//! Async filesystem primitives.
//!
//! This crate is an async version of [`std::fs`].
//!
//! # Implementation
//!
//! This crate uses [`blocking`] to offload blocking I/O onto a thread pool.
//!
//! [`blocking`]: https://docs.rs/blocking
//!
//! # Examples
//!
//! Create a new file and write some bytes to it:
//!
//! ```no_run
//! use async_fs::File;
//! use futures_lite::io::AsyncWriteExt;
//!
//! # futures_lite::future::block_on(async {
//! let mut file = File::create("a.txt").await?;
//! file.write_all(b"Hello, world!").await?;
//! file.flush().await?;
//! # std::io::Result::Ok(()) });
//! ```

#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]

use std::ffi::OsString;
use std::fmt;
use std::future::Future;
use std::io::{self, SeekFrom};
use std::path::{Path, PathBuf};
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll};

use blocking::{unblock, Unblock};
use futures_lite::io::{AsyncRead, AsyncSeek, AsyncWrite, AsyncWriteExt};
use futures_lite::stream::Stream;
use futures_lite::{future, ready};

#[doc(no_inline)]
pub use std::fs::{FileType, Metadata, Permissions};

/// Returns the canonical form of a path.
///
/// The returned path is in absolute form with all intermediate components normalized and symbolic
/// links resolved.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file or directory.
/// * A non-final component in `path` is not a directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let path = async_fs::canonicalize(".").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn canonicalize<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::canonicalize(&path)).await
}

/// Copies a file to a new location.
///
/// On success, the total number of bytes copied is returned and equals the length of the `dst`
/// file after this operation.
///
/// The old contents of `dst` will be overwritten. If `src` and `dst` both point to the same
/// file, then the file will likely get truncated as a result of this operation.
///
/// If you're working with open [`File`]s and want to copy contents through those types, use
/// [`futures_lite::io::copy()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `src` does not point to an existing file.
/// * The current process lacks permissions to read `src` or write `dst`.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let num_bytes = async_fs::copy("a.txt", "b.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn copy<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<u64> {
    let src = src.as_ref().to_owned();
    let dst = dst.as_ref().to_owned();
    unblock(move || std::fs::copy(&src, &dst)).await
}

/// Creates a directory.
///
/// Note that this function will only create the final directory in `path`. If you want to create
/// all of its missing parent directories too, use [`create_dir_all()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` already points to an existing file or directory.
/// * A parent directory in `path` does not exist.
/// * The current process lacks permissions to create the directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::create_dir("./some/directory").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::create_dir(&path)).await
}

/// Creates a directory and its parent directories if they are missing.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` already points to an existing file or directory.
/// * The current process lacks permissions to create the directory or its missing parents.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::create_dir_all("./some/directory").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn create_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::create_dir_all(&path)).await
}

/// Creates a hard link on the filesystem.
///
/// The `dst` path will be a link pointing to the `src` path. Note that operating systems often
/// require these two paths to be located on the same filesystem.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `src` does not point to an existing file.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::hard_link("a.txt", "b.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn hard_link<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
    let src = src.as_ref().to_owned();
    let dst = dst.as_ref().to_owned();
    unblock(move || std::fs::hard_link(&src, &dst)).await
}

/// Reads metadata for a path.
///
/// This function will traverse symbolic links to read metadata for the target file or directory.
/// If you want to read metadata without following symbolic links, use [`symlink_metadata()`]
/// instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file or directory.
/// * The current process lacks permissions to read metadata for the path.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let perm = async_fs::metadata("a.txt").await?.permissions();
/// # std::io::Result::Ok(()) });
/// ```
pub async fn metadata<P: AsRef<Path>>(path: P) -> io::Result<Metadata> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::metadata(path)).await
}

/// Reads the entire contents of a file as raw bytes.
///
/// This is a convenience function for reading entire files. It pre-allocates a buffer based on the
/// file size when available, so it is typically faster than manually opening a file and reading
/// from it.
///
/// If you want to read the contents as a string, use [`read_to_string()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file.
/// * The current process lacks permissions to read the file.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let contents = async_fs::read("a.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn read<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::read(&path)).await
}

