/*!
A lazily initialized value for safe sharing between threads.
The principal type in this module is `Lazy`, which makes it easy to construct
values that are shared safely across multiple threads simultaneously.
*/
use core::fmt;
/// A lazily initialized value that implements `Deref` for `T`.
///
/// A `Lazy` takes an initialization function and permits callers from any
/// thread to access the result of that initialization function in a safe
/// manner. In effect, this permits one-time initialization of global resources
/// in a (possibly) multi-threaded program.
///
/// This type and its functionality are available even when neither the `alloc`
/// nor the `std` features are enabled. In exchange, a `Lazy` does **not**
/// guarantee that the given `create` function is called at most once. It
/// might be called multiple times. Moreover, a call to `Lazy::get` (either
/// explicitly or implicitly via `Lazy`'s `Deref` impl) may block until a `T`
/// is available.
///
/// This is very similar to `lazy_static` or `once_cell`, except it doesn't
/// guarantee that the initialization function will be run once and it works
/// in no-alloc no-std environments. With that said, if you need stronger
/// guarantees or a more flexible API, then it is recommended to use either
/// `lazy_static` or `once_cell`.
///
/// # Warning: may use a spin lock
///
/// When this crate is compiled _without_ the `alloc` feature, then this type
/// may used a spin lock internally. This can have subtle effects that may
/// be undesirable. See [Spinlocks Considered Harmful][spinharm] for a more
/// thorough treatment of this topic.
///
/// [spinharm]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
///
/// # Example
///
/// This type is useful for creating regexes once, and then using them from
/// multiple threads simultaneously without worrying about synchronization.
///
/// ```
/// use regex_automata::{dfa::regex::Regex, util::lazy::Lazy, Match};
///
/// static RE: Lazy<Regex> = Lazy::new(|| Regex::new("foo[0-9]+bar").unwrap());
///
/// let expected = Some(Match::must(0, 3..14));
/// assert_eq!(expected, RE.find(b"zzzfoo12345barzzz"));
/// ```
pub struct Lazy<T, F = fn() -> T>(lazy::Lazy<T, F>);
impl<T, F> Lazy<T, F> {
/// Create a new `Lazy` value that is initialized via the given function.
///
/// The `T` type is automatically inferred from the return type of the
/// `create` function given.
pub const fn new(create: F) -> Lazy<T, F> {
Lazy(lazy::Lazy::new(create))
}
}
impl<T, F: Fn() -> T> Lazy<T, F> {
/// Return a reference to the lazily initialized value.
///
/// This routine may block if another thread is initializing a `T`.
///
/// Note that given a `x` which has type `Lazy`, this must be called via
/// `Lazy::get(x)` and not `x.get()`. This routine is defined this way
/// because `Lazy` impls `Deref` with a target of `T`.
///
/// # Panics
///
/// This panics if the `create` function inside this lazy value panics.
/// If the panic occurred in another thread, then this routine _may_ also
/// panic (but is not guaranteed to do so).
pub fn get(this: &Lazy<T, F>) -> &T {
this.0.get()
}
}
impl<T, F: Fn() -> T> core::ops::Deref for Lazy<T, F> {
type Target = T;
fn deref(&self) -> &T {
Lazy::get(self)
}
}
impl<T: fmt::Debug, F: Fn() -> T> fmt::Debug for Lazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.0.fmt(f)
}
}
#[cfg(feature = "alloc")]
mod lazy {
use core::{
fmt,
marker::PhantomData,
sync::atomic::{AtomicPtr, Ordering},
};
use alloc::boxed::Box;
/// A non-std lazy initialized value.
///
/// This might run the initialization function more than once, but will
/// never block.
///
/// I wish I could get these semantics into the non-alloc non-std Lazy
/// type below, but I'm not sure how to do it. If you can do an alloc,
/// then the implementation becomes very simple if you don't care about
/// redundant work precisely because a pointer can be atomically swapped.
