Partial downloads misbehave with libtorrent 2.0 `POSIX-compliant` mode

[HanabishiRecca]

qBittorrent & operating system versions

qBittorrent v4.6.3
OS: Arch Linux
Qt: 6.6.1
Libtorrent: 2.0.9

What is the problem?

Partial downloads has weird behavior with libtorrent 2.0 POSIX-compliant mode.
Edit: this is because POSIX storage do not restore file priorities arvidn/libtorrent#7593

Steps to reproduce

0. Set Disk IO type to POSIX-compliant.
1. Add a torrent with many files, keep all files selected for now, but do not start the torrent (i.e. uncheck the Start torrent option).
2. Go into the Content tab and uncheck some arbitrary files to make the task partial.
3. Start the download. Unwanted parts will be saved into .parts file.
4. Wait until the download is 100% completed.
5. Exit the client.
6. Start the client again.

You will see Missing Files error on that task as it wants actual file parts to be present instead of the .parts file now.

You can also force recheck it and see that the progress is not 100% anymore. Because after the client restart that task has switched into the different mode for some reason.

Additional context

I found this bug because you can't uncheck files while adding a torrent in WebUI. So you add full torrent first (paused) and only then uncheck the files.

Log(s) & preferences file(s)

No response

[thalieht]

3. Start the download. Unwanted parts will be saved into .parts file. (I guess this is already wrong behavior?)

No, cross file pieces have to be saved somewhere.

[MikaelKW]

Same here. It's the .PARTS files that it has to download. Would be nice if qBittorrent gave a notification about it being .PARTS file that it has to download.

[HanabishiRecca]

No, cross file pieces have to be saved somewhere.

Well, then the sudden mode switch after the client restart is a bug here.

I actually caught this on the current master first. With the option from 2a20764 enabled, this behavior can be clearly seen. The client uses .parts initially, but switches into .unwanted mode after the restart.

[HanabishiRecca]

I have found the cause. This only happens with libtorrent 2.0 POSIX-compliant mode.

Presumably because mmap storage and posix storage checks are different here.
https://github.com/arvidn/libtorrent/blob/18f821ea1a2aaafed3cf530120293043ee0bfa94/src/mmap_storage.cpp#L339-L352
https://github.com/arvidn/libtorrent/blob/18f821ea1a2aaafed3cf530120293043ee0bfa94/src/posix_storage.cpp#L409-L416

Note the lack of size > 0 and use_partfile(i, true); in the posix storage. I think this is the root of the issue.
I will try to patch libtorrent and report on results.

[HanabishiRecca]

Previous assumption turned to be wrong, but I found the actual place where it fails.

Removing this lines fixes the issue:
https://github.com/arvidn/libtorrent/blob/18f821ea1a2aaafed3cf530120293043ee0bfa94/src/storage_utils.cpp#L565-L566

I don't know why though. Reporting it to libtorrent repo anyway.

[HanabishiRecca]

Found the actual cause. See arvidn/libtorrent#7593 for details.

[HanabishiRecca]

Fixed upstream.