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// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! Thread local storage #![unstable(feature = "thread_local_internals", issue = "0")] use cell::UnsafeCell; use fmt; use hint; use mem; /// 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. /// /// The [`with`] method yields a reference to the contained value which cannot be /// sent across threads or escape the given closure. /// /// # Initialization and Destruction /// /// Initialization is dynamically performed on the first call to [`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, and using /// a `LocalKey` in this way will cause the initializer to infinitely recurse /// on the first call to `with`. /// /// # Examples /// /// ``` /// use std::cell::RefCell; /// use std::thread; /// /// thread_local!(static FOO: RefCell<u32> = RefCell::new(1)); /// /// FOO.with(|f| { /// assert_eq!(*f.borrow(), 1); /// *f.borrow_mut() = 2; /// }); /// /// // each thread starts out with the initial value of 1 /// thread::spawn(move|| { /// FOO.with(|f| { /// assert_eq!(*f.borrow(), 1); /// *f.borrow_mut() = 3; /// }); /// }); /// /// // we retain our original value of 2 despite the child thread /// FOO.with(|f| { /// assert_eq!(*f.borrow(), 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. On macOS, initializing TLS during destruction of other TLS slots can /// sometimes cancel *all* destructors for the current thread, whether or not /// the slots have already had their destructors run or not. /// /// [`with`]: ../../std/thread/struct.LocalKey.html#method.with /// [`thread_local!`]: ../../std/macro.thread_local.html /// [`Drop`]: ../../std/ops/trait.Drop.html #[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<&'static UnsafeCell<Option<T>>>, // initialization routine to invoke to create a value init: fn() -> 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.pad("LocalKey { .. }") } } /// 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); /// /// #[allow(unused)] /// static BAR: RefCell<f32> = RefCell::new(1.0); /// } /// # fn main() {} /// ``` /// /// See [LocalKey documentation][`std::thread::LocalKey`] for more /// information. /// /// [`std::thread::LocalKey`]: ../std/thread/struct.LocalKey.html #[macro_export] #[stable(feature = "rust1", since = "1.0.0")] #[allow_internal_unstable] macro_rules! thread_local { // empty (base case for the recursion) () => {}; // process multiple declarations ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => ( $crate::__thread_local_inner!($(#[$attr])* $vis $name, $t, $init); $crate::thread_local!($($rest)*); ); // handle a single declaration ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr) => ( $crate::__thread_local_inner!($(#[$attr])* $vis $name, $t, $init); ); } #[doc(hidden)] #[unstable(feature = "thread_local_internals", reason = "should not be necessary", issue = "0")] #[macro_export] #[allow_internal_unstable] #[allow_internal_unsafe] macro_rules! __thread_local_inner { (@key $(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => { { #[inline] fn __init() -> $t { $init } unsafe fn __getit() -> $crate::option::Option< &'static $crate::cell::UnsafeCell< $crate::option::Option<$t>>> { #[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))] static __KEY: $crate::thread::__StaticLocalKeyInner<$t> = $crate::thread::__StaticLocalKeyInner::new(); #[thread_local] #[cfg(all( target_thread_local, not(all(target_arch = "wasm32", not(target_feature = "atomics"))), ))] static __KEY: $crate::thread::__FastLocalKeyInner<$t> = $crate::thread::__FastLocalKeyInner::new(); #[cfg(all( not(target_thread_local), not(all(target_arch = "wasm32", not(target_feature = "atomics"))), ))] static __KEY: $crate::thread::__OsLocalKeyInner<$t> = $crate::thread::__OsLocalKeyInner::new(); __KEY.get() } unsafe { $crate::thread::LocalKey::new(__getit, __init) } } }; ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => { $(#[$attr])* $vis const $name: $crate::thread::LocalKey<$t> = $crate::__thread_local_inner!