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//! Thread local storage
#![unstable(feature = "thread_local_internals", issue = "none")]
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
#[cfg(test)]
mod dynamic_tests;
use crate::cell::{Cell, RefCell};
use crate::error::Error;
use crate::fmt;
/// A thread local storage key which owns its contents.
///
/// This key uses the fastest possible implementation available to it for the
/// target platform. It is instantiated with the [`thread_local!`] macro and the
/// primary method is the [`with`] method, though there are helpers to make
/// working with [`Cell`] types easier.
///
/// The [`with`] method yields a reference to the contained value which cannot
/// outlive the current thread or escape the given closure.
///
/// [`thread_local!`]: crate::thread_local
///
/// # Initialization and Destruction
///
/// Initialization is dynamically performed on the first call to a setter (e.g.
/// [`with`]) within a thread, and values that implement [`Drop`] get
/// destructed when a thread exits. Some caveats apply, which are explained below.
///
/// A `LocalKey`'s initializer cannot recursively depend on itself. Using a
/// `LocalKey` in this way may cause panics, aborts or infinite recursion on
/// the first call to `with`.
///
/// # Single-thread Synchronization
///
/// Though there is no potential race with other threads, it is still possible to
/// obtain multiple references to the thread-local data in different places on
/// the call stack. For this reason, only shared (`&T`) references may be obtained.
///
/// To allow obtaining an exclusive mutable reference (`&mut T`), typically a
/// [`Cell`] or [`RefCell`] is used (see the [`std::cell`] for more information
/// on how exactly this works). To make this easier there are specialized
/// implementations for [`LocalKey<Cell<T>>`] and [`LocalKey<RefCell<T>>`].
///
/// [`std::cell`]: `crate::cell`
/// [`LocalKey<Cell<T>>`]: struct.LocalKey.html#impl-LocalKey<Cell<T>>
/// [`LocalKey<RefCell<T>>`]: struct.LocalKey.html#impl-LocalKey<RefCell<T>>
///
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
/// use std::thread;
///
/// thread_local!(static FOO: RefCell<u32> = RefCell::new(1));
///
/// FOO.with_borrow(|v| assert_eq!(*v, 1));
/// FOO.with_borrow_mut(|v| *v = 2);
///
/// // each thread starts out with the initial value of 1
/// let t = thread::spawn(move|| {
/// FOO.with_borrow(|v| assert_eq!(*v, 1));
/// FOO.with_borrow_mut(|v| *v = 3);
/// });
///
/// // wait for the thread to complete and bail out on panic
/// t.join().unwrap();
///
/// // we retain our original value of 2 despite the child thread
/// FOO.with_borrow(|v| assert_eq!(*v, 2));
/// ```
///
/// # Platform-specific behavior
///
/// Note that a "best effort" is made to ensure that destructors for types
/// stored in thread local storage are run, but not all platforms can guarantee
/// that destructors will be run for all types in thread local storage. For
/// example, there are a number of known caveats where destructors are not run:
///
/// 1. On Unix systems when pthread-based TLS is being used, destructors will
/// not be run for TLS values on the main thread when it exits. Note that the
/// application will exit immediately after the main thread exits as well.
/// 2. On all platforms it's possible for TLS to re-initialize other TLS slots
/// during destruction. Some platforms ensure that this cannot happen
/// infinitely by preventing re-initialization of any slot that has been
/// destroyed, but not all platforms have this guard. Those platforms that do
/// not guard typically have a synthetic limit after which point no more
/// destructors are run.
/// 3. When the process exits on Windows systems, TLS destructors may only be
/// run on the thread that causes the process to exit. This is because the
/// other threads may be forcibly terminated.
///
/// ## Synchronization in thread-local destructors
///
/// On Windows, synchronization operations (such as [`JoinHandle::join`]) in
/// thread local destructors are prone to deadlocks and so should be avoided.
/// This is because the [loader lock] is held while a destructor is run. The
/// lock is acquired whenever a thread starts or exits or when a DLL is loaded
/// or unloaded. Therefore these events are blocked for as long as a thread
/// local destructor is running.
///
/// [loader lock]: https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices
/// [`JoinHandle::join`]: crate::thread::JoinHandle::join
/// [`with`]: LocalKey::with
#[cfg_attr(not(test), rustc_diagnostic_item = "LocalKey")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct LocalKey<T: 'static> {
// This outer `LocalKey<T>` type is what's going to be stored in statics,
// but actual data inside will sometimes be tagged with #[thread_local].
// It's not valid for a true static to reference a #[thread_local] static,
// so we get around that by exposing an accessor through a layer of function
// indirection (this thunk).
//
// Note that the thunk is itself unsafe because the returned lifetime of the
// slot where data lives, `'static`, is not actually valid. The lifetime
// here is actually slightly shorter than the currently running thread!
