// Copyright 2020 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! An Executor and future combinators based on operations that block on file descriptors.
//!
//! This crate is meant to be used with the `futures-rs` crate that provides further combinators
//! and utility functions to combine and manage futures. All futures will run until they block on a
//! file descriptor becoming readable or writable. Facilities are provided to register future
//! wakers based on such events.
//!
//! # Running top-level futures.
//!
//! Use helper functions based the desired behavior of your application.
//!
//! ## Running one future.
//!
//! If there is only one top-level future to run, use the [`run_one`](fn.run_one.html) function.
//!
//! ## Completing one of several futures.
//!
//! If there are several top level tasks that should run until any one completes, use the "select"
//! family of executor constructors. These return an [`Executor`](trait.Executor.html) whose `run`
//! function will return when the first future completes. The uncompleted futures will also be
//! returned so they can be run further or otherwise cleaned up. These functions are inspired by
//! the `select_all` function from futures-rs, but built to be run inside an FD based executor and
//! to poll only when necessary. See the docs for [`select2`](fn.select2.html),
//! [`select3`](fn.select3.html), [`select4`](fn.select4.html), and [`select5`](fn.select5.html).
//!
//! ## Completing all of several futures.
//!
//! If there are several top level tasks that all need to be completed, use the "complete" family
//! of executor constructors. These return an [`Executor`](trait.Executor.html) whose `run`
//! function will return only once all the futures passed to it have completed. These functions are
//! inspired by the `join_all` function from futures-rs, but built to be run inside an FD based
//! executor and to poll only when necessary. See the docs for [`complete2`](fn.complete2.html),
//! [`complete3`](fn.complete3.html), [`complete4`](fn.complete4.html), and
//! [`complete5`](fn.complete5.html).
//!
//! # Implementing new FD-based futures.
//!
//! For URing implementations should provide an implementation of the `IoSource` trait.
//! For the FD executor, new futures can use the existing ability to poll a source to build async
//! functionality on top of.
//!
//! # Implementations
//!
//! Currently there are two paths for using the asynchronous IO. One uses a PollContext and drivers
//! futures based on the FDs signaling they are ready for the opteration. This method will exist so
//! long as kernels < 5.4 are supported.
//! The other method submits operations to io_uring and is signaled when they complete. This is more
//! efficient, but only supported on kernel 5.4+.
//! If `IoSourceExt::new` is used to interface with async IO, then the correct backend will be chosen
//! automatically.
//!
//! # Examples
//!
//! See the docs for `IoSourceExt` if support for kernels <5.4 is required. Focus on `UringSource` if
//! all systems have support for io_uring.

mod complete;
mod event;
mod executor;
mod fd_executor;
mod io_ext;
pub mod mem;
mod poll_source;
mod queue;
mod select;
pub mod sync;
mod timer;
mod uring_executor;
mod uring_source;
mod waker;

pub use event::EventAsync;
pub use executor::Executor;
pub use fd_executor::FdExecutor;
pub use io_ext::{
    Error as AsyncError, IntoAsync, IoSourceExt, ReadAsync, Result as AsyncResult, WriteAsync,
};
pub use mem::{BackingMemory, MemRegion};
pub use poll_source::PollSource;
pub use select::SelectResult;
pub use timer::TimerAsync;
pub use uring_executor::URingExecutor;
pub use uring_source::UringSource;

use std::future::Future;
use std::marker::PhantomData;
use std::pin::Pin;
use std::task::{Context, Poll};

use thiserror::Error as ThisError;

#[derive(ThisError, Debug)]
pub enum Error {
    /// Error from the FD executor.
    #[error("Failure in the FD executor: {0}")]
    FdExecutor(fd_executor::Error),
    /// Error from the uring executor.
    #[error("Failure in the uring executor: {0}")]
    URingExecutor(uring_executor::Error),
}
pub type Result<T> = std::result::Result<T, Error>;

// A Future that never completes.
pub struct Empty<T> {
    phantom: PhantomData<T>,
}

impl<T> Future for Empty<T> {
    type Output = T;

    fn poll(self: Pin<&mut Self>, _: &mut Context) -> Poll<T> {
        Poll::Pending
    }
}

pub fn empty<T>() -> Empty<T> {
    Empty {
        phantom: PhantomData,
    }
}

/// Creates an Executor that runs one future to completion.
///
///  # Example
///
///    ```
///    use cros_async::run_one;
///
///    let fut = async { 55 };
///    assert_eq!(55, run_one(fut).unwrap());
///    ```
pub fn run_one<F: Future>(fut: F) -> Result<F::Output> {
    if uring_executor::use_uring() {
        run_one_uring(fut)
    } else {
        run_one_poll(fut)
    }
}

/// Creates a URingExecutor that runs one future to completion.
///
///  # Example
///
///    ```
///    use cros_async::run_one_uring;
///
///    let fut = async { 55 };
///    assert_eq!(55, run_one_uring(fut).unwrap());
///    ```
pub fn run_one_uring<F: Future>(fut: F) -> Result<F::Output> {
    URingExecutor::new()
        .and_then(|ex| ex.run_until(fut))
        .map_err(Error::URingExecutor)
}

/// Creates a FdExecutor that runs one future to completion.
///
///  # Example
///
///    ```
///    use cros_async::run_one_poll;
///
///    let fut = async { 55 };
///    assert_eq!(55, run_one_poll(fut).unwrap());
///    ```
pub fn run_one_poll<F: Future>(fut: F) -> Result<F::Output> {
    FdExecutor::new()
        .and_then(|ex| ex.run_until(fut))
        .map_err(Error::FdExecutor)
}

// Select helpers to run until any future completes.