///
/// Perhaps making this approach work in the non-alloc non-std case
/// requires asking the caller for a pointer? It would make the API less
/// convenient I think.
pub(super) struct Lazy<T, F> {
data: AtomicPtr<T>,
create: F,
// This indicates to the compiler that this type can drop T. It's not
// totally clear how the absence of this marker could lead to trouble,
// but putting here doesn't have any downsides so we hedge until somone
// can from the Unsafe Working Group can tell us definitively that we
// don't need it.
//
// See: https://github.com/BurntSushi/regex-automata/issues/30
owned: PhantomData<Box<T>>,
}
// SAFETY: So long as T and &T (and F and &F) can themselves be safely
// shared among threads, so to can a Lazy<T, _>. Namely, the Lazy API only
// permits accessing a &T and initialization is free of data races. So if T
// is thread safe, then so to is Lazy<T, _>.
//
// We specifically require that T: Send in order for Lazy<T> to be Sync.
// Without that requirement, it's possible to send a T from one thread to
// another via Lazy's destructor.
//
// It's not clear whether we need F: Send+Sync for Lazy to be Sync. But
// we're conservative for now and keep both.
unsafe impl<T: Send + Sync, F: Send + Sync> Sync for Lazy<T, F> {}
impl<T, F> Lazy<T, F> {
/// Create a new alloc but non-std lazy value that is racily
/// initialized. That is, the 'create' function may be called more than
/// once.
pub(super) const fn new(create: F) -> Lazy<T, F> {
Lazy {
data: AtomicPtr::new(core::ptr::null_mut()),
create,
owned: PhantomData,
}
}
}
impl<T, F: Fn() -> T> Lazy<T, F> {
/// Get the underlying lazy value. If it hasn't been initialized
/// yet, then always attempt to initialize it (even if some other
/// thread is initializing it) and atomically attach it to this lazy
/// value before returning it.
pub(super) fn get(&self) -> &T {
if let Some(data) = self.poll() {
return data;
}
let data = (self.create)();
let mut ptr = Box::into_raw(Box::new(data));
// We attempt to stuff our initialized value into our atomic
// pointer. Upon success, we don't need to do anything. But if
// someone else beat us to the punch, then we need to make sure
// our newly created value is dropped.
let result = self.data.compare_exchange(
core::ptr::null_mut(),
ptr,
Ordering::AcqRel,
Ordering::Acquire,
);
if let Err(old) = result {
// SAFETY: We created 'ptr' via Box::into_raw above, so turning
// it back into a Box via from_raw is safe.
drop(unsafe { Box::from_raw(ptr) });
ptr = old;
}
// SAFETY: We just set the pointer above to a non-null value, even
// in the error case, and set it to a fully initialized value
// returned by 'create'.
unsafe { &*ptr }
}
/// If this lazy value has been initialized successfully, then return
/// that value. Otherwise return None immediately. This never attempts
/// to run initialization itself.
fn poll(&self) -> Option<&T> {
let ptr = self.data.load(Ordering::Acquire);
if ptr.is_null() {
return None;
}
// SAFETY: We just checked that the pointer is not null. Since it's
// not null, it must have been fully initialized by 'get' at some
// point.
Some(unsafe { &*ptr })
}
}
impl<T: fmt::Debug, F: Fn() -> T> fmt::Debug for Lazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Lazy").field("data", &self.poll()).finish()
}
}
impl<T, F> Drop for Lazy<T, F> {
fn drop(&mut self) {
let ptr = *self.data.get_mut();
if !ptr.is_null() {
// SAFETY: We just checked that 'ptr' is not null. And since
// we have exclusive access, there are no races to worry about.
drop(unsafe { Box::from_raw(ptr) });
}
}
}
}
#[cfg(not(feature = "alloc"))]
mod lazy {
use core::{
cell::Cell,
fmt,
mem::MaybeUninit,
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicU8, Ordering},
};
/// Our 'Lazy' value can be in one of three states:
///
/// * INIT is where it starts, and also ends up back here if the
/// 'create' routine panics.
