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#![deny(missing_docs)]
//! A crate for things that are
//! 1) Lazily initialized
//! 2) Expensive to create
//! 3) Immutable after creation
//! 4) Used on multiple threads
//!
//! `Lazy<T>` is better than `Mutex<Option<T>>` because after creation accessing
//! `T` does not require any locking, just a single boolean load with
//! `Ordering::Acquire` (which on x86 is just a compiler barrier, not an actual
//! memory barrier).
use std::cell::UnsafeCell;
use std::fmt;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Mutex;
#[derive(Clone)]
enum ThisOrThat<T, U> {
This(T),
That(U),
}
/// `LazyTransform<T, U>` is a synchronized holder type, that holds a value of
/// type T until it is lazily converted into a value of type U.
pub struct LazyTransform<T, U> {
initialized: AtomicBool,
lock: Mutex<()>,
value: UnsafeCell<Option<ThisOrThat<T, U>>>,
}
// Implementation details.
impl<T, U> LazyTransform<T, U> {
fn extract(&self) -> Option<&U> {
// Make sure we're initialized first!
match unsafe { (*self.value.get()).as_ref() } {
None => None,
Some(&ThisOrThat::This(_)) => panic!(), // Should already be initialized!
Some(&ThisOrThat::That(ref that)) => Some(that),
}
}
}
// Public API.
impl<T, U> LazyTransform<T, U> {
/// Construct a new, untransformed `LazyTransform<T, U>` with an argument of
/// type T.
pub fn new(t: T) -> LazyTransform<T, U> {
LazyTransform {
initialized: AtomicBool::new(false),
lock: Mutex::new(()),
value: UnsafeCell::new(Some(ThisOrThat::This(t))),
}
}
/// Unwrap the contained value, returning `Ok(U)` if the `LazyTransform<T, U>` has been
/// transformed or `Err(T)` if it has not.
pub fn into_inner(self) -> Result<U, T> {
// We don't need to inspect `self.initialized` since `self` is owned
// so it is guaranteed that no other threads are accessing its data.
match self.value.into_inner().unwrap() {
ThisOrThat::This(t) => Err(t),
ThisOrThat::That(u) => Ok(u),
}
}
}
// Public API.
impl<T, U> LazyTransform<T, U> {
/// Get a reference to the transformed value, invoking `f` to transform it
/// if the `LazyTransform<T, U>` has yet to be transformed. It is
/// guaranteed that if multiple calls to `get_or_create` race, only one
/// will invoke its closure, and every call will receive a reference to the
/// newly transformed value.
///
/// The closure can only ever be called once, so think carefully about what
/// transformation you want to apply!
pub fn get_or_create<F>(&self, f: F) -> &U
where
F: FnOnce(T) -> U,
{
// In addition to being correct, this pattern is vouched for by Hans Boehm
// (http://schd.ws/hosted_files/cppcon2016/74/HansWeakAtomics.pdf Page 27)
if !self.initialized.load(Ordering::Acquire) {
// We *may* not be initialized. We have to block to be certain.
let _lock = self.lock.lock().unwrap();
if !self.initialized.load(Ordering::Relaxed) {
// Ok, we're definitely uninitialized.
// Safe to fiddle with the UnsafeCell now, because we're locked,
// and there can't be any outstanding references.
let value = unsafe { &mut *self.value.get() };
let this = match value.take().unwrap() {
ThisOrThat::This(t) => t,
ThisOrThat::That(_) => panic!(), // Can't already be initialized!
};
*value = Some(ThisOrThat::That(f(this)));
self.initialized.store(true, Ordering::Release);
} else {
// We raced, and someone else initialized us. We can fall
// through now.
}
}
// We're initialized, our value is immutable, no synchronization needed.
self.extract().unwrap()
}
/// Get a reference to the transformed value, returning `Some(&U)` if the
/// `LazyTransform<T, U>` has been transformed or `None` if it has not. It
/// is guaranteed that if a reference is returned it is to the transformed
/// value inside the the `LazyTransform<T>`.
pub fn get(&self) -> Option<&U> {
if self.initialized.load(Ordering::Acquire) {
// We're initialized, our value is immutable, no synchronization needed.
self.extract()
} else {
None
}
}
}
// As `T` is only ever accessed when locked, it's enough if it's `Send` for `Self` to be `Sync`.
unsafe impl<T, U> Sync for LazyTransform<T, U>
where
T: Send,
U: Send + Sync,
{
}
impl<T, U> Clone for LazyTransform<T, U>
where
T: Clone,
U: Clone,
{
fn clone(&self) -> Self {
// Overall, this method is very similar to `get_or_create` and uses the same
// soundness reasoning.
if self.initialized.load(Ordering::Acquire) {
Self {
initialized: true.into(),
lock: Mutex::default(),
value: UnsafeCell::new(unsafe {
// SAFETY:
// Everything is initialized and immutable here, so lockless cloning is safe.
(&*self.value.get()).clone()
}),
}
} else {
// We *may* not be initialized. We have to block here before accessing `value`,
// which also synchronises the `initialized` load.
let _lock = self.lock.lock().unwrap();
Self {
initialized: self.initialized.load(Ordering::Relaxed).into(),
lock: Mutex::default(),
value: UnsafeCell::new(unsafe {
// SAFETY:
// Exclusive access while `_lock` is held.
