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//! Management of asynchronous tasks in an FFI context.
//!
//! The *Task API* is very similar to the that exposed by Rust [futures], with
//! the aim to be usable from other languages.
//!
//! The main idea is your C users will:
//!
//! 1. Create a `Task` struct (who's `run()` method will execute the job)
//! 2. Spawn the `Task` on a background thread, receiving an opaque `TaskHandle`
//! through which the task can be monitored
//! 3. Periodically `poll()` the `TaskHandle` to see whether it's done or if
//! there was an error
//! 4. Retrieve the result when the `Task` completes
//! 5. destroy the original `TaskHandle` when it's longer need it
//!
//! # Implementing The Task API
//!
//! To use the *Task API* you just need to create a struct which implements the
//! `Task` trait. This is essentially just a trait with a `run()` function
//! that'll be given a [`CancellationToken`].
//!
//!
//! Implementors of the `Task` trait should periodically check the provided
//! [`CancellationToken`] to see whether the caller wants them to stop early.
//!
//! # Examples
//!
//! Once you have a type implementing `Task` you can use the [`export_task!()`]
//! macro to generate `extern "C"` functions for spawning the task and
//! monitoring its progress. This is usually the most
//! annoying/error-prone/tedious part of exposing running a `Task` in the
//! background using just a C API.
//!
//! For this example we're defining a `Spin` task which will count up until it
//! receives a cancel signal, then return the number of spins.
//!
//! ```rust
//! # use anyhow::Error;
//! # use ffi_helpers::task::CancellationToken;
//! # use ffi_helpers::Task;
//! # use ffi_helpers::error_handling::*;
//! # use std::thread;
//! # use std::time::Duration;
//! #[derive(Debug, Clone, Copy)]
//! pub struct Spin;
//!
//! impl Task for Spin {
//! type Output = usize;
//!
//! fn run(&self, cancel_tok: &CancellationToken) -> Result<Self::Output, Error> {
//! let mut spins = 0;
//!
//! while !cancel_tok.cancelled() {
//! thread::sleep(Duration::from_millis(10));
//! spins += 1;
//! }
//!
//! Ok(spins)
//! }
//! }
//!
//! // Generate the various `extern "C"` utility functions for working with the
//! // `Spin` task. The `spawn` function will be called `spin_spawn`, and so on.
//! ffi_helpers::export_task! {
//! Task: Spin;
//! spawn: spin_spawn;
//! wait: spin_wait;
//! poll: spin_poll;
//! cancel: spin_cancel;
//! cancelled: spin_cancelled;
//! handle_destroy: spin_handle_destroy;
//! result_destroy: spin_result_destroy;
//! }
//!
//! // create our `Spin` task
//! let s = Spin;
//!
//! unsafe {
//! // spawn the task in the background and get a handle to it
//! let handle = spin_spawn(&s);
//! assert_eq!(spin_cancelled(handle), 0,
//! "The spin shouldn't have been cancelled yet");
//!
//! // poll the task. The result can vary depending on the outcome:
//! // - If the task completed, get a pointer to the `Output`
//! // - If it completed with an error, return `null` and update the
//! // LAST_ERROR appropriately
//! // - Return `null` and *don't* set LAST_ERROR if the task isn't done
//! clear_last_error();
//! let ret = spin_poll(handle);
//! assert_eq!(last_error_length(), 0, "There shouldn't have been any errors");
//! assert!(ret.is_null(), "The task should still be running");
//!
//! // tell the task to stop spinning by sending the cancel signal
//! spin_cancel(handle);
//!
//! // wait for the task to finish and retrieve a pointer to its result
//! // Note: this will automatically free the handle, so we don't need
//! // to manually call `spin_handle_destroy()`.
//! let got = spin_wait(handle);
//!
//! assert_eq!(last_error_length(), 0, "There shouldn't have been any errors");
//! assert!(!got.is_null(), "Oops!");
//!
//! let num_spins: usize = *got;
//!