/// * BUSY is where it sits while initialization is running in exactly
/// one thread.
/// * DONE is where it sits after 'create' has completed and 'data' has
/// been fully initialized.
const LAZY_STATE_INIT: u8 = 0;
const LAZY_STATE_BUSY: u8 = 1;
const LAZY_STATE_DONE: u8 = 2;
/// A non-alloc non-std lazy initialized value.
///
/// This guarantees initialization only happens once, but uses a spinlock
/// to block in the case of simultaneous access. Blocking occurs so that
/// one thread waits while another thread initializes the value.
///
/// I would much rather have the semantics of the 'alloc' Lazy type above.
/// Namely, that we might run the initialization function more than once,
/// but we never otherwise block. However, I don't know how to do that in
/// a non-alloc non-std context.
pub(super) struct Lazy<T, F> {
state: AtomicU8,
create: Cell<Option<F>>,
data: Cell<MaybeUninit<T>>,
}
// SAFETY: So long as T and &T (and F and &F) can themselves be safely
// shared among threads, so to can a Lazy<T, _>. Namely, the Lazy API only
// permits accessing a &T and initialization is free of data races. So if T
// is thread safe, then so to is Lazy<T, _>.
unsafe impl<T: Send + Sync, F: Send + Sync> Sync for Lazy<T, F> {}
// A reference to a Lazy is unwind safe because we specifically take
// precautions to poison all accesses to a Lazy if the caller-provided
// 'create' function panics.
impl<T: UnwindSafe, F: UnwindSafe + RefUnwindSafe> RefUnwindSafe
for Lazy<T, F>
{
}
impl<T, F> Lazy<T, F> {
/// Create a new non-alloc non-std lazy value that is initialized
/// exactly once on first use using the given function.
pub(super) const fn new(create: F) -> Lazy<T, F> {
Lazy {
state: AtomicU8::new(LAZY_STATE_INIT),
create: Cell::new(Some(create)),
data: Cell::new(MaybeUninit::uninit()),
}
}
}
impl<T, F: FnOnce() -> T> Lazy<T, F> {
/// Get the underlying lazy value. If it isn't been initialized
/// yet, then either initialize it or block until some other thread
/// initializes it. If the 'create' function given to Lazy::new panics
/// (even in another thread), then this panics too.
pub(super) fn get(&self) -> &T {
// This is effectively a spinlock. We loop until we enter a DONE
// state, and if possible, initialize it ourselves. The only way
// we exit the loop is if 'create' panics, we initialize 'data' or
// some other thread initializes 'data'.
//
// Yes, I have read spinlocks considered harmful[1]. And that
// article is why this spinlock is only active when 'alloc' isn't
// enabled. I did this because I don't think there is really
// another choice without 'alloc', other than not providing this at
// all. But I think that's a big bummer.
//
// [1]: https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html
while self.state.load(Ordering::Acquire) != LAZY_STATE_DONE {
// Check if we're the first ones to get here. If so, we'll be
// the ones who initialize.
let result = self.state.compare_exchange(
LAZY_STATE_INIT,
LAZY_STATE_BUSY,
Ordering::AcqRel,
Ordering::Acquire,
);
// This means we saw the INIT state and nobody else can. So we
// must take responsibility for initializing. And by virtue of
// observing INIT, we have also told anyone else trying to
// get here that we are BUSY. If someone else sees BUSY, then
// they will spin until we finish initialization.
if let Ok(_) = result {
// Since we are guaranteed to be the only ones here, we
// know that 'create' is there... Unless someone else got
// here before us and 'create' panicked. In which case,
// 'self.create' is now 'None' and we forward the panic
// to the caller. (i.e., We implement poisoning.)
