You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: buffered_file.rs
    path of file: ./repos/glommio/glommio/src/io
    
    the code of the file until where you have to start completion: // Unless explicitly stated otherwise all files in this repository are licensed
// under the MIT/Apache-2.0 License, at your convenience
//
// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2020 Datadog, Inc.
//

use crate::{
    io::{glommio_file::GlommioFile, read_result::ReadResult, OpenOptions},
    GlommioError,
};
use std::{
    cell::Ref,
    os::unix::io::{AsRawFd, FromRawFd, RawFd},
    path::{Path
    
    pub async fn write_at(&self, buf: Vec<u8>, pos: u64) -> Result<usize> {
        let source =
            self.file
                .reactor
                .upgrade()
                .unwrap()
                .write_buffered(self.as_raw_fd(), buf, pos);
        source.collect_rw().await.map_err(|source| {
            GlommioError::create_enhanced(
                source,
                "Writing",
                self.file.path.borrow().as_ref(),
                Some(self.as_raw_fd()),
            )
        })
    }