/// Creates a combinator that runs the two given futures until one completes, returning a tuple
/// containing the result of the finished future and the still pending future.
///
///  # Example
///
///    ```
///    use cros_async::{SelectResult, select2, run_one};
///    use futures::future::pending;
///    use futures::pin_mut;
///
///    let first = async {5};
///    let second = async {let () = pending().await;};
///    pin_mut!(first);
///    pin_mut!(second);
///    match run_one(select2(first, second)) {
///        Ok((SelectResult::Finished(5), SelectResult::Pending(_second))) => (),
///        _ => panic!("Select didn't return the first future"),
///    };
///    ```
pub async fn select2<F1: Future + Unpin, F2: Future + Unpin>(
    f1: F1,
    f2: F2,
) -> (SelectResult<F1>, SelectResult<F2>) {
    select::Select2::new(f1, f2).await
}

/// Creates a combinator that runs the three given futures until one or more completes, returning a
/// tuple containing the result of the finished future(s) and the still pending future(s).
///
///  # Example
///
///    ```
///    use cros_async::{SelectResult, select3, run_one};
///    use futures::future::pending;
///    use futures::pin_mut;
///
///    let first = async {4};
///    let second = async {let () = pending().await;};
///    let third = async {5};
///    pin_mut!(first);
///    pin_mut!(second);
///    pin_mut!(third);
///    match run_one(select3(first, second, third)) {
///        Ok((SelectResult::Finished(4),
///            SelectResult::Pending(_second),
///            SelectResult::Finished(5))) => (),
///        _ => panic!("Select didn't return the futures"),
///    };
///    ```
pub async fn select3<F1: Future + Unpin, F2: Future + Unpin, F3: Future + Unpin>(
    f1: F1,
    f2: F2,
    f3: F3,
) -> (SelectResult<F1>, SelectResult<F2>, SelectResult<F3>) {
    select::Select3::new(f1, f2, f3).await
}

/// Creates a combinator that runs the four given futures until one or more completes, returning a
/// tuple containing the result of the finished future(s) and the still pending future(s).
///
///  # Example
///
///    ```
///    use cros_async::{SelectResult, select4, run_one};
///    use futures::future::pending;
///    use futures::pin_mut;
///
///    let first = async {4};
///    let second = async {let () = pending().await;};
///    let third = async {5};
///    let fourth = async {let () = pending().await;};
///    pin_mut!(first);
///    pin_mut!(second);
///    pin_mut!(third);
///    pin_mut!(fourth);
///    match run_one(select4(first, second, third, fourth)) {
///        Ok((SelectResult::Finished(4), SelectResult::Pending(_second),
///            SelectResult::Finished(5), SelectResult::Pending(_fourth))) => (),
///        _ => panic!("Select didn't return the futures"),
///    };
///    ```
pub async fn select4<
    F1: Future + Unpin,
    F2: Future + Unpin,
    F3: Future + Unpin,
    F4: Future + Unpin,
>(
    f1: F1,
    f2: F2,
    f3: F3,
    f4: F4,
) -> (
    SelectResult<F1>,
    SelectResult<F2>,
    SelectResult<F3>,
    SelectResult<F4>,
) {
    select::Select4::new(f1, f2, f3, f4).await
}

/// Creates a combinator that runs the five given futures until one or more completes, returning a
/// tuple containing the result of the finished future(s) and the still pending future(s).
///
///  # Example
///
///    ```
///    use cros_async::{SelectResult, select5, run_one};
///    use futures::future::pending;
///    use futures::pin_mut;
///
///    let first = async {4};
///    let second = async {let () = pending().await;};
///    let third = async {5};
///    let fourth = async {let () = pending().await;};
///    let fifth = async {6};
///    pin_mut!(first);
///    pin_mut!(second);
///    pin_mut!(third);
///    pin_mut!(fourth);
///    pin_mut!(fifth);
///    match run_one(select5(first, second, third, fourth, fifth)) {
///        Ok((SelectResult::Finished(4), SelectResult::Pending(_second),
///            SelectResult::Finished(5), SelectResult::Pending(_fourth),
///            SelectResult::Finished(6))) => (),
///        _ => panic!("Select didn't return the futures"),
///    };
///    ```
pub async fn select5<
    F1: Future + Unpin,
    F2: Future + Unpin,
    F3: Future + Unpin,
    F4: Future + Unpin,
    F5: Future + Unpin,
>(
    f1: F1,
    f2: F2,
    f3: F3,
    f4: F4,
    f5: F5,
) -> (
    SelectResult<F1>,
    SelectResult<F2>,
    SelectResult<F3>,
    SelectResult<F4>,
    SelectResult<F5>,
) {
    select::Select5::new(f1, f2, f3, f4, f5).await
}