//
// SAFETY: Our use of 'self.state' guarantees that we are
// the only thread executing this line, and thus there are
// no races.
let create = unsafe {
(*self.create.as_ptr()).take().expect(
"Lazy's create function panicked, \
preventing initialization,
poisoning current thread",
)
};
let guard = Guard { state: &self.state };
// SAFETY: Our use of 'self.state' guarantees that we are
// the only thread executing this line, and thus there are
// no races.
unsafe {
(*self.data.as_ptr()).as_mut_ptr().write(create());
}
// All is well. 'self.create' ran successfully, so we
// forget the guard.
core::mem::forget(guard);
// Everything is initialized, so we can declare success.
self.state.store(LAZY_STATE_DONE, Ordering::Release);
break;
}
core::hint::spin_loop();
}
// We only get here if data is fully initialized, and thus poll
// will always return something.
self.poll().unwrap()
}
/// If this lazy value has been initialized successfully, then return
/// that value. Otherwise return None immediately. This never blocks.
fn poll(&self) -> Option<&T> {
if self.state.load(Ordering::Acquire) == LAZY_STATE_DONE {
// SAFETY: The DONE state only occurs when data has been fully
// initialized.
Some(unsafe { &*(*self.data.as_ptr()).as_ptr() })
} else {
None
}
}
}
impl<T: fmt::Debug, F: FnMut() -> T> fmt::Debug for Lazy<T, F> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Lazy")
.field("state", &self.state.load(Ordering::Acquire))
.field("create", &"<closure>")
.field("data", &self.poll())
.finish()
}
}
impl<T, F> Drop for Lazy<T, F> {
fn drop(&mut self) {
if *self.state.get_mut() == LAZY_STATE_DONE {
// SAFETY: state is DONE if and only if data has been fully
// initialized. At which point, it is safe to drop.
unsafe {
self.data.get_mut().assume_init_drop();
}
}
}
}
/// A guard that will reset a Lazy's state back to INIT when dropped. The
/// idea here is to 'forget' this guard on success. On failure (when a
/// panic occurs), the Drop impl runs and causes all in-progress and future
/// 'get' calls to panic. Without this guard, all in-progress and future
/// 'get' calls would spin forever. Crashing is much better than getting
/// stuck in an infinite loop.
struct Guard<'a> {
state: &'a AtomicU8,
}
impl<'a> Drop for Guard<'a> {
fn drop(&mut self) {
// We force ourselves back into an INIT state. This will in turn
// cause any future 'get' calls to attempt calling 'self.create'
// again which will in turn panic because 'self.create' will now
// be 'None'.
self.state.store(LAZY_STATE_INIT, Ordering::Release);
}
}
}
#[cfg(test)]
mod tests {
use super::*;
fn assert_send<T: Send>() {}
fn assert_sync<T: Sync>() {}
fn assert_unwind<T: core::panic::UnwindSafe>() {}
fn assert_refunwind<T: core::panic::RefUnwindSafe>() {}
#[test]
fn oibits() {
assert_send::<Lazy<u64>>();
assert_sync::<Lazy<u64>>();
assert_unwind::<Lazy<u64>>();
assert_refunwind::<Lazy<u64>>();
}
// This is a regression test because we used to rely on the inferred Sync
// impl for the Lazy type defined above (for 'alloc' mode). In the
// inferred impl, it only requires that T: Sync for Lazy<T>: Sync. But
// if we have that, we can actually make use of the fact that Lazy<T> drops
// T to create a value on one thread and drop it on another. This *should*
// require T: Send, but our missing bounds before let it sneak by.
//
// Basically, this test should not compile, so we... comment it out. We
// don't have a great way of testing compile-fail tests right now.
//
// See: https://github.com/BurntSushi/regex-automata/issues/30
/*
#[test]
fn sync_not_send() {
#[allow(dead_code)]
fn inner<T: Sync + Default>() {
let lazy = Lazy::new(move || T::default());
std::thread::scope(|scope| {
scope.spawn(|| {
Lazy::get(&lazy); // We create T in this thread
});
});
// And drop in this thread.
drop(lazy);
// So we have send a !Send type over threads. (with some more
// legwork, its possible to even sneak the value out of drop
// through thread local)
}
}
*/
}
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