xref: /aosp_15_r20/external/cronet/third_party/rust/chromium_crates_io/vendor/getrandom-0.2.14/src/lazy.rs (revision 6777b5387eb2ff775bb5750e3f5d96f37fb7352b)
1 use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
2 
3 // This structure represents a lazily initialized static usize value. Useful
4 // when it is preferable to just rerun initialization instead of locking.
5 // unsync_init will invoke an init() function until it succeeds, then return the
6 // cached value for future calls.
7 //
8 // unsync_init supports init() "failing". If the init() method returns UNINIT,
9 // that value will be returned as normal, but will not be cached.
10 //
11 // Users should only depend on the _value_ returned by init() functions.
12 // Specifically, for the following init() function:
13 //      fn init() -> usize {
14 //          a();
15 //          let v = b();
16 //          c();
17 //          v
18 //      }
19 // the effects of c() or writes to shared memory will not necessarily be
20 // observed and additional synchronization methods may be needed.
21 pub(crate) struct LazyUsize(AtomicUsize);
22 
23 impl LazyUsize {
new() -> Self24     pub const fn new() -> Self {
25         Self(AtomicUsize::new(Self::UNINIT))
26     }
27 
28     // The initialization is not completed.
29     pub const UNINIT: usize = usize::max_value();
30 
31     // Runs the init() function at most once, returning the value of some run of
32     // init(). Multiple callers can run their init() functions in parallel.
33     // init() should always return the same value, if it succeeds.
unsync_init(&self, init: impl FnOnce() -> usize) -> usize34     pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
35         // Relaxed ordering is fine, as we only have a single atomic variable.
36         let mut val = self.0.load(Relaxed);
37         if val == Self::UNINIT {
38             val = init();
39             self.0.store(val, Relaxed);
40         }
41         val
42     }
43 }
44 
45 // Identical to LazyUsize except with bool instead of usize.
46 pub(crate) struct LazyBool(LazyUsize);
47 
48 impl LazyBool {
new() -> Self49     pub const fn new() -> Self {
50         Self(LazyUsize::new())
51     }
52 
unsync_init(&self, init: impl FnOnce() -> bool) -> bool53     pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
54         self.0.unsync_init(|| init() as usize) != 0
55     }
56 }
57