/// Creates a combinator that runs the six given futures until one or more completes, returning a
/// tuple containing the result of the finished future(s) and the still pending future(s).
///
///  # Example
///
///    ```
///    use cros_async::{SelectResult, select6, run_one};
///    use futures::future::pending;
///    use futures::pin_mut;
///
///    let first = async {1};
///    let second = async {let () = pending().await;};
///    let third = async {3};
///    let fourth = async {let () = pending().await;};
///    let fifth = async {5};
///    let sixth = async {6};
///    pin_mut!(first);
///    pin_mut!(second);
///    pin_mut!(third);
///    pin_mut!(fourth);
///    pin_mut!(fifth);
///    pin_mut!(sixth);
///    match run_one(select6(first, second, third, fourth, fifth, sixth)) {
///        Ok((SelectResult::Finished(1), SelectResult::Pending(_second),
///            SelectResult::Finished(3), SelectResult::Pending(_fourth),
///            SelectResult::Finished(5), SelectResult::Finished(6))) => (),
///        _ => panic!("Select didn't return the futures"),
///    };
///    ```
pub async fn select6<
    F1: Future + Unpin,
    F2: Future + Unpin,
    F3: Future + Unpin,
    F4: Future + Unpin,
    F5: Future + Unpin,
    F6: Future + Unpin,
>(
    f1: F1,
    f2: F2,
    f3: F3,
    f4: F4,
    f5: F5,
    f6: F6,
) -> (
    SelectResult<F1>,
    SelectResult<F2>,
    SelectResult<F3>,
    SelectResult<F4>,
    SelectResult<F5>,
    SelectResult<F6>,
) {
    select::Select6::new(f1, f2, f3, f4, f5, f6).await
}

// Combination helpers to run until all futures are complete.

/// Creates a combinator that runs the two given futures to completion, returning a tuple of the
/// outputs each yields.
///
///  # Example
///
///    ```
///    use cros_async::{complete2, run_one};
///
///    let first = async {5};
///    let second = async {6};
///    assert_eq!(run_one(complete2(first, second)).unwrap_or((0,0)), (5,6));
///    ```
pub async fn complete2<F1, F2>(f1: F1, f2: F2) -> (F1::Output, F2::Output)
where
    F1: Future,
    F2: Future,
{
    complete::Complete2::new(f1, f2).await
}

/// Creates a combinator that runs the three given futures to completion, returning a tuple of the
/// outputs each yields.
///
///  # Example
///
///    ```
///    use cros_async::{complete3, run_one};
///
///    let first = async {5};
///    let second = async {6};
///    let third = async {7};
///    assert_eq!(run_one(complete3(first, second, third)).unwrap_or((0,0,0)), (5,6,7));
///    ```
pub async fn complete3<F1, F2, F3>(f1: F1, f2: F2, f3: F3) -> (F1::Output, F2::Output, F3::Output)
where
    F1: Future,
    F2: Future,
    F3: Future,
{
    complete::Complete3::new(f1, f2, f3).await
}

/// Creates a combinator that runs the four given futures to completion, returning a tuple of the
/// outputs each yields.
///
///  # Example
///
///    ```
///    use cros_async::{complete4, run_one};
///
///    let first = async {5};
///    let second = async {6};
///    let third = async {7};
///    let fourth = async {8};
///    assert_eq!(run_one(complete4(first, second, third, fourth)).unwrap_or((0,0,0,0)), (5,6,7,8));
///    ```
pub async fn complete4<F1, F2, F3, F4>(
    f1: F1,
    f2: F2,
    f3: F3,
    f4: F4,
) -> (F1::Output, F2::Output, F3::Output, F4::Output)
where
    F1: Future,
    F2: Future,
    F3: Future,
    F4: Future,
{
    complete::Complete4::new(f1, f2, f3, f4).await
}

/// Creates a combinator that runs the five given futures to completion, returning a tuple of the
/// outputs each yields.
///
///  # Example
///
///    ```
///    use cros_async::{complete5, run_one};
///
///    let first = async {5};
///    let second = async {6};
///    let third = async {7};
///    let fourth = async {8};
///    let fifth = async {9};
///    assert_eq!(run_one(complete5(first, second, third, fourth, fifth)).unwrap_or((0,0,0,0,0)),
///               (5,6,7,8,9));
///    ```
pub async fn complete5<F1, F2, F3, F4, F5>(
    f1: F1,
    f2: F2,
    f3: F3,
    f4: F4,
    f5: F5,
) -> (F1::Output, F2::Output, F3::Output, F4::Output, F5::Output)
where
    F1: Future,
    F2: Future,
    F3: Future,
    F4: Future,
    F5: Future,
{
    complete::Complete5::new(f1, f2, f3, f4, f5).await
}