Sort tempfile logic opens up extra files

[karlmcdowall]

As part of investigating this issue, found that we're using extra file descriptors to mange our tempfile logic for batched-sort operations. This is a problem because some of the GNU compatibility tests run with ulimit enforcing strict limits on the number of files that sort can open.

Looking at the open files when starting a batched sort, I see...

$ ls -l /proc/1778302/fd
total 0
lrwx------ 1 karl karl 64 Dec  8 19:43 0 -> /dev/pts/3
l-wx------ 1 karl karl 64 Dec  8 19:43 1 -> /home/karl/Rust/gnu/tests/sort/sort-merge-fdlimit.log
l-wx------ 1 karl karl 64 Dec  8 19:43 2 -> /home/karl/Rust/gnu/gt-sort-merge-fdlimit.sh.A1OJ/err/merge-random-err
lr-x------ 1 karl karl 64 Dec  8 19:43 3 -> 'pipe:[3021894]'
l-wx------ 1 karl karl 64 Dec  8 19:43 4 -> 'pipe:[3021894]'

It's the last two of these that are problematic. These two pipes are created when we set up the Tempfile logic...

    fn init_tmp_dir(&mut self) -> UResult<()> {
<snip>
        let path = self.temp_dir.as_ref().unwrap().path().to_owned();
        let lock = self.lock.clone();
// *** The line below will open the two pipes.
        ctrlc::set_handler(move || {
            // Take the lock so that `next_file_path` returns no new file path,
            // and the program doesn't terminate before the handler has finished
            let _lock = lock.lock().unwrap();
            if let Err(e) = remove_tmp_dir(&path) {
                show_error!("failed to delete temporary directory: {}", e);
            }
            std::process::exit(2)
        })
        .map_err(|e| USimpleError::new(2, format!("failed to set up signal handler: {e}")))
    }

To fix this, I propose to just simplify this logic and just remove the Tempdir altogether. Just put any temporary files directly into /tmp (which is what GNU-sort does) and have the OS do the cleanup automatically rather than create a dedicated folder for them. We can then remove this code above that uses the ctrlc crate.

I don't know the history on this, so maybe there's a good reason to use a separate folder - If so, please let me know!

[sylvestre]

I don't think we have good reason for this. If we do, we should have a test covering it. :)

[sylvestre]

So, don't hesitate to remove it

[karlmcdowall]

Did a bit of thinking about this...

• My initial proposal to remove the Tempdir folder and just use files directly in /tmp won't solve the problem. We would still need a way to manually track all the temporary files we create so we can clean them up in the event of a CTRL+C, and it'll just be harder to do if we don't put them all in one directory. So, let's keep the Tempdir!
• Considered switching from using the ctrlc crate to signal-hook. This doesn't help at all though! ctrlc uses 2 file descriptors for pipes (this is the problem, we don't want to use any more descriptors than we have to). Signal-hook still uses 2 file descriptors, it just uses sockets rather than pipes. So switching to signal-Hook wouldn't help at all alas.
• Write our own VERY lightweight signal handler, directly using the nix::sys::signal API. I think this is unfortunately the way we'll have to go.

If anyone has other ideas please let me know.
Karl