrent::storage_interface::write calls performs as expected.

In my implementation of storage interface "write_resume_data" is just empty function that always returns true.
Thanks in forward!
[wargaming.net]
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[Arvid Norberg]

Post by Egor Orekhov
Hi
I have a problem with using resume data.
My lib version is 0.16.19. I have a huge amount of files packed in big
compressed pack and implemented custom read/write operations for it. So I
want to use them through my own implementation of
libtorrent::storage_interface.
After each piece finished I save restore data using same way as described
in documentations.
[...]
If resume data exist I have bad sequence for calls of
libtorrent::storage_interface::write function. As I understand this
function should be called for each block of piece sequentially. But when
resume data exist write calls alternate for different pieces. If there is
no resume data all of libtorrent::storage_interface::write calls performs
as expected.

There is no guarantee that blocks will be written in order. There is an
_affinity_ towards downloading blocks in order and to cache blocks such
that they are written in order. This is to enable the optimization of
calculating the piece hash from data in RAM, rather than reading it back
from disk again once the piece is done.

Post by Egor Orekhov
In my implementation of storage interface "write_resume_data" is just
empty function that always returns true.

I don't think fundamentally this problem has anything to do with resume
data. resume data may be more likely to trigger the condition you don't
handle.