How to determine if a file is seekable in Rust?

Question

In order to be able to infer a schema of a csv file passed via /dev/stdin before converting the whole csv file to parquet, I have implemented a wrapper that buffers the input and implements Seek as required by the crate arrow2. This all works.

This wrapper is, however, not necessary in certain situations like when a file is redirected to stdin: my_binary /dev/stdin <test.csv. I would like to only use that wrapper when it is really necessary, as in this case zstdcat test.csv.zst | my_binary /dev/stdin. As a result, I need to know whether the file I open errors on seek or not.

The following method I came up with seems to work. Is there a better way? Is this idiomatic for Rust?

fn main() {
    let mut file = std::fs::File::open("/dev/stdin").unwrap();
    let seekable = match std::io::Seek::rewind(&mut file) {
        Ok(_) => true,
        Err(_) => false,
    };
    println!("File is seekable: {:?}", seekable);
}

There is a similar question for C, but the solution doesn't transfer blindly to Rust: How to determine if a file descriptor is seekable? - or is this effectively what file.rewind() does under the hood?

Answer 1

There is a similar question for C, but the solution doesn't seem to transfer to Rust: How to determine if a file descriptor is seekable? - or is this effectively what file.rewind() does under the hood?

rewind actually performs a lseek(fd, 0, SEEK_SET), so it'll have the side-effect of, well, rewinding (hence the name) the fd's cursor. I assume the reason the original uses SEEK_CUR is to avoid moving the cursor on seekable files for maximum generality.

If you want to match the original question exactly you should use seek(SeekFrom::Current(0)). If it doesn't matter then rewind is fine.

Additionally:

• you don't need a match, just call is_ok on the result of rewind (/ seek)
• you don't need to call std::io::Seek::rewind(&mut file), if you use std::io::Seek then you can just call the provided methods on any seekable objects, like files

So:

use std::io::{Seek, SeekFrom::Current};

fn main() {
    let mut file = std::fs::File::open("/dev/stdin").unwrap();
    let seekable = file.seek(Current(0)).is_ok();
    println!("File is seekable: {:?}", seekable);
}

matches the C answer exactly.

Though for what it's worth on my mac the device files are seekable by default.

Only way I can get it to fail is if I pipe (not redirect):

> ./test
File is seekable: true
> </dev/null ./test
File is seekable: true
> </dev/null cat | ./test
File is seekable: false