//! // don't forget the result is heap allocated so we need to free it
//! spin_result_destroy(got);
//! }
//! ```
//!
//! # Managing Task Output Lifetimes
//!
//! The result of a `Task` will be allocated on the heap and then a pointer
//! returned to the user from the `poll` and `wait` functions. It is the
//! caller's responsibility to ensure this gets free'd once you're done with it.
//!
//! The `export_task!()` macro lets you define a `results_destroy` function
//! which will free the object for you.
//!
//! Zero-sized types (like `()` - Rust's equivalent of the C `void` or Python
//! `None`) won't incur an allocation, meaning the `results_destroy` function
//! will be a noop.
//!
//! [futures]: https://github.com/rust-lang-nursery/futures-rs
//! [`CancellationToken`]: struct.CancellationToken.html
//! [`export_task!()`]: ../macro.export_task.html
use anyhow::Error;
use std::{
panic::UnwindSafe,
sync::{
atomic::{AtomicBool, Ordering},
mpsc::{self, Receiver, TryRecvError},
Arc,
},
thread,
};
use thiserror::Error;
use crate::{error_handling, panic};
/// Convenience macro to define the FFI bindings for working with a [`Task`].
///
/// This is implemented as an incremental TT muncher which lets you define the
/// functions you'll need. These are:
///
/// - `spawn`: The function for spawning a task on a background thread,
/// returning a [`TaskHandle`]
/// - `poll`: A function for receiving the result if it's available
/// - `wait`: Block the current thread until we get either a result or an error
/// - `cancel`: Cancel the background task
/// - `cancelled`: Has the task already been cancelled?
/// - `result_destroy`: A destructor for the task's result
/// - `handle_destroy`: A destructor for the [`TaskHandle`], for cleaning up the
/// task once you're done with it
///
/// You'll always need to provide the concrete [`Task`] type in the macro's
/// first "argument".
///
/// [`Task`]: task/trait.Task.html
/// [`TaskHandle`]: task/struct.TaskHandle.html
#[macro_export]
macro_rules! export_task {
($( #[$attr:meta] )* Task: $Task:ty; spawn: $spawn:ident; $( $tokens:tt )*) => {
/// Spawn a task in the background, returning a pointer to the task
/// handle.
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $spawn(task: *const $Task) -> *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output> {
$crate::null_pointer_check!(task);
let task = (&*task).clone();
let handle = $crate::task::TaskHandle::spawn(task);
Box::into_raw(Box::new(handle))
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; poll: $poll:ident; $( $tokens:tt )*) => {
/// Poll the task handle and retrieve the result it's ready.
///
/// # Note
///
/// This will return `null` if there was no result **or** if there was
/// an error. If there is an error, we update the last error accordingly.
///
/// You probably want to call `ffi_helpers::error_handling::clear_last_error()`
/// beforehand to make sure there
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $poll(handle: *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output>) -> *mut <$Task as $crate::Task>::Output {
$crate::null_pointer_check!(handle);
match (&*handle).poll() {
Some(Ok(value)) => Box::into_raw(Box::new(value)),
Some(Err(e)) => {
$crate::error_handling::update_last_error(e);
::std::ptr::null_mut()
}
None => ::std::ptr::null_mut()
}
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; handle_destroy: $handle_destructor:ident; $( $tokens:tt )*) => {
/// Destroy a task handle once you no longer need it, cancelling the
/// task if it hasn't yet completed.
///
/// # Warning
///
/// This conflicts with the `wait` function, which also destroys its
/// task handle.
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $handle_destructor(handle: *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output>) {
$crate::null_pointer_check!(handle);
let handle = Box::from_raw(handle);
drop(handle);
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; result_destroy: $result_destroy:ident; $( $tokens:tt )*) => {
/// Destroy the result of a task once you are done with it.
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $result_destroy(result: *mut <$Task as $crate::Task>::Output) {
$crate::null_pointer_check!(result);
let result = Box::from_raw(result);
drop(result);
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; wait: $wait:ident; $( $tokens:tt )*) => {
/// Wait for the task to finish, returning the boxed result and consuming
/// the task handle in the process.
