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use parking_lot::{Mutex, MutexGuard};
use std::{
future::Future,
sync::{
atomic::{AtomicBool, Ordering},
Arc,
},
task::{Poll, Waker},
};
pub struct Promise<T> {
shared: Arc<PromiseShared<T>>,
}
pub struct PromiseFuture<T> {
shared: Arc<PromiseShared<T>>,
}
struct PromiseShared<T> {
inner: Mutex<PromiseInner<T>>,
promise_dropped: AtomicBool,
}
impl<T> Default for PromiseShared<T> {
fn default() -> Self {
Self {
inner: Default::default(),
promise_dropped: Default::default(),
}
}
}
impl<T> PromiseShared<T> {
fn inner(&self) -> MutexGuard<'_, PromiseInner<T>> {
self.inner.lock()
}
}
struct PromiseInner<T> {
result: Option<T>,
waker: Option<Waker>,
merged: Option<Promise<T>>,
}
impl<T> Default for PromiseInner<T> {
fn default() -> Self {
Self {
result: None,
waker: None,
merged: None,
}
}
}
unsafe impl<T: Send> Send for PromiseInner<T> {}
unsafe impl<T: Send> Sync for PromiseInner<T> {}
#[derive(Debug, PartialEq, Eq)]
pub struct PromiseDropped;
impl std::error::Error for PromiseDropped {}
impl std::fmt::Display for PromiseDropped {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.write_str("The promise was dropped without being fulfilled")
}
}
impl<T> Drop for Promise<T> {
fn drop(&mut self) {
self.shared.promise_dropped.store(true, Ordering::Release);
let mut data = self.shared.inner();
drop(data.merged.take());
if let Some(waker) = data.waker.take() {
waker.wake();
}
}
}
impl<T> Promise<T> {
pub fn new() -> (Promise<T>, PromiseFuture<T>) {
let shared = Default::default();
(
Self {
shared: Arc::clone(&shared),
},
PromiseFuture { shared },
)
}
pub fn fulfill_unmergable(self, result: T) {
let mut data = self.shared.inner();
data.result.replace(result);
}
}
impl<T: Clone> Promise<T> {
pub fn fulfill(self, result: T) {
let mut data = self.shared.inner();
if let Some(merged) = data.merged.take() {
merged.fulfill(result.clone());
}
data.result.replace(result);
}
pub fn merge(&mut self, other: Self) {
let mut data = self.shared.inner();
if let Some(merged) = &mut data.merged {
merged.merge(other);
} else {
data.merged.replace(other);
}
}
}
impl<T> Future for PromiseFuture<T> {
type Output = Result<T, PromiseDropped>;
fn poll(self: std::pin::Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
let mut data = self.shared.inner();
if let Some(result) = data.result.take() {
Poll::Ready(Ok(result))
} else if self.shared.promise_dropped.load(Ordering::Acquire) {
Poll::Ready(Err(PromiseDropped))
} else {
data.waker.replace(cx.waker().clone());
Poll::Pending
}
}
}
#[cfg(test)]
mod test {
use std::thread;
use futures::executor::block_on;
use crate::{Job, MergeResult};
use super::*;
struct MyJob(Promise<String>, String);
impl Job for MyJob {
type Exclusion = ();
fn exclusion(&self) -> Self::Exclusion {}
type Priority = ();
fn priority(&self) -> Self::Priority {}
fn execute(self) {
self.0.fulfill(self.1)
}
}
fn merge(this: MyJob, target: &mut MyJob) -> crate::MergeResult<MyJob> {
target.1.push_str(&this.1);
target.0.merge(this.0);
MergeResult::Success
}
#[test]
fn test_with_result() {
let (promise, fut) = Promise::new();
let job = MyJob(promise, "hello".to_string());
thread::spawn(move || job.execute());
assert_eq!(Ok("hello".to_string()), block_on(fut));
}
#[test]
fn test_with_result_already() {
let (promise, fut) = Promise::new();
let job = MyJob(promise, "hello".to_string());
job.execute();
assert_eq!(Ok("hello".to_string()), block_on(fut));
}
#[test]
fn test_with_drop() {
let (promise, fut) = Promise::new();
let job = MyJob(promise, "hello".to_string());
thread::spawn(move || drop(job));
assert_eq!(Err(PromiseDropped), block_on(fut));
}
#[test]
fn test_with_drop_already() {
let (promise, fut) = Promise::new();
let job = MyJob(promise, "hello".to_string());
drop(job);
assert_eq!(Err(PromiseDropped), block_on(fut));
}
#[test]
fn test_merged_with_result() {
let (promise1, fut1) = Promise::new();
let (promise2, fut2) = Promise::new();
let mut job = MyJob(promise1, "hello".to_string());
let job2 = MyJob(promise2, "world".to_string());
merge(job2, &mut job);
thread::spawn(move || job.execute());
assert_eq!(Ok("helloworld".to_string()), block_on(fut1));
assert_eq!(Ok("helloworld".to_string()), block_on(fut2));
}
#[test]
fn test_merged_with_result_reverse() {
let (promise1, fut1) = Promise::new();
let (promise2, fut2) = Promise::new();
let mut job = MyJob(promise1, "hello".to_string());
let job2 = MyJob(promise2, "world".to_string());
merge(job2, &mut job);
thread::spawn(move || job.execute());
assert_eq!(Ok("helloworld".to_string()), block_on(fut2));
assert_eq!(Ok("helloworld".to_string()), block_on(fut1));
}
#[test]
fn test_merged_with_drop() {
let (promise1, fut1) = Promise::new();
let (promise2, fut2) = Promise::new();
let mut job = MyJob(promise1, "hello".to_string());
let job2 = MyJob(promise2, "world".to_string());
merge(job2, &mut job);
thread::spawn(move || drop(job));
assert!(block_on(fut1).is_err());
assert!(block_on(fut2).is_err());
}
#[test]
fn test_merged_with_drop_reverse() {
let (promise1, fut1) = Promise::new();
let (promise2, fut2) = Promise::new();
let mut job = MyJob(promise1, "hello".to_string());
let job2 = MyJob(promise2, "world".to_string());
merge(job2, &mut job);
thread::spawn(move || drop(job));
assert!(block_on(fut2).is_err());
assert!(block_on(fut1).is_err());
}
}