//
// Although this is an extra layer of indirection, it should in theory be
// trivially devirtualizable by LLVM because the value of `inner` never
// changes and the constant should be readonly within a crate. This mainly
// only runs into problems when TLS statics are exported across crates.
inner: unsafe fn(Option<&mut Option<T>>) -> Option<&'static T>,
}
#[stable(feature = "std_debug", since = "1.16.0")]
impl<T: 'static> fmt::Debug for LocalKey<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("LocalKey").finish_non_exhaustive()
}
}
/// Declare a new thread local storage key of type [`std::thread::LocalKey`].
///
/// # Syntax
///
/// The macro wraps any number of static declarations and makes them thread local.
/// Publicity and attributes for each static are allowed. Example:
///
/// ```
/// use std::cell::RefCell;
/// thread_local! {
/// pub static FOO: RefCell<u32> = RefCell::new(1);
///
/// static BAR: RefCell<f32> = RefCell::new(1.0);
/// }
///
/// FOO.with_borrow(|v| assert_eq!(*v, 1));
/// BAR.with_borrow(|v| assert_eq!(*v, 1.0));
/// ```
///
/// Note that only shared references (`&T`) to the inner data may be obtained, so a
/// type such as [`Cell`] or [`RefCell`] is typically used to allow mutating access.
///
/// This macro supports a special `const {}` syntax that can be used
/// when the initialization expression can be evaluated as a constant.
/// This can enable a more efficient thread local implementation that
/// can avoid lazy initialization. For types that do not
/// [need to be dropped][crate::mem::needs_drop], this can enable an
/// even more efficient implementation that does not need to
/// track any additional state.
///
/// ```
/// use std::cell::Cell;
/// thread_local! {
/// pub static FOO: Cell<u32> = const { Cell::new(1) };
/// }
///
/// assert_eq!(FOO.get(), 1);
/// ```
///
/// See [`LocalKey` documentation][`std::thread::LocalKey`] for more
/// information.
///
/// [`std::thread::LocalKey`]: crate::thread::LocalKey
#[macro_export]
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "thread_local_macro")]
#[allow_internal_unstable(thread_local_internals)]
// FIXME: Use `SyncUnsafeCell` instead of allowing `static_mut_refs` lint
#[cfg_attr(not(bootstrap), allow(static_mut_refs))]
macro_rules! thread_local {
// empty (base case for the recursion)
() => {};
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const $init:block; $($rest:tt)*) => (
$crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, const $init);
$crate::thread_local!($($rest)*);
);
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const $init:block) => (
$crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, const $init);
);
// process multiple declarations
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => (
$crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, $init);
$crate::thread_local!($($rest)*);
);
// handle a single declaration
($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr) => (
$crate::thread::local_impl::thread_local_inner!($(#[$attr])* $vis $name, $t, $init);
);
}
/// An error returned by [`LocalKey::try_with`](struct.LocalKey.html#method.try_with).
#[stable(feature = "thread_local_try_with", since = "1.26.0")]
#[non_exhaustive]
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct AccessError;
#[stable(feature = "thread_local_try_with", since = "1.26.0")]
impl fmt::Debug for AccessError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("AccessError").finish()
}
}
#[stable(feature = "thread_local_try_with", since = "1.26.0")]
impl fmt::Display for AccessError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt("already destroyed", f)
}
}
#[stable(feature = "thread_local_try_with", since = "1.26.0")]
impl Error for AccessError {}
impl<T: 'static> LocalKey<T> {
#[doc(hidden)]
#[unstable(
feature = "thread_local_internals",
reason = "recently added to create a key",
issue = "none"
)]
#[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
pub const unsafe fn new(
inner: unsafe fn(Option<&mut Option<T>>) -> Option<&'static T>,
) -> LocalKey<T> {
LocalKey { inner }
}
/// Acquires a reference to the value in this TLS key.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// This function will `panic!()` if the key currently has its
/// destructor running, and it **may** panic if the destructor has
/// previously been run for this thread.
#[stable(feature = "rust1", since = "1.0.0")]
pub fn with<F, R>(&'static self, f: F) -> R
where
F: FnOnce(&T) -> R,
{
self.try_with(f).expect(
"cannot access a Thread Local Storage value \
during or after destruction",
)
}
/// Acquires a reference to the value in this TLS key.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet. If the key has been destroyed (which may happen if this is called
/// in a destructor), this function will return an [`AccessError`].
///
/// # Panics
///
/// This function will still `panic!()` if the key is uninitialized and the
/// key's initializer panics.
#[stable(feature = "thread_local_try_with", since = "1.26.0")]
#[inline]
pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
where
F: FnOnce(&T) -> R,
{
unsafe {
let thread_local = (self.inner)(None).ok_or(AccessError)?;
Ok(f(thread_local))
}
}
/// Acquires a reference to the value in this TLS key, initializing it with
/// `init` if it wasn't already initialized on this thread.
///
/// If `init` was used to initialize the thread local variable, `None` is
/// passed as the first argument to `f`. If it was already initialized,
/// `Some(init)` is passed to `f`.