///
/// # Warning
///
/// This will consume the task handle, meaning you **should not** call
/// the handle destructor afterwards.
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $wait(handle: *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output>)
-> *mut <$Task as $crate::Task>::Output
{
$crate::null_pointer_check!(handle);
let handle = Box::from_raw(handle);
let result = handle.wait();
match result {
Ok(value) => Box::into_raw(Box::new(value)),
Err(e) => {
$crate::update_last_error(e);
::std::ptr::null_mut()
}
}
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; cancel: $cancel:ident; $( $tokens:tt )*) => {
/// Cancel the task.
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $cancel(handle: *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output>) {
$crate::null_pointer_check!(handle);
(&*handle).cancel();
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty; cancelled: $cancelled:ident; $( $tokens:tt )*) => {
/// Has the task already been cancelled?
#[allow(dead_code)]
#[no_mangle]
$( #[$attr] )*
pub unsafe extern "C" fn $cancelled(handle: *mut $crate::task::TaskHandle<<$Task as $crate::Task>::Output>) -> ::std::os::raw::c_int {
$crate::null_pointer_check!(handle);
if (&*handle).cancelled() {
1
} else {
0
}
}
$crate::export_task!($( #[$attr] )* Task: $Task; $( $tokens )*);
};
($( #[$attr:meta] )* Task: $Task:ty;) => {};
}
/// A cancellable task which is meant to be run in a background thread.
///
/// For more information on the *Task API*, refer to the [module documentation].
///
/// [module documentation]: ./index.html
pub trait Task: Send + Sync + Clone {
type Output: Send + Sync;
/// Run this task to completion *synchronously*, exiting early if the
/// provided `CancellationToken` is triggered.
///
/// You probably shouldn't call this function directly. Instead prefer
/// higher level abstractions like [`TaskHandle::spawn()`] or bindings
/// generated by the [`export_task!()`] macro.
///
/// [`TaskHandle::spawn()`]: struct.TaskHandle.html#method.spawn
/// [`export_task!()`]: ../macro.export_task.html
fn run(
&self,
cancel_tok: &CancellationToken,
) -> Result<Self::Output, Error>;
}
/// A shareable token to let you notify other tasks they should stop what they
/// are doing and exit early.
#[derive(Debug, Clone)]
pub struct CancellationToken(Arc<AtomicBool>);
impl CancellationToken {
/// Create a new `CancellationToken`.
pub fn new() -> CancellationToken {
CancellationToken(Arc::new(AtomicBool::new(false)))
}
/// Has this token already been cancelled?
pub fn cancelled(&self) -> bool { self.0.load(Ordering::SeqCst) }
/// Cancel the token, notifying anyone else listening that they should halt
/// what they are doing.
pub fn cancel(&self) { self.0.store(true, Ordering::SeqCst); }
pub fn is_done(&self) -> Result<(), Cancelled> {
if self.cancelled() {
Err(Cancelled)
} else {
Ok(())
}
}
}
impl Default for CancellationToken {
fn default() -> CancellationToken { CancellationToken::new() }
}
/// An error to indicate a task was cancelled.
#[derive(Debug, Clone, Copy, PartialEq, Error)]
#[error("The task was cancelled")]
pub struct Cancelled;
/// An opaque handle to some task which is running in the background.
pub struct TaskHandle<T> {
result: Receiver<Result<T, Error>>,
token: CancellationToken,
}
impl<T> TaskHandle<T> {
/// Spawn a `Task` in the background, returning the a `TaskHandle` so you
/// can cancel it or retrieve the result later on.
pub fn spawn<K>(task: K) -> TaskHandle<T>
where
K: Task<Output = T> + UnwindSafe + Send + Sync + 'static,
T: Send + Sync + 'static,
{
let (tx, rx) = mpsc::channel();
let cancel_tok = CancellationToken::new();
let tok_2 = cancel_tok.clone();
thread::spawn(move || {
error_handling::clear_last_error();
let got =
panic::catch_panic(move || task.run(&tok_2)).map_err(|_| {
// we want to preserve panic messages and pass them back to
// the main thread so we manually take
// LAST_ERROR
let e = error_handling::take_last_error();
e.unwrap_or_else(|| anyhow::anyhow!("The task failed"))
});
tx.send(got).ok();
});
TaskHandle {
result: rx,
token: cancel_tok,
}
}
/// Check if the background task has finished.