(&*self.value.get()).clone()
}),
}
}
}
fn clone_from(&mut self, source: &Self) {
// Overall, this method is very similar to `get_or_create` and uses the same
// soundness reasoning. It's implemented explicitly here to avoid a `Mutex` drop/new.
if self.initialized.load(Ordering::Acquire) {
unsafe {
// SAFETY:
// Everything is initialized and immutable here, so lockless cloning is safe.
// It's still important to store `initialized` with correct ordering, though.
*self.value.get() = (&*source.value.get()).clone();
self.initialized.store(true, Ordering::Release);
}
} else {
// `source` *may* not be initialized. We have to block here before accessing `value`,
// which also synchronises the `initialized` load (and incidentally also the `initialized`
// store due to the exclusive reference to `self`, so that can be `Relaxed` here too).
let _lock = source.lock.lock().unwrap();
unsafe {
// SAFETY:
// Exclusive access to `source` while `_lock` is held.
*self.value.get() = (&*source.value.get()).clone();
self.initialized.store(
source.initialized.load(Ordering::Relaxed),
Ordering::Relaxed,
);
}
}
}
}
impl<T, U> Default for LazyTransform<T, U>
where
T: Default,
{
fn default() -> Self {
LazyTransform::new(T::default())
}
}
/// `Lazy<T>` is a lazily initialized synchronized holder type. You can think
/// of it as a `LazyTransform` where the initial type doesn't exist.
#[derive(Clone)]
pub struct Lazy<T> {
inner: LazyTransform<(), T>,
}
impl<T> Lazy<T> {
/// Construct a new, uninitialized `Lazy<T>`.
pub fn new() -> Lazy<T> {
Self::default()
}
/// Unwrap the contained value, returning `Some` if the `Lazy<T>` has been initialized
/// or `None` if it has not.
pub fn into_inner(self) -> Option<T> {
self.inner.into_inner().ok()
}
}
impl<T> Lazy<T> {
/// Get a reference to the contained value, invoking `f` to create it
/// if the `Lazy<T>` is uninitialized. It is guaranteed that if multiple
/// calls to `get_or_create` race, only one will invoke its closure, and
/// every call will receive a reference to the newly created value.
///
/// The value stored in the `Lazy<T>` is immutable after the closure returns
/// it, so think carefully about what you want to put inside!
pub fn get_or_create<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
self.inner.get_or_create(|_| f())
}
/// Get a reference to the contained value, returning `Some(ref)` if the
/// `Lazy<T>` has been initialized or `None` if it has not. It is
/// guaranteed that if a reference is returned it is to the value inside
/// the `Lazy<T>`.
pub fn get(&self) -> Option<&T> {
self.inner.get()
}
}
// `#[derive(Default)]` automatically adds `T: Default` trait bound, but that
// is too restrictive, because `Lazy<T>` always has a default value for any `T`.
impl<T> Default for Lazy<T> {
fn default() -> Self {
Lazy {
inner: LazyTransform::new(()),
}
}
}
impl<T> fmt::Debug for Lazy<T>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
if let Some(v) = self.get() {
f.write_fmt(format_args!("Lazy({:?})", v))
} else {
f.write_str("Lazy(<uninitialized>)")
}
}
}
#[cfg(test)]
extern crate rayon;
#[cfg(test)]
mod tests {
use super::{Lazy, LazyTransform};
use rayon::ThreadPoolBuilder;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::{thread, time};
#[test]
fn test_lazy() {
let lazy_value: Lazy<u8> = Lazy::new();
assert_eq!(lazy_value.get(), None);
let n = AtomicUsize::new(0);
let pool = ThreadPoolBuilder::new().num_threads(100).build().unwrap();
pool.scope(|scope| {
for _ in 0..100 {
let lazy_ref = &lazy_value;
let n_ref = &n;
scope.spawn(move |_| {
let ten_millis = time::Duration::from_millis(10);
thread::sleep(ten_millis);
let value = *lazy_ref.get_or_create(|| {
// Make everybody else wait on me, because I'm a jerk.
thread::sleep(ten_millis);
// Make this relaxed so it doesn't interfere with
// Lazy internals at all.
n_ref.fetch_add(1, Ordering::Relaxed);
42
});
assert_eq!(value, 42);
let value = lazy_ref.get();
assert_eq!(value, Some(&42));
});
}
});
assert_eq!(n.load(Ordering::SeqCst), 1);
}
#[test]
fn test_lazy_transform() {
let lazy_value: LazyTransform<u8, u8> = LazyTransform::new(21);
assert_eq!(lazy_value.get(), None);
let n = AtomicUsize::new(0);
let pool = ThreadPoolBuilder::new().num_threads(100).build().unwrap();
pool.scope(|scope| {
for _ in 0..100 {
let lazy_ref = &lazy_value;
let n_ref = &n;
scope.spawn(move |_| {
let ten_millis = time::Duration::from_millis(10);
thread::sleep(ten_millis);
let value = *lazy_ref.get_or_create(|v| {
// Make everybody else wait on me, because I'm a jerk.
thread::sleep(ten_millis);
// Make this relaxed so it doesn't interfere with
// Lazy internals at all.
n_ref.fetch_add(1, Ordering::Relaxed);
v * 2
});
assert_eq!(value, 42);
let value = lazy_ref.get();
assert_eq!(value, Some(&42));
});
}
});
assert_eq!(n.load(Ordering::SeqCst), 1);
}
}