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-rw-r--r--rand/src/rngs/thread.rs124
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-// Copyright 2018 Developers of the Rand project.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-//! Thread-local random number generator
-
-use std::cell::UnsafeCell;
-use std::ptr::NonNull;
-
-use crate::{RngCore, CryptoRng, SeedableRng, Error};
-use crate::rngs::adapter::ReseedingRng;
-use crate::rngs::OsRng;
-use super::std::Core;
-
-// Rationale for using `UnsafeCell` in `ThreadRng`:
-//
-// Previously we used a `RefCell`, with an overhead of ~15%. There will only
-// ever be one mutable reference to the interior of the `UnsafeCell`, because
-// we only have such a reference inside `next_u32`, `next_u64`, etc. Within a
-// single thread (which is the definition of `ThreadRng`), there will only ever
-// be one of these methods active at a time.
-//
-// A possible scenario where there could be multiple mutable references is if
-// `ThreadRng` is used inside `next_u32` and co. But the implementation is
-// completely under our control. We just have to ensure none of them use
-// `ThreadRng` internally, which is nonsensical anyway. We should also never run
-// `ThreadRng` in destructors of its implementation, which is also nonsensical.
-
-
-// Number of generated bytes after which to reseed `ThreadRng`.
-// According to benchmarks, reseeding has a noticable impact with thresholds
-// of 32 kB and less. We choose 64 kB to avoid significant overhead.
-const THREAD_RNG_RESEED_THRESHOLD: u64 = 1024 * 64;
-
-/// The type returned by [`thread_rng`], essentially just a reference to the
-/// PRNG in thread-local memory.
-///
-/// `ThreadRng` uses the same PRNG as [`StdRng`] for security and performance.
-/// As hinted by the name, the generator is thread-local. `ThreadRng` is a
-/// handle to this generator and thus supports `Copy`, but not `Send` or `Sync`.
-///
-/// Unlike `StdRng`, `ThreadRng` uses the [`ReseedingRng`] wrapper to reseed
-/// the PRNG from fresh entropy every 64 kiB of random data.
-/// [`OsRng`] is used to provide seed data.
-///
-/// Note that the reseeding is done as an extra precaution against side-channel
-/// attacks and mis-use (e.g. if somehow weak entropy were supplied initially).
-/// The PRNG algorithms used are assumed to be secure.
-///
-/// [`ReseedingRng`]: crate::rngs::adapter::ReseedingRng
-/// [`StdRng`]: crate::rngs::StdRng
-#[derive(Copy, Clone, Debug)]
-pub struct ThreadRng {
- // inner raw pointer implies type is neither Send nor Sync
- rng: NonNull<ReseedingRng<Core, OsRng>>,
-}
-
-thread_local!(
- static THREAD_RNG_KEY: UnsafeCell<ReseedingRng<Core, OsRng>> = {
- let r = Core::from_rng(OsRng).unwrap_or_else(|err|
- panic!("could not initialize thread_rng: {}", err));
- let rng = ReseedingRng::new(r,
- THREAD_RNG_RESEED_THRESHOLD,
- OsRng);
- UnsafeCell::new(rng)
- }
-);
-
-/// Retrieve the lazily-initialized thread-local random number generator,
-/// seeded by the system. Intended to be used in method chaining style,
-/// e.g. `thread_rng().gen::<i32>()`, or cached locally, e.g.
-/// `let mut rng = thread_rng();`. Invoked by the `Default` trait, making
-/// `ThreadRng::default()` equivalent.
-///
-/// For more information see [`ThreadRng`].
-pub fn thread_rng() -> ThreadRng {
- let raw = THREAD_RNG_KEY.with(|t| t.get());
- let nn = NonNull::new(raw).unwrap();
- ThreadRng { rng: nn }
-}
-
-impl Default for ThreadRng {
- fn default() -> ThreadRng {
- crate::prelude::thread_rng()
- }
-}
-
-impl RngCore for ThreadRng {
- #[inline(always)]
- fn next_u32(&mut self) -> u32 {
- unsafe { self.rng.as_mut().next_u32() }
- }
-
- #[inline(always)]
- fn next_u64(&mut self) -> u64 {
- unsafe { self.rng.as_mut().next_u64() }
- }
-
- fn fill_bytes(&mut self, dest: &mut [u8]) {
- unsafe { self.rng.as_mut().fill_bytes(dest) }
- }
-
- fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
- unsafe { self.rng.as_mut().try_fill_bytes(dest) }
- }
-}
-
-impl CryptoRng for ThreadRng {}
-
-
-#[cfg(test)]
-mod test {
- #[test]
- fn test_thread_rng() {
- use crate::Rng;
- let mut r = crate::thread_rng();
- r.gen::<i32>();
- assert_eq!(r.gen_range(0, 1), 0);
- }
-}