///
/// If the other end hangs up for whatever reason this will return an error.
pub fn poll(&self) -> Option<Result<T, Error>> {
// This looks an awful lot like the Futures API, doesn't it?
match self.result.try_recv() {
Ok(value) => Some(value),
Err(TryRecvError::Empty) => None,
Err(e) => Some(Err(e.into())),
}
}
/// Block the current thread until the task has finished and returned a
/// result.
pub fn wait(self) -> Result<T, Error> {
match self.result.recv() {
Ok(Ok(value)) => Ok(value),
Ok(Err(e)) => Err(e),
Err(recv_err) => Err(recv_err.into()),
}
}
/// Cancel the background task.
pub fn cancel(&self) { self.token.cancel(); }
/// Has this task been cancelled?
pub fn cancelled(&self) -> bool { self.token.cancelled() }
}
impl<T> Drop for TaskHandle<T> {
fn drop(&mut self) { self.token.cancel(); }
}
#[cfg(test)]
mod tests {
use super::*;
use crate::panic::Panic;
use std::time::Duration;
#[derive(Debug, Clone, Copy)]
pub struct Spin;
impl Task for Spin {
type Output = usize;
fn run(
&self,
cancel_tok: &CancellationToken,
) -> Result<Self::Output, Error> {
let mut spins = 0;
while !cancel_tok.cancelled() {
thread::sleep(Duration::from_millis(10));
spins += 1;
}
Ok(spins)
}
}
#[test]
fn spawn_a_task() {
let task = Spin;
let handle = TaskHandle::spawn(task);
// wait for about 100 ms
for _ in 0..10 {
thread::sleep(Duration::from_millis(10));
let got = handle.poll();
assert!(got.is_none());
}
handle.cancel();
let got = handle.wait().unwrap();
// the task should have spun 9-12 times (depending on the OS's
// scheduler)
assert!(9 <= got && got <= 12);
}
export_task! {
Task: Spin;
spawn: spin_spawn;
wait: spin_wait;
poll: spin_poll;
cancel: spin_cancel;
cancelled: spin_cancelled;
handle_destroy: spin_handle_destroy;
result_destroy: spin_result_destroy;
}
#[test]
fn use_the_c_api() {
use crate::error_handling::*;
let s = Spin;
unsafe {
let handle = spin_spawn(&s);
assert_eq!(
spin_cancelled(handle),
0,
"The spin shouldn't have been cancelled yet"
);
// poll the task
clear_last_error();
let ret = spin_poll(handle);
assert!(ret.is_null(), "The task should still be running");
assert_eq!(
last_error_length(),
0,
"There shouldn't have been any errors"
);
// tell the task to stop spinning
spin_cancel(handle);
// wait for the task to finish and retrieve its result
let got = spin_wait(handle);
assert_eq!(
last_error_length(),
0,
"There shouldn't have been any errors"
);
assert!(!got.is_null(), "Oops!");
}
}
#[derive(Copy, Clone)]
struct PanicTask;
const PANIC_MESSAGE: &str = "Oops";
impl Task for PanicTask {
type Output = ();
fn run(&self, _: &CancellationToken) -> Result<Self::Output, Error> {
panic!("{}", PANIC_MESSAGE)
}
}
#[test]
fn task_can_catch_panic_messages() {
let task = PanicTask;
let err = TaskHandle::spawn(task).wait().unwrap_err();
if let Some(p) = err.downcast_ref::<Panic>() {
assert_eq!(p.message, PANIC_MESSAGE);
} else {
panic!("Expected a panic failure, got {}", err);
}
}
}