(@key $(#[$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")] pub struct AccessError { _private: (), } #[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) } } impl<T: 'static> LocalKey<T> { #[doc(hidden)] #[unstable(feature = "thread_local_internals", reason = "recently added to create a key", issue = "0")] pub const unsafe fn new(inner: unsafe fn() -> Option<&'static UnsafeCell<Option<T>>>, init: fn() -> T) -> LocalKey<T> { LocalKey { inner, init, } } /// 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 TLS value during or \ after it is destroyed") } unsafe fn init(&self, slot: &UnsafeCell<Option<T>>) -> &T { // Execute the initialization up front, *then* move it into our slot, // just in case initialization fails. let value = (self.init)(); let ptr = slot.get(); // note that this can in theory just be `*ptr = Some(value)`, but due to // the compiler will currently codegen that pattern with something like: // // ptr::drop_in_place(ptr) // ptr::write(ptr, Some(value)) // // Due to this pattern it's possible for the destructor of the value in // `ptr` (e.g. if this is being recursively initialized) to re-access // TLS, in which case there will be a `&` and `&mut` pointer to the same // value (an aliasing violation). To avoid setting the "I'm running a // destructor" flag we just use `mem::replace` which should sequence the // operations a little differently and make this safe to call. mem::replace(&mut *ptr, Some(value)); // After storing `Some` we want to get a reference to the contents of // what we just stored. While we could use `unwrap` here and it should // always work it empirically doesn't seem to always get optimized away, // which means that using something like `try_with` can pull in // panicking code and cause a large size bloat. match *ptr { Some(ref x) => x, None => hint::unreachable_unchecked(), } } /// 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`](struct.AccessError.html). /// /// # 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")] pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError> where F: FnOnce(&T) -> R, { unsafe { let slot = (self.inner)().ok_or(AccessError { _private: (), })?; Ok(f(match *slot.get() { Some(ref inner) => inner, None => self.init(slot), })) } } } /// On some platforms like wasm32 there's no threads, so no need to generate /// thread locals and we can instead just use plain statics! #[doc(hidden)] #[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))] pub mod statik { use cell::UnsafeCell; use fmt; pub struct Key<T> { inner: UnsafeCell<Option<T>>, } unsafe impl<T> ::marker::Sync for Key<T> { } impl<T> fmt::Debug for Key<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("Key { .. }") } } impl<T> Key<T> { pub const fn new() -> Key<T> { Key { inner: UnsafeCell::new(None), } } pub unsafe fn get(&self) -> Option<&'static UnsafeCell<Option<T>>> { Some(&*(&self.inner as *const _)) } } } #[doc(hidden)] #[cfg(target_thread_local)] pub mod fast { use cell::{Cell, UnsafeCell}; use fmt; use mem; use ptr; use sys::fast_thread_local::{register_dtor, requires_move_before_drop}; pub struct Key<T> { inner: UnsafeCell<Option<T>>, // Metadata to keep track of the state of the destructor. Remember that // these variables are thread-local, not global. dtor_registered: Cell<bool>, dtor_running: Cell<bool>, } impl<T> fmt::Debug for Key<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("Key { .. }") } } impl<T> Key<T> { pub const fn new() -> Key<T> { Key { inner: UnsafeCell::new(None), dtor_registered: Cell::new(false), dtor_running: Cell::new(false) } } pub unsafe fn get(&self) -> Option<&'static UnsafeCell<Option<T>>> { if mem::needs_drop::<T>() && self.dtor_running.get() { return None } self.register_dtor(); Some(&*(&self.inner as *const _)) } unsafe fn register_dtor(&self) { if !mem::needs_drop::<T>() || self.dtor_registered.get() { return } register_dtor(self as *const _ as *mut u8, destroy_value::<T>); self.dtor_registered.set(true); } } unsafe extern fn destroy_value<T>(ptr: *mut u8) { let ptr = ptr as *mut Key<T>; // Right before we run the user destructor be sure to flag the // destructor as running for this thread so calls to `get` will return // `None`. (*ptr).dtor_running.set(true); // Some implementations may require us to move the value before we drop // it as it could get re-initialized in-place during destruction. // // Hence, we use `ptr::read` on those platforms (to move to a "safe" // location) instead of drop_in_place. if