API for deleted files / raw FD

[tatref]

Hi,

If a file is deleted while still opened by a process, the fd under /proc/$pid/fd/$fd will show up as

[tatref@srv dev]$ ls -ld /proc/12779/fd/*
lr-x------ 1 tatref tatref 64 Oct 16 01:00 /proc/12779/fd/4 -> '/home/tatref/dev/tes (deleted)'

We can get that info with

use std::fs::File;
use std::fs::Metadata;
use std::os::unix::fs::MetadataExt;

let f = File::open(&format!("/proc/{}/fd/{}", p.pid, fd.fd)).unwrap();
let metadata = f.metadata().unwrap();

if metadata.nlink() == 0 {}

So this means format a String, open a file, and do a syscall to get metadata.

Currently it seems there is no easier API in procfs to access this information (please correct me if I'm wrong).
FDTarget has a Path variant, but there is no guarantee that another file with the same name hasn't be created.

Maybe we could store a File in the FDInfo, and add it where it is created:

procfs/procfs/src/process/mod.rs

Lines 149 to 175 in e303757

	fn from_process_at<P: AsRef<Path>, Q: AsRef<Path>>(
	base: P,
	dirfd: BorrowedFd,
	path: Q,
	fd: i32,
	) -> ProcResult<Self> {
	let p = path.as_ref();
	let root = base.as_ref().join(p);
	// for 2.6.39 <= kernel < 3.6 fstat doesn't support O_PATH see https://github.com/eminence/procfs/issues/265
	let flags = match Version::cached() {
	Ok(v) if v < KernelVersion::new(3, 6, 0) => OFlags::NOFOLLOW | OFlags::CLOEXEC,
	Ok(_) => OFlags::NOFOLLOW | OFlags::PATH | OFlags::CLOEXEC,
	Err(_) => OFlags::NOFOLLOW | OFlags::PATH | OFlags::CLOEXEC,
	};
	let file = wrap_io_error!(root, rustix::fs::openat(dirfd, p, flags, Mode::empty()))?;
	let link = rustix::fs::readlinkat(&file, "", Vec::new()).map_err(io::Error::from)?;
	let md =
	rustix::fs::statat(&file, "", AtFlags::SYMLINK_NOFOLLOW | AtFlags::EMPTY_PATH).map_err(io::Error::from)?;
	let link_os = link.to_string_lossy();
	let target = FDTarget::from_str(link_os.as_ref())?;
	Ok(FDInfo {
	fd,
	mode: (md.st_mode & Mode::RWXU.bits()) as u16,
	target,
	})
	}

What do you think?