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use crate::primitive::sync::{Arc, Condvar, Mutex};
use std::fmt;
/// Enables threads to synchronize the beginning or end of some computation.
///
/// # Wait groups vs barriers
///
/// `WaitGroup` is very similar to [`Barrier`], but there are a few differences:
///
/// * [`Barrier`] needs to know the number of threads at construction, while `WaitGroup` is cloned to
/// register more threads.
///
/// * A [`Barrier`] can be reused even after all threads have synchronized, while a `WaitGroup`
/// synchronizes threads only once.
///
/// * All threads wait for others to reach the [`Barrier`]. With `WaitGroup`, each thread can choose
/// to either wait for other threads or to continue without blocking.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::WaitGroup;
/// use std::thread;
///
/// // Create a new wait group.
/// let wg = WaitGroup::new();
///
/// for _ in 0..4 {
/// // Create another reference to the wait group.
/// let wg = wg.clone();
///
/// thread::spawn(move || {
/// // Do some work.
///
/// // Drop the reference to the wait group.
/// drop(wg);
/// });
/// }
///
/// // Block until all threads have finished their work.
/// wg.wait();
/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
/// ```
///
/// [`Barrier`]: std::sync::Barrier
pub struct WaitGroup {
inner: Arc<Inner>,
}
/// Inner state of a `WaitGroup`.
struct Inner {
cvar: Condvar,
count: Mutex<usize>,
}
impl Default for WaitGroup {
fn default() -> Self {
Self {
inner: Arc::new(Inner {
cvar: Condvar::new(),
count: Mutex::new(1),
}),
}
}
}
impl WaitGroup {
/// Creates a new wait group and returns the single reference to it.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::WaitGroup;
///
/// let wg = WaitGroup::new();
/// ```
pub fn new() -> Self {
Self::default()
}
/// Drops this reference and waits until all other references are dropped.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::WaitGroup;
/// use std::thread;
///
/// let wg = WaitGroup::new();
///
/// thread::spawn({
/// let wg = wg.clone();
/// move || {
/// // Block until both threads have reached `wait()`.
/// wg.wait();
/// }
/// });
///
/// // Block until both threads have reached `wait()`.
/// wg.wait();
/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
/// ```
pub fn wait(self) {
if *self.inner.count.lock().unwrap() == 1 {
return;
}
let inner = self.inner.clone();
drop(self);
let mut count = inner.count.lock().unwrap();
while *count > 0 {
count = inner.cvar.wait(count).unwrap();
}
}
}
impl Drop for WaitGroup {
fn drop(&mut self) {
let mut count = self.inner.count.lock().unwrap();
*count -= 1;
if *count == 0 {
self.inner.cvar.notify_all();
}
}
}
impl Clone for WaitGroup {
fn clone(&self) -> WaitGroup {
let mut count = self.inner.count.lock().unwrap();
*count += 1;
WaitGroup {
inner: self.inner.clone(),
}
}
}
impl fmt::Debug for WaitGroup {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let count: &usize = &*self.inner.count.lock().unwrap();
f.debug_struct("WaitGroup").field("count", count).finish()
}
}