requires_move_before_drop() { ptr::read((*ptr).inner.get()); } else { ptr::drop_in_place((*ptr).inner.get()); } } } #[doc(hidden)] pub mod os { use cell::{Cell, UnsafeCell}; use fmt; use marker; use ptr; use sys_common::thread_local::StaticKey as OsStaticKey; pub struct Key<T> { // OS-TLS key that we'll use to key off. os: OsStaticKey, marker: marker::PhantomData<Cell<T>>, } impl<T> fmt::Debug for Key<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("Key { .. }") } } unsafe impl<T> ::marker::Sync for Key<T> { } struct Value<T: 'static> { key: &'static Key<T>, value: UnsafeCell<Option<T>>, } impl<T: 'static> Key<T> { pub const fn new() -> Key<T> { Key { os: OsStaticKey::new(Some(destroy_value::<T>)), marker: marker::PhantomData } } pub unsafe fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> { let ptr = self.os.get() as *mut Value<T>; if !ptr.is_null() { if ptr as usize == 1 { return None } return Some(&(*ptr).value); } // If the lookup returned null, we haven't initialized our own // local copy, so do that now. let ptr: Box<Value<T>> = box Value { key: self, value: UnsafeCell::new(None), }; let ptr = Box::into_raw(ptr); self.os.set(ptr as *mut u8); Some(&(*ptr).value) } } unsafe extern fn destroy_value<T: 'static>(ptr: *mut u8) { // The OS TLS ensures that this key contains a NULL value when this // destructor starts to run. We set it back to a sentinel value of 1 to // ensure that any future calls to `get` for this thread will return // `None`. // // Note that to prevent an infinite loop we reset it back to null right // before we return from the destructor ourselves. let ptr = Box::from_raw(ptr as *mut Value<T>); let key = ptr.key; key.os.set(1 as *mut u8); drop(ptr); key.os.set(ptr::null_mut()); } } #[cfg(all(test, not(target_os = "emscripten")))] mod tests { use sync::mpsc::{channel, Sender}; use cell::{Cell, UnsafeCell}; use thread; struct Foo(Sender<()>); impl Drop for Foo { fn drop(&mut self) { let Foo(ref s) = *self; s.send(()).unwrap(); } } #[test] fn smoke_no_dtor() { thread_local!(static FOO: Cell<i32> = Cell::new(1)); FOO.with(|f| { assert_eq!(f.get(), 1); f.set(2); }); let (tx, rx) = channel(); let _t = thread::spawn(move|| { FOO.with(|f| { assert_eq!(f.get(), 1); }); tx.send(()).unwrap(); }); rx.recv().unwrap(); FOO.with(|f| { assert_eq!(f.get(), 2); }); } #[test] fn states() { struct Foo; impl Drop for Foo { fn drop(&mut self) { assert!(FOO.try_with(|_| ()).is_err()); } } thread_local!(static FOO: Foo = Foo); thread::spawn(|| { assert!(FOO.try_with(|_| ()).is_ok()); }).join().ok().unwrap(); } #[test] fn smoke_dtor() { thread_local!(static FOO: UnsafeCell<Option<Foo>> = UnsafeCell::new(None)); let (tx, rx) = channel(); let _t = thread::spawn(move|| unsafe { let mut tx = Some(tx); FOO.with(|f| { *f.get() = Some(Foo(tx.take().unwrap())); }); }); rx.recv().unwrap(); } #[test] fn circular() { struct S1; struct S2; thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None)); thread_local!(static K2: UnsafeCell<Option<S2>> = UnsafeCell::new(None)); static mut HITS: u32 = 0; impl Drop for S1 { fn drop(&mut self) { unsafe { HITS += 1; if K2.try_with(|_| ()).is_err() { assert_eq!(HITS, 3); } else { if HITS == 1 { K2.with(|s| *s.get() = Some(S2)); } else { assert_eq!(HITS, 3); } } } } } impl Drop for S2 { fn drop(&mut self) { unsafe { HITS += 1; assert!(K1.try_with(|_| ()).is_ok()); assert_eq!(HITS, 2); K1.with(|s| *s.get() = Some(S1)); } } } thread::spawn(move|| { drop(S1); }).join().ok().unwrap(); } #[test] fn self_referential() { struct S1; thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None)); impl Drop for S1 { fn drop(&mut self) { assert!(K1.try_with(|_| ()).is_err()); } } thread::spawn(move|| unsafe { K1.with(|s| *s.get() = Some(S1)); }).join().ok().unwrap(); } // Note that this test will deadlock if TLS destructors aren't run (this // requires the destructor to be run to pass the test). macOS has a known bug // where dtors-in-dtors may cancel other destructors, so we just ignore this // test on macOS. #[test] #[cfg_attr(target_os = "macos", ignore)] fn dtors_in_dtors_in_dtors() { struct S1(Sender<()>); thread_local!(static K1: UnsafeCell<Option<S1>> = UnsafeCell::new(None)); thread_local!(static K2: UnsafeCell<Option<Foo>> = UnsafeCell::new(None)