/// Returns a stream of entries in a directory.
///
/// The stream yields items of type [`io::Result`]`<`[`DirEntry`]`>`. Note that I/O errors can
/// occur while reading from the stream.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing directory.
/// * The current process lacks permissions to read the contents of the directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// use futures_lite::stream::StreamExt;
///
/// let mut entries = async_fs::read_dir(".").await?;
///
/// while let Some(res) = entries.next().await {
///     let entry = res?;
///     println!("{}", entry.file_name().to_string_lossy());
/// }
/// # std::io::Result::Ok(()) });
/// ```
pub async fn read_dir<P: AsRef<Path>>(path: P) -> io::Result<ReadDir> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::read_dir(&path).map(|inner| ReadDir(State::Idle(Some(inner))))).await
}

/// A stream of entries in a directory.
///
/// This stream is returned by [`read_dir()`] and yields items of type
/// [`io::Result`]`<`[`DirEntry`]`>`. Each [`DirEntry`] can then retrieve information like entry's
/// path or metadata.
pub struct ReadDir(State);

/// The state of an asynchronous `ReadDir`.
///
/// The `ReadDir` can be either idle or busy performing an asynchronous operation.
enum State {
    Idle(Option<std::fs::ReadDir>),
    Busy(future::Boxed<(std::fs::ReadDir, Option<io::Result<std::fs::DirEntry>>)>),
}

impl fmt::Debug for ReadDir {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ReadDir").finish()
    }
}

impl Stream for ReadDir {
    type Item = io::Result<DirEntry>;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        loop {
            match &mut self.0 {
                State::Idle(opt) => {
                    let mut inner = opt.take().unwrap();

                    // Start the operation asynchronously.
                    self.0 = State::Busy(Box::pin(unblock(move || {
                        let next = inner.next();
                        (inner, next)
                    })));
                }
                // Poll the asynchronous operation the file is currently blocked on.
                State::Busy(task) => {
                    let (inner, opt) = ready!(task.as_mut().poll(cx));
                    self.0 = State::Idle(Some(inner));
                    return Poll::Ready(opt.map(|res| res.map(|inner| DirEntry(Arc::new(inner)))));
                }
            }
        }
    }
}

/// An entry in a directory.
///
/// A stream of entries in a directory is returned by [`read_dir()`].
///
/// For Unix-specific options, import the [`DirEntryExt`][`std::os::unix::fs::DirEntryExt`] trait.
pub struct DirEntry(Arc<std::fs::DirEntry>);

impl DirEntry {
    /// Returns the full path to this entry.
    ///
    /// The full path is created by joining the original path passed to [`read_dir()`] with the
    /// name of this entry.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use futures_lite::stream::StreamExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut dir = async_fs::read_dir(".").await?;
    ///
    /// while let Some(res) = dir.next().await {
    ///     let entry = res?;
    ///     println!("{:?}", entry.path());
    /// }
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn path(&self) -> PathBuf {
        self.0.path().into()
    }

    /// Reads the metadata for this entry.
    ///
    /// This function will traverse symbolic links to read the metadata.
    ///
    /// If you want to read metadata without following symbolic links, use [`symlink_metadata()`]
    /// instead.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * This entry does not point to an existing file or directory anymore.
    /// * The current process lacks permissions to read the metadata.
    /// * Some other I/O error occurred.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use futures_lite::stream::StreamExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut dir = async_fs::read_dir(".").await?;
    ///
    /// while let Some(res) = dir.next().await {
    ///     let entry = res?;
    ///     println!("{:?}", entry.metadata().await?);
    /// }
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn metadata(&self) -> io::Result<Metadata> {
        let inner = self.0.clone();
        unblock(move || inner.metadata()).await
    }

    /// Reads the file type for this entry.
    ///
    /// This function will not traverse symbolic links if this entry points at one.
    ///
    /// If you want to read metadata with following symbolic links, use [`metadata()`] instead.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * This entry does not point to an existing file or directory anymore.
    /// * The current process lacks permissions to read this entry's metadata.
    /// * Some other I/O error occurred.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use futures_lite::stream::StreamExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut dir = async_fs::read_dir(".").await?;
    ///
    /// while let Some(res) = dir.next().await {
    ///     let entry = res?;
    ///     println!("{:?}", entry.file_type().await?);
    /// }
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn file_type(&self) -> io::Result<FileType> {
        let inner = self.0.clone();
        unblock(move || inner.file_type()).await
    }

