Support File::seek for Hermit

[thaliaarchi]

lseek was added in hermit-abi in commit 87dd201 (add missing interface for lseek, 2024-07-15), which was just released in version 0.5.0.

cc @mkroening, @stlankes

Fixes hermit-os/hermit-rs#652

[rustbot]

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[rustbot]

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[thaliaarchi]

I have not setup Hermit for runtime testing, so although this compiles with x check library/std --target=x86_64-unknown-hermit, I do not know whether it is fully correct. I copied fn seek from Unix with minor type adjustments.

To the Hermit maintainers: Please check that these invariants are also good for Hermit.

rust/library/std/src/sys/pal/unix/fs.rs

Lines 1428 to 1438 in 30f168e

	pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {
	let (whence, pos) = match pos {
	// Casting to `i64` is fine, too large values will end up as
	// negative which will cause an error in `lseek64`.
	SeekFrom::Start(off) => (libc::SEEK_SET, off as i64),
	SeekFrom::End(off) => (libc::SEEK_END, off),
	SeekFrom::Current(off) => (libc::SEEK_CUR, off),
	};
	let n = cvt(unsafe { lseek64(self.as_raw_fd(), pos as off64_t, whence) })?;
	Ok(n as u64)
	}

[mkroening]

This looks good to me. Thanks a lot!

I have also tested this with Hermit and the kernel receives all system calls as expected. 👍

[thaliaarchi]

@mkroening Is the negative cast fine too?

[thaliaarchi]

It looks like all Hermit targets are 64-bit, so the i64 to isize cast should be fine.

[mkroening]

@mkroening Is the negative cast fine too?

Do you mean casting SeekFrom::Start(off) to i64? That's fine, yes. I have tested this with Hermit's ramfs, and we do explicitly check for negative values.

In any case, since this appears to be fine to do on all Unixes, it should also be fine to do on Hermit, since we aim for POSIX compatibility for most file descriptor related things.

[bors]

☔ The latest upstream changes (presumably #138279) made this pull request unmergeable. Please resolve the merge conflicts.

[ChrisDenton]

If a target maintainer is fine with it then I am too 🙂

@bors r+

[bors]

📌 Commit 1abaacd has been approved by ChrisDenton

It is now in the queue for this repository.