File preallocation does not work after its priority is changed from "do not download"

[HanabishiRecca]

libtorrent version (or branch): 2.0.10
platform/architecture: Linux

File preallocation does not work after its priority is changed from "do not download".
It is reproducible if you try to download additional files on a partial torrent.

I see that open_file_impl intentionally checks for dont_download priority.

libtorrent/src/mmap_storage.cpp

Lines 899 to 901 in 2ab8fed

	// files with priority 0 should always be sparse
	if (m_file_priority.end_index() > file && m_file_priority[file] == dont_download)
	mode |= aux::open_mode::sparse;

But the problem is, set_file_priority opens the file before updating the priority.

libtorrent/src/mmap_storage.cpp

Lines 164 to 172 in 2ab8fed

	if (old_prio == dont_download && new_prio != dont_download)
	{
	// move stuff out of the part file
	std::shared_ptr<aux::file_mapping> f = open_file(sett, i, aux::open_mode::write, ec);
	if (ec)
	{
	prio = m_file_priority;
	return;
	}

---

This patch fixes the issue:

--- a/src/mmap_storage.cpp
+++ b/src/mmap_storage.cpp
@@ -164,6 +164,7 @@ error_code translate_error(std::error_code const& err, bool const write)
 			if (old_prio == dont_download && new_prio != dont_download)
 			{
 				// move stuff out of the part file
+				m_file_priority[i] = new_prio;
 				std::shared_ptr<aux::file_mapping> f = open_file(sett, i, aux::open_mode::write, ec);
 				if (ec)
 				{

But I'm not sure how appropriate it is. What do you think, @arvidn?

[ValeZAA]

I had a torrent 100%, then switched off file, then deleted file, then enabled file. It allocates 0 byte file on the disk with sparse metadata of correct size of that file. There is no bug...

Qbittorrent 5.0.0beta1 still has those issues: no rehashing of specific folders or files without full rehashing, Tixati has it; issues with restarting torrents if you delete specific files without first disabling them...

[HanabishiRecca]

It allocates 0 byte file on the disk with sparse metadata of correct size of that file. There is no bug...

I don't really get what you are trying to say. Preallocation is supposed to create a file with its full size. Like fallocate -l SIZE file.
There is a bug and my patch fixes it.

[ValeZAA]

Preallocation is supposed to create a file with its full size.

I am not sure such a mode makes any sense, but sure, if there is undefined behaviour where you first open file and then change open properties... Can you submit a patch? Fork this repo, click edit in the file in your repo, click pull request on this repo, submit changes.

[HanabishiRecca]

I am not sure such a mode makes any sense

That's how it works right now. Add a new torrent (without checking or unchecking the files), start it, and you'll see that full-sized files are created on first access.

Can you submit a patch?

Yes. I just waited for @arvidn's response on it. But I guess it could be reviewed as a PR.

[HanabishiRecca]

• Make setting the file priority early #7733

[HanabishiRecca]

Fixed in #7738.