    /// Returns the bare name of this entry without the leading path.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use futures_lite::stream::StreamExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut dir = async_fs::read_dir(".").await?;
    ///
    /// while let Some(res) = dir.next().await {
    ///     let entry = res?;
    ///     println!("{}", entry.file_name().to_string_lossy());
    /// }
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn file_name(&self) -> OsString {
        self.0.file_name()
    }
}

impl fmt::Debug for DirEntry {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("DirEntry").field(&self.path()).finish()
    }
}

impl Clone for DirEntry {
    fn clone(&self) -> Self {
        DirEntry(self.0.clone())
    }
}

#[cfg(unix)]
impl std::os::unix::fs::DirEntryExt for DirEntry {
    fn ino(&self) -> u64 {
        self.0.ino()
    }
}

/// Reads a symbolic link and returns the path it points to.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing link.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let path = async_fs::read_link("a.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn read_link<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::read_link(&path)).await
}

/// Reads the entire contents of a file as a string.
///
/// This is a convenience function for reading entire files. It pre-allocates a string based on the
/// file size when available, so it is typically faster than manually opening a file and reading
/// from it.
///
/// If you want to read the contents as raw bytes, use [`read()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file.
/// * The current process lacks permissions to read the file.
/// * The contents of the file cannot be read as a UTF-8 string.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let contents = async_fs::read_to_string("a.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn read_to_string<P: AsRef<Path>>(path: P) -> io::Result<String> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::read_to_string(&path)).await
}

/// Removes an empty directory.
///
/// Note that this function can only delete an empty directory. If you want to delete a directory
/// and all of its contents, use [`remove_dir_all()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` is not an existing and empty directory.
/// * The current process lacks permissions to remove the directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::remove_dir("./some/directory").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn remove_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::remove_dir(&path)).await
}

/// Removes a directory and all of its contents.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` is not an existing and empty directory.
/// * The current process lacks permissions to remove the directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::remove_dir_all("./some/directory").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn remove_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::remove_dir_all(&path)).await
}

/// Removes a file.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file.
/// * The current process lacks permissions to remove the file.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::remove_file("a.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn remove_file<P: AsRef<Path>>(path: P) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::remove_file(&path)).await
}

/// Renames a file or directory to a new location.
///
/// If a file or directory already exists at the target location, it will be overwritten by this
/// operation.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `src` does not point to an existing file or directory.
/// * `src` and `dst` are on different filesystems.
/// * The current process lacks permissions to do the rename operation.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::rename("a.txt", "b.txt").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn rename<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
    let src = src.as_ref().to_owned();
    let dst = dst.as_ref().to_owned();
    unblock(move || std::fs::rename(&src, &dst)).await
}

/// Changes the permissions of a file or directory.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file or directory.
/// * The current process lacks permissions to change attributes on the file or directory.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let mut perm = async_fs::metadata("a.txt").await?.permissions();
/// perm.set_readonly(true);
/// async_fs::set_permissions("a.txt", perm).await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn set_permissions<P: AsRef<Path>>(path: P, perm: Permissions) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::set_permissions(path, perm)).await
}

/// Reads metadata for a path without following symbolic links.
///
/// If you want to follow symbolic links before reading metadata of the target file or directory,
/// use [`metadata()`] instead.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * `path` does not point to an existing file or directory.
/// * The current process lacks permissions to read metadata for the path.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// let perm = async_fs::symlink_metadata("a.txt").await?.permissions();
/// # std::io::Result::Ok(()) });
/// ```
pub async fn symlink_metadata<P: AsRef<Path>>(path: P) -> io::Result<Metadata> {
    let path = path.as_ref().to_owned();
    unblock(move || std::fs::symlink_metadata(path)).await
}

