1use crate::primitive::sync::{Arc, Condvar, Mutex};
2use std::fmt;
3
4/// Enables threads to synchronize the beginning or end of some computation.
5///
6/// # Wait groups vs barriers
7///
8/// `WaitGroup` is very similar to [`Barrier`], but there are a few differences:
9///
10/// * [`Barrier`] needs to know the number of threads at construction, while `WaitGroup` is cloned to
11/// register more threads.
12///
13/// * A [`Barrier`] can be reused even after all threads have synchronized, while a `WaitGroup`
14/// synchronizes threads only once.
15///
16/// * All threads wait for others to reach the [`Barrier`]. With `WaitGroup`, each thread can choose
17/// to either wait for other threads or to continue without blocking.
18///
19/// # Examples
20///
21/// ```
22/// use crossbeam_utils::sync::WaitGroup;
23/// use std::thread;
24///
25/// // Create a new wait group.
26/// let wg = WaitGroup::new();
27///
28/// for _ in 0..4 {
29/// // Create another reference to the wait group.
30/// let wg = wg.clone();
31///
32/// thread::spawn(move || {
33/// // Do some work.
34///
35/// // Drop the reference to the wait group.
36/// drop(wg);
37/// });
38/// }
39///
40/// // Block until all threads have finished their work.
41/// wg.wait();
42/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
43/// ```
44///
45/// [`Barrier`]: std::sync::Barrier
46pub struct WaitGroup {
47 inner: Arc<Inner>,
48}
49
50/// Inner state of a `WaitGroup`.
51struct Inner {
52 cvar: Condvar,
53 count: Mutex<usize>,
54}
55
56impl Default for WaitGroup {
57 fn default() -> Self {
58 Self {
59 inner: Arc::new(data:Inner {
60 cvar: Condvar::new(),
61 count: Mutex::new(1),
62 }),
63 }
64 }
65}
66
67impl WaitGroup {
68 /// Creates a new wait group and returns the single reference to it.
69 ///
70 /// # Examples
71 ///
72 /// ```
73 /// use crossbeam_utils::sync::WaitGroup;
74 ///
75 /// let wg = WaitGroup::new();
76 /// ```
77 pub fn new() -> Self {
78 Self::default()
79 }
80
81 /// Drops this reference and waits until all other references are dropped.
82 ///
83 /// # Examples
84 ///
85 /// ```
86 /// use crossbeam_utils::sync::WaitGroup;
87 /// use std::thread;
88 ///
89 /// let wg = WaitGroup::new();
90 ///
91 /// thread::spawn({
92 /// let wg = wg.clone();
93 /// move || {
94 /// // Block until both threads have reached `wait()`.
95 /// wg.wait();
96 /// }
97 /// });
98 ///
99 /// // Block until both threads have reached `wait()`.
100 /// wg.wait();
101 /// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
102 /// ```
103 pub fn wait(self) {
104 if *self.inner.count.lock().unwrap() == 1 {
105 return;
106 }
107
108 let inner = self.inner.clone();
109 drop(self);
110
111 let mut count = inner.count.lock().unwrap();
112 while *count > 0 {
113 count = inner.cvar.wait(count).unwrap();
114 }
115 }
116}
117
118impl Drop for WaitGroup {
119 fn drop(&mut self) {
120 let mut count: MutexGuard<'_, usize> = self.inner.count.lock().unwrap();
121 *count -= 1;
122
123 if *count == 0 {
124 self.inner.cvar.notify_all();
125 }
126 }
127}
128
129impl Clone for WaitGroup {
130 fn clone(&self) -> WaitGroup {
131 let mut count: MutexGuard<'_, usize> = self.inner.count.lock().unwrap();
132 *count += 1;
133
134 WaitGroup {
135 inner: self.inner.clone(),
136 }
137 }
138}
139
140impl fmt::Debug for WaitGroup {
141 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
142 let count: &usize = &*self.inner.count.lock().unwrap();
143 f.debug_struct("WaitGroup").field(name:"count", value:count).finish()
144 }
145}
146