///
/// # Panics
///
/// This function will panic if the key currently has its destructor
/// running, and it **may** panic if the destructor has previously been run
/// for this thread.
fn initialize_with<F, R>(&'static self, init: T, f: F) -> R
where
F: FnOnce(Option<T>, &T) -> R,
{
unsafe {
let mut init = Some(init);
let reference = (self.inner)(Some(&mut init)).expect(
"cannot access a Thread Local Storage value \
during or after destruction",
);
f(init, reference)
}
}
}
impl<T: 'static> LocalKey<Cell<T>> {
/// Sets or initializes the contained value.
///
/// Unlike the other methods, this will *not* run the lazy initializer of
/// the thread local. Instead, it will be directly initialized with the
/// given value if it wasn't initialized yet.
///
/// # Panics
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// thread_local! {
/// static X: Cell<i32> = panic!("!");
/// }
///
/// // Calling X.get() here would result in a panic.
///
/// X.set(123); // But X.set() is fine, as it skips the initializer above.
///
/// assert_eq!(X.get(), 123);
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn set(&'static self, value: T) {
self.initialize_with(Cell::new(value), |value, cell| {
if let Some(value) = value {
// The cell was already initialized, so `value` wasn't used to
// initialize it. So we overwrite the current value with the
// new one instead.
cell.set(value.into_inner());
}
});
}
/// Returns a copy of the contained value.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// thread_local! {
/// static X: Cell<i32> = Cell::new(1);
/// }
///
/// assert_eq!(X.get(), 1);
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn get(&'static self) -> T
where
T: Copy,
{
self.with(|cell| cell.get())
}
/// Takes the contained value, leaving `Default::default()` in its place.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// thread_local! {
/// static X: Cell<Option<i32>> = Cell::new(Some(1));
/// }
///
/// assert_eq!(X.take(), Some(1));
/// assert_eq!(X.take(), None);
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn take(&'static self) -> T
where
T: Default,
{
self.with(|cell| cell.take())
}
/// Replaces the contained value, returning the old value.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// thread_local! {
/// static X: Cell<i32> = Cell::new(1);
/// }
///
/// assert_eq!(X.replace(2), 1);
/// assert_eq!(X.replace(3), 2);
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn replace(&'static self, value: T) -> T {
self.with(|cell| cell.replace(value))
}
}
impl<T: 'static> LocalKey<RefCell<T>> {
/// Acquires a reference to the contained value.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the value is currently mutably borrowed.
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Example
///
/// ```
/// use std::cell::RefCell;
///
/// thread_local! {
/// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
/// }
///
/// X.with_borrow(|v| assert!(v.is_empty()));
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn with_borrow<F, R>(&'static self, f: F) -> R
where
F: FnOnce(&T) -> R,
{
self.with(|cell| f(&cell.borrow()))
}
/// Acquires a mutable reference to the contained value.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Example
///
/// ```
/// use std::cell::RefCell;
///
/// thread_local! {
/// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
/// }
///
/// X.with_borrow_mut(|v| v.push(1));
///
/// X.with_borrow(|v| assert_eq!(*v, vec![1]));
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn with_borrow_mut<F, R>(&'static self, f: F) -> R
where
F: FnOnce(&mut T) -> R,
{
self.with(|cell| f(&mut cell.borrow_mut()))
}
/// Sets or initializes the contained value.
///
/// Unlike the other methods, this will *not* run the lazy initializer of
/// the thread local. Instead, it will be directly initialized with the
/// given value if it wasn't initialized yet.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// thread_local! {
/// static X: RefCell<Vec<i32>> = panic!("!");
/// }
///
/// // Calling X.with() here would result in a panic.
///
/// X.set(vec![1, 2, 3]); // But X.set() is fine, as it skips the initializer above.
///
/// X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn set(&'static self, value: T) {
self.initialize_with(RefCell::new(value), |value, cell| {
if let Some(value) = value {
// The cell was already initialized, so `value` wasn't used to
// initialize it. So we overwrite the current value with the
// new one instead.
*cell.borrow_mut() = value.into_inner();
}
});
}
/// Takes the contained value, leaving `Default::default()` in its place.
///
/// This will lazily initialize the value if this thread has not referenced
/// this key yet.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// thread_local! {
/// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
/// }
///
/// X.with_borrow_mut(|v| v.push(1));
///
/// let a = X.take();
///
/// assert_eq!(a, vec![1]);
///
/// X.with_borrow(|v| assert!(v.is_empty()));
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn take(&'static self) -> T
where
T: Default,
{
self.with(|cell| cell.take())
}
/// Replaces the contained value, returning the old value.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// Panics if the key currently has its destructor running,
/// and it **may** panic if the destructor has previously been run for this thread.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// thread_local! {
/// static X: RefCell<Vec<i32>> = RefCell::new(Vec::new());
/// }
///
/// let prev = X.replace(vec![1, 2, 3]);
/// assert!(prev.is_empty());
///
/// X.with_borrow(|v| assert_eq!(*v, vec![1, 2, 3]));
/// ```
#[stable(feature = "local_key_cell_methods", since = "1.73.0")]
pub fn replace(&'static self, value: T) -> T {
self.with(|cell| cell.replace(value))
}
}