/// Writes a slice of bytes as the new contents of a file.
///
/// This function will create a file if it does not exist, and will entirely replace its contents
/// if it does.
///
/// # Errors
///
/// An error will be returned in the following situations:
///
/// * The file's parent directory does not exist.
/// * The current process lacks permissions to write to the file.
/// * Some other I/O error occurred.
///
/// # Examples
///
/// ```no_run
/// # futures_lite::future::block_on(async {
/// async_fs::write("a.txt", b"Hello world!").await?;
/// # std::io::Result::Ok(()) });
/// ```
pub async fn write<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> io::Result<()> {
    let path = path.as_ref().to_owned();
    let contents = contents.as_ref().to_owned();
    unblock(move || std::fs::write(&path, contents)).await
}

/// A builder for creating directories with configurable options.
///
/// For Unix-specific options, import the [`DirBuilderExt`][`std::os::unix::fs::DirBuilderExt`]
/// trait.
#[derive(Debug, Default)]
pub struct DirBuilder {
    /// Set to `true` if non-existent parent directories should be created.
    recursive: bool,

    /// Unix mode for newly created directories.
    #[cfg(unix)]
    mode: Option<u32>,
}

impl DirBuilder {
    /// Creates a blank set of options.
    ///
    /// The [`recursive()`][`DirBuilder::recursive()`] option is initially set to `false`.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_fs::DirBuilder;
    ///
    /// let builder = DirBuilder::new();
    /// ```
    pub fn new() -> DirBuilder {
        #[cfg(not(unix))]
        let builder = DirBuilder { recursive: false };

        #[cfg(unix)]
        let builder = DirBuilder {
            recursive: false,
            mode: None,
        };

        builder
    }

    /// Sets the option for recursive mode.
    ///
    /// When set to `true`, this option means all parent directories should be created recursively
    /// if they don't exist. Parents are created with the same permissions as the final directory.
    ///
    /// This option is initially set to `false`.
    ///
    /// # Examples
    ///
    /// ```
    /// use async_fs::DirBuilder;
    ///
    /// let mut builder = DirBuilder::new();
    /// builder.recursive(true);
    /// ```
    pub fn recursive(&mut self, recursive: bool) -> &mut Self {
        self.recursive = recursive;
        self
    }

    /// Creates a directory with the configured options.
    ///
    /// It is considered an error if the directory already exists unless recursive mode is enabled.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * `path` already points to an existing file or directory.
    /// * The current process lacks permissions to create the directory or its missing parents.
    /// * Some other I/O error occurred.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::DirBuilder;
    ///
    /// # futures_lite::future::block_on(async {
    /// DirBuilder::new()
    ///     .recursive(true)
    ///     .create("./some/directory")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn create<P: AsRef<Path>>(&self, path: P) -> impl Future<Output = io::Result<()>> {
        let mut builder = std::fs::DirBuilder::new();
        builder.recursive(self.recursive);

        #[cfg(unix)]
        {
            if let Some(mode) = self.mode {
                std::os::unix::fs::DirBuilderExt::mode(&mut builder, mode);
            }
        }

        let path = path.as_ref().to_owned();
        unblock(move || builder.create(path))
    }
}

#[cfg(unix)]
impl std::os::unix::fs::DirBuilderExt for DirBuilder {
    fn mode(&mut self, mode: u32) -> &mut Self {
        self.mode = Some(mode);
        self
    }
}

/// An open file on the filesystem.
///
/// Depending on what options the file was opened with, this type can be used for reading and/or
/// writing.
///
/// Files are automatically closed when they get dropped and any errors detected on closing are
/// ignored. Use the [`sync_all()`][`File::sync_all()`] method before dropping a file if such
/// errors need to be handled.
///
/// **NOTE:** If writing to a file, make sure to call
/// [`flush()`][`futures_lite::io::AsyncWriteExt::flush()`], [`sync_data()`][`File::sync_data()`],
/// or [`sync_all()`][`File::sync_all()`] before dropping the file or else some written data
/// might get lost!
///
/// # Examples
///
/// Create a new file and write some bytes to it:
///
/// ```no_run
/// use async_fs::File;
/// use futures_lite::io::AsyncWriteExt;
///
/// # futures_lite::future::block_on(async {
/// let mut file = File::create("a.txt").await?;
///
/// file.write_all(b"Hello, world!").await?;
/// file.flush().await?;
/// # std::io::Result::Ok(()) });
/// ```
///
/// Read the contents of a file into a vector of bytes:
///
/// ```no_run
/// use async_fs::File;
/// use futures_lite::io::AsyncReadExt;
///
/// # futures_lite::future::block_on(async {
/// let mut file = File::open("a.txt").await?;
///
/// let mut contents = Vec::new();
/// file.read_to_end(&mut contents).await?;
/// # std::io::Result::Ok(()) });
/// ```
pub struct File {
    /// Always accessible reference to the file.
    file: Arc<std::fs::File>,

