Chih-Hung Hsieh | cfc3a23 | 2020-06-10 20:13:05 -0700 | [diff] [blame] | 1 | //! Lazily initialized data. |
| 2 | //! Used in generated code. |
| 3 | |
Haibo Huang | 52aa785 | 2020-07-10 20:23:55 -0700 | [diff] [blame] | 4 | // Avoid deprecation warnings when compiling rust-protobuf |
| 5 | #![allow(deprecated)] |
| 6 | |
Chih-Hung Hsieh | cfc3a23 | 2020-06-10 20:13:05 -0700 | [diff] [blame] | 7 | use std::mem; |
| 8 | use std::sync; |
| 9 | |
| 10 | /// Lasily initialized data. |
Haibo Huang | 52aa785 | 2020-07-10 20:23:55 -0700 | [diff] [blame] | 11 | #[deprecated( |
| 12 | since = "2.16", |
| 13 | note = "Please regenerate .rs files from .proto files to use newer APIs" |
| 14 | )] |
Chih-Hung Hsieh | cfc3a23 | 2020-06-10 20:13:05 -0700 | [diff] [blame] | 15 | pub struct Lazy<T> { |
| 16 | #[doc(hidden)] |
| 17 | pub lock: sync::Once, |
| 18 | #[doc(hidden)] |
| 19 | pub ptr: *const T, |
| 20 | } |
| 21 | |
| 22 | impl<T> Lazy<T> { |
| 23 | /// Uninitialized `Lazy` object. |
| 24 | /// |
| 25 | /// The initializer is added in rust-protobuf 2.11, for compatibility with |
| 26 | /// previously generated code, existing fields are kept public. |
| 27 | pub const INIT: Lazy<T> = Lazy { |
| 28 | lock: sync::Once::new(), |
| 29 | ptr: 0 as *const T, |
| 30 | }; |
| 31 | |
| 32 | /// Get lazy field value, initialize it with given function if not yet. |
| 33 | pub fn get<F>(&'static mut self, init: F) -> &'static T |
| 34 | where |
| 35 | F: FnOnce() -> T, |
| 36 | { |
| 37 | // ~ decouple the lifetimes of 'self' and 'self.lock' such we |
| 38 | // can initialize self.ptr in the call_once closure (note: we |
| 39 | // do have to initialize self.ptr in the closure to guarantee |
| 40 | // the ptr is valid for all calling threads at any point in |
| 41 | // time) |
| 42 | let lock: &sync::Once = unsafe { mem::transmute(&self.lock) }; |
| 43 | lock.call_once(|| unsafe { |
| 44 | self.ptr = mem::transmute(Box::new(init())); |
| 45 | }); |
| 46 | unsafe { &*self.ptr } |
| 47 | } |
| 48 | } |
| 49 | |
| 50 | /// Used to initialize `lock` field in `Lazy` struct. |
| 51 | #[deprecated( |
| 52 | since = "2.11", |
| 53 | note = "Regenerate .proto files to use safer initializer" |
| 54 | )] |
| 55 | pub const ONCE_INIT: sync::Once = sync::Once::new(); |
| 56 | |
| 57 | #[cfg(test)] |
| 58 | mod test { |
Chih-Hung Hsieh | cfc3a23 | 2020-06-10 20:13:05 -0700 | [diff] [blame] | 59 | use std::sync::atomic::AtomicIsize; |
| 60 | use std::sync::atomic::Ordering; |
| 61 | use std::sync::Arc; |
| 62 | use std::sync::Barrier; |
| 63 | use std::thread; |
| 64 | |
David LeGare | 793d84b | 2022-03-02 16:21:10 +0000 | [diff] [blame] | 65 | use super::Lazy; |
| 66 | |
Chih-Hung Hsieh | cfc3a23 | 2020-06-10 20:13:05 -0700 | [diff] [blame] | 67 | #[test] |
| 68 | fn many_threads_calling_get() { |
| 69 | const N_THREADS: usize = 32; |
| 70 | const N_ITERS_IN_THREAD: usize = 32; |
| 71 | const N_ITERS: usize = 16; |
| 72 | |
| 73 | static mut LAZY: Lazy<String> = Lazy::INIT; |
| 74 | static CALL_COUNT: AtomicIsize = AtomicIsize::new(0); |
| 75 | |
| 76 | let value = "Hello, world!".to_owned(); |
| 77 | |
| 78 | for _ in 0..N_ITERS { |
| 79 | // Reset mutable state. |
| 80 | unsafe { |
| 81 | LAZY = Lazy::INIT; |
| 82 | } |
| 83 | CALL_COUNT.store(0, Ordering::SeqCst); |
| 84 | |
| 85 | // Create a bunch of threads, all calling .get() at the same time. |
| 86 | let mut threads = vec![]; |
| 87 | let barrier = Arc::new(Barrier::new(N_THREADS)); |
| 88 | |
| 89 | for _ in 0..N_THREADS { |
| 90 | let cloned_value_thread = value.clone(); |
| 91 | let cloned_barrier = barrier.clone(); |
| 92 | threads.push(thread::spawn(move || { |
| 93 | // Ensure all threads start at once to maximise contention. |
| 94 | cloned_barrier.wait(); |
| 95 | for _ in 0..N_ITERS_IN_THREAD { |
| 96 | assert_eq!(&cloned_value_thread, unsafe { |
| 97 | LAZY.get(|| { |
| 98 | CALL_COUNT.fetch_add(1, Ordering::SeqCst); |
| 99 | cloned_value_thread.clone() |
| 100 | }) |
| 101 | }); |
| 102 | } |
| 103 | })); |
| 104 | } |
| 105 | |
| 106 | for thread in threads { |
| 107 | thread.join().unwrap(); |
| 108 | } |
| 109 | |
| 110 | assert_eq!(CALL_COUNT.load(Ordering::SeqCst), 1); |
| 111 | } |
| 112 | } |
| 113 | } |