    /// Performs blocking I/O operations on a thread pool.
    unblock: Unblock<ArcFile>,

    /// Logical file cursor, tracked when reading from the file.
    ///
    /// This will be set to an error if the file is not seekable.
    read_pos: Option<io::Result<u64>>,

    /// Set to `true` if the file needs flushing.
    is_dirty: bool,
}

impl File {
    /// Creates an async file from a blocking file.
    fn new(inner: std::fs::File, is_dirty: bool) -> File {
        let file = Arc::new(inner);
        let unblock = Unblock::new(ArcFile(file.clone()));
        let read_pos = None;
        File {
            file,
            unblock,
            read_pos,
            is_dirty,
        }
    }

    /// Opens a file in read-only mode.
    ///
    /// See the [`OpenOptions::open()`] function for more options.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * `path` does not point to an existing file.
    /// * The current process lacks permissions to read the file.
    /// * Some other I/O error occurred.
    ///
    /// For more details, see the list of errors documented by [`OpenOptions::open()`].
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = File::open("a.txt").await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn open<P: AsRef<Path>>(path: P) -> io::Result<File> {
        let path = path.as_ref().to_owned();
        let file = unblock(move || std::fs::File::open(&path)).await?;
        Ok(File::new(file, false))
    }

    /// Opens a file in write-only mode.
    ///
    /// This method will create a file if it does not exist, and will truncate it if it does.
    ///
    /// See the [`OpenOptions::open`] function for more options.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * The file's parent directory does not exist.
    /// * The current process lacks permissions to write to the file.
    /// * Some other I/O error occurred.
    ///
    /// For more details, see the list of errors documented by [`OpenOptions::open()`].
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = File::create("a.txt").await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn create<P: AsRef<Path>>(path: P) -> io::Result<File> {
        let path = path.as_ref().to_owned();
        let file = unblock(move || std::fs::File::create(&path)).await?;
        Ok(File::new(file, false))
    }

    /// Synchronizes OS-internal buffered contents and metadata to disk.
    ///
    /// This function will ensure that all in-memory data reaches the filesystem.
    ///
    /// This can be used to handle errors that would otherwise only be caught by closing the file.
    /// When a file is dropped, errors in synchronizing this in-memory data are ignored.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    /// use futures_lite::io::AsyncWriteExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut file = File::create("a.txt").await?;
    ///
    /// file.write_all(b"Hello, world!").await?;
    /// file.sync_all().await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn sync_all(&mut self) -> io::Result<()> {
        self.flush().await?;
        let file = self.file.clone();
        unblock(move || file.sync_all()).await
    }

    /// Synchronizes OS-internal buffered contents to disk.
    ///
    /// This is similar to [`sync_all()`][`File::sync_data()`], except that file metadata may not
    /// be synchronized.
    ///
    /// This is intended for use cases that must synchronize the contents of the file, but don't
    /// need the file metadata synchronized to disk.
    ///
    /// Note that some platforms may simply implement this in terms of
    /// [`sync_all()`][`File::sync_data()`].
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    /// use futures_lite::io::AsyncWriteExt;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut file = File::create("a.txt").await?;
    ///
    /// file.write_all(b"Hello, world!").await?;
    /// file.sync_data().await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn sync_data(&mut self) -> io::Result<()> {
        self.flush().await?;
        let file = self.file.clone();
        unblock(move || file.sync_data()).await
    }

    /// Truncates or extends the file.
    ///
    /// If `size` is less than the current file size, then the file will be truncated. If it is
    /// greater than the current file size, then the file will be extended to `size` and have all
    /// intermediate data filled with zeros.
    ///
    /// The file's cursor stays at the same position, even if the cursor ends up being past the end
    /// of the file after this operation.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    ///
    /// # futures_lite::future::block_on(async {
    /// let mut file = File::create("a.txt").await?;
    /// file.set_len(10).await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn set_len(&mut self, size: u64) -> io::Result<()> {
        self.flush().await?;
        let file = self.file.clone();
        unblock(move || file.set_len(size)).await
    }

    /// Reads the file's metadata.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = File::open("a.txt").await?;
    /// let metadata = file.metadata().await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn metadata(&self) -> io::Result<Metadata> {
        let file = self.file.clone();
        unblock(move || file.metadata()).await
    }

    /// Changes the permissions on the file.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * The current process lacks permissions to change attributes on the file.
    /// * Some other I/O error occurred.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::File;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = File::create("a.txt").await?;
    ///
    /// let mut perms = file.metadata().await?.permissions();
    /// perms.set_readonly(true);
    /// file.set_permissions(perms).await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
        let file = self.file.clone();
        unblock(move || file.set_permissions(perm)).await
    }

    /// Repositions the cursor after reading.
    ///
    /// When reading from a file, actual file reads run asynchronously in the background, which
    /// means the real file cursor is usually ahead of the logical cursor, and the data between
    /// them is buffered in memory. This kind of buffering is an important optimization.
    ///
    /// After reading ends, if we decide to perform a write or a seek operation, the real file
    /// cursor must first be repositioned back to the correct logical position.
    fn poll_reposition(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        if let Some(Ok(read_pos)) = self.read_pos {
            ready!(Pin::new(&mut self.unblock).poll_seek(cx, SeekFrom::Start(read_pos)))?;
        }
        self.read_pos = None;
        Poll::Ready(Ok(()))
    }
}

impl fmt::Debug for File {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.file.fmt(f)
    }
}

impl From<std::fs::File> for File {
    fn from(inner: std::fs::File) -> File {
        File::new(inner, true)
    }
}

#[cfg(unix)]
impl std::os::unix::io::FromRawFd for File {
    unsafe fn from_raw_fd(raw: std::os::unix::io::RawFd) -> File {
        File::from(std::fs::File::from_raw_fd(raw))
    }
}

#[cfg(windows)]
impl std::os::windows::io::FromRawHandle for File {
    unsafe fn from_raw_handle(raw: std::os::windows::io::RawHandle) -> File {
        File::from(std::fs::File::from_raw_handle(raw))
    }
}

#[cfg(unix)]
impl std::os::unix::io::AsRawFd for File {
    fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
        self.file.as_raw_fd()
    }
}

#[cfg(windows)]
impl std::os::windows::io::AsRawHandle for File {
    fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
        self.file.as_raw_handle()
    }
}

impl AsyncRead for File {
    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        // Before reading begins, remember the current cursor position.
        if self.read_pos.is_none() {
            // Initialize the logical cursor to the current position in the file.
            self.read_pos = Some(ready!(self.as_mut().poll_seek(cx, SeekFrom::Current(0))));
        }

        let n = ready!(Pin::new(&mut self.unblock).poll_read(cx, buf))?;

        // Update the logical cursor if the file is seekable.
        if let Some(Ok(pos)) = self.read_pos.as_mut() {
            *pos += n as u64;
        }

        Poll::Ready(Ok(n))
    }
}

impl AsyncWrite for File {
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        ready!(self.poll_reposition(cx))?;
        self.is_dirty = true;
        Pin::new(&mut self.unblock).poll_write(cx, buf)
    }

    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        if self.is_dirty {
            ready!(Pin::new(&mut self.unblock).poll_flush(cx))?;
            self.is_dirty = false;
        }
        Poll::Ready(Ok(()))
    }

    fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        Pin::new(&mut self.unblock).poll_close(cx)
    }
}

impl AsyncSeek for File {
    fn poll_seek(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        pos: SeekFrom,
    ) -> Poll<io::Result<u64>> {
        ready!(self.poll_reposition(cx))?;
        Pin::new(&mut self.unblock).poll_seek(cx, pos)
    }
}

/// A wrapper around `Arc<std::fs::File>` that implements `Read`, `Write`, and `Seek`.
struct ArcFile(Arc<std::fs::File>);

impl io::Read for ArcFile {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        (&*self.0).read(buf)
    }
}

impl io::Write for ArcFile {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        (&*self.0).write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        (&*self.0).flush()
    }
}

impl io::Seek for ArcFile {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
        (&*self.0).seek(pos)
    }
}

/// A builder for opening files with configurable options.
///
/// Files can be opened in [`read`][`OpenOptions::read()`] and/or
/// [`write`][`OpenOptions::write()`] mode.
///
/// The [`append`][`OpenOptions::append()`] option opens files in a special writing mode that
/// moves the file cursor to the end of file before every write operation.
///
/// It is also possible to [`truncate`][`OpenOptions::truncate()`] the file right after opening,
/// to [`create`][`OpenOptions::create()`] a file if it doesn't exist yet, or to always create a
/// new file with [`create_new`][`OpenOptions::create_new()`].
///
/// # Examples
///
/// Open a file for reading:
///
/// ```no_run
/// use async_fs::OpenOptions;
///
/// # futures_lite::future::block_on(async {
/// let file = OpenOptions::new()
///     .read(true)
///     .open("a.txt")
///     .await?;
/// # std::io::Result::Ok(()) });
/// ```
///
/// Open a file for both reading and writing, and create it if it doesn't exist yet:
///
/// ```no_run
/// use async_fs::OpenOptions;
///
/// # futures_lite::future::block_on(async {
/// let file = OpenOptions::new()
///     .read(true)
///     .write(true)
///     .create(true)
///     .open("a.txt")
///     .await?;
/// # std::io::Result::Ok(()) });
/// ```
#[derive(Clone, Debug)]
pub struct OpenOptions(std::fs::OpenOptions);

impl OpenOptions {
    /// Creates a blank set of options.
    ///
    /// All options are initially set to `false`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .read(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn new() -> OpenOptions {
        OpenOptions(std::fs::OpenOptions::new())
    }

    /// Configures the option for read mode.
    ///
    /// When set to `true`, this option means the file will be readable after opening.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .read(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn read(&mut self, read: bool) -> &mut OpenOptions {
        self.0.read(read);
        self
    }

    /// Configures the option for write mode.
    ///
    /// When set to `true`, this option means the file will be writable after opening.
    ///
    /// If the file already exists, write calls on it will overwrite the previous contents without
    /// truncating it.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .write(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn write(&mut self, write: bool) -> &mut OpenOptions {
        self.0.write(write);
        self
    }

    /// Configures the option for append mode.
    ///
    /// When set to `true`, this option means the file will be writable after opening and the file
    /// cursor will be moved to the end of file before every write operaiton.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .append(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn append(&mut self, append: bool) -> &mut OpenOptions {
        self.0.append(append);
        self
    }

    /// Configures the option for truncating the previous file.
    ///
    /// When set to `true`, the file will be truncated to the length of 0 bytes.
    ///
    /// The file must be opened in [`write`][`OpenOptions::write()`] or
    /// [`append`][`OpenOptions::append()`] mode for truncation to work.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .write(true)
    ///     .truncate(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn truncate(&mut self, truncate: bool) -> &mut OpenOptions {
        self.0.truncate(truncate);
        self
    }

    /// Configures the option for creating a new file if it doesn't exist.
    ///
    /// When set to `true`, this option means a new file will be created if it doesn't exist.
    ///
    /// The file must be opened in [`write`][`OpenOptions::write()`] or
    /// [`append`][`OpenOptions::append()`] mode for file creation to work.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .write(true)
    ///     .create(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn create(&mut self, create: bool) -> &mut OpenOptions {
        self.0.create(create);
        self
    }

    /// Configures the option for creating a new file or failing if it already exists.
    ///
    /// When set to `true`, this option means a new file will be created, or the open operation
    /// will fail if the file already exists.
    ///
    /// The file must be opened in [`write`][`OpenOptions::write()`] or
    /// [`append`][`OpenOptions::append()`] mode for file creation to work.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .write(true)
    ///     .create_new(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn create_new(&mut self, create_new: bool) -> &mut OpenOptions {
        self.0.create_new(create_new);
        self
    }

    /// Opens a file with the configured options.
    ///
    /// # Errors
    ///
    /// An error will be returned in the following situations:
    ///
    /// * The file does not exist and neither [`create`] nor [`create_new`] were set.
    /// * The file's parent directory does not exist.
    /// * The current process lacks permissions to open the file in the configured mode.
    /// * The file already exists and [`create_new`] was set.
    /// * Invalid combination of options was used, like [`truncate`] was set but [`write`] wasn't,
    ///   or none of [`read`], [`write`], and [`append`] modes was set.
    /// * An OS-level occurred, like too many files are open or the file name is too long.
    /// * Some other I/O error occurred.
    ///
    /// [`read`]: `OpenOptions::read()`
    /// [`write`]: `OpenOptions::write()`
    /// [`append`]: `OpenOptions::append()`
    /// [`truncate`]: `OpenOptions::truncate()`
    /// [`create`]: `OpenOptions::create()`
    /// [`create_new`]: `OpenOptions::create_new()`
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use async_fs::OpenOptions;
    ///
    /// # futures_lite::future::block_on(async {
    /// let file = OpenOptions::new()
    ///     .read(true)
    ///     .open("a.txt")
    ///     .await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub fn open<P: AsRef<Path>>(&self, path: P) -> impl Future<Output = io::Result<File>> {
        let path = path.as_ref().to_owned();
        let options = self.0.clone();
        async move {
            let file = unblock(move || options.open(path)).await?;
            Ok(File::new(file, false))
        }
    }
}

impl Default for OpenOptions {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(unix)]
impl std::os::unix::fs::OpenOptionsExt for OpenOptions {
    fn mode(&mut self, mode: u32) -> &mut Self {
        self.0.mode(mode);
        self
    }

    fn custom_flags(&mut self, flags: i32) -> &mut Self {
        self.0.custom_flags(flags);
        self
    }
}

#[cfg(windows)]
impl std::os::windows::fs::OpenOptionsExt for OpenOptions {
    fn access_mode(&mut self, access: u32) -> &mut Self {
        self.0.access_mode(access);
        self
    }

    fn share_mode(&mut self, val: u32) -> &mut Self {
        self.0.share_mode(val);
        self
    }

    fn custom_flags(&mut self, flags: u32) -> &mut Self {
        self.0.custom_flags(flags);
        self
    }

    fn attributes(&mut self, val: u32) -> &mut Self {
        self.0.attributes(val);
        self
    }

    fn security_qos_flags(&mut self, flags: u32) -> &mut Self {
        self.0.security_qos_flags(flags);
        self
    }
}

/// Unix-specific extensions.
#[cfg(unix)]
pub mod unix {
    use super::*;

    #[doc(no_inline)]
    pub use std::os::unix::fs::{
        DirBuilderExt, DirEntryExt, FileTypeExt, MetadataExt, OpenOptionsExt, PermissionsExt,
    };

    /// Creates a new symbolic link on the filesystem.
    ///
    /// The `dst` path will be a symbolic link pointing to the `src` path.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// # futures_lite::future::block_on(async {
    /// async_fs::unix::symlink("a.txt", "b.txt").await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn symlink<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
        let src = src.as_ref().to_owned();
        let dst = dst.as_ref().to_owned();
        unblock(move || std::os::unix::fs::symlink(&src, &dst)).await
    }
}

/// Windows-specific extensions.
#[cfg(windows)]
pub mod windows {
    use super::*;

    #[doc(no_inline)]
    pub use std::os::windows::fs::{MetadataExt, OpenOptionsExt};

    /// Creates a new directory symbolic link on the filesystem.
    ///
    /// The `dst` path will be a directory symbolic link pointing to the `src` path.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// # futures_lite::future::block_on(async {
    /// async_fs::windows::symlink_dir("a", "b").await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn symlink_dir<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
        let src = src.as_ref().to_owned();
        let dst = dst.as_ref().to_owned();
        unblock(move || std::os::windows::fs::symlink_dir(&src, &dst)).await
    }

    /// Creates a new file symbolic link on the filesystem.
    ///
    /// The `dst` path will be a file symbolic link pointing to the `src` path.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// # futures_lite::future::block_on(async {
    /// async_fs::windows::symlink_file("a.txt", "b.txt").await?;
    /// # std::io::Result::Ok(()) });
    /// ```
    pub async fn symlink_file<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> io::Result<()> {
        let src = src.as_ref().to_owned();
        let dst = dst.as_ref().to_owned();
        unblock(move || std::os::windows::fs::symlink_file(&src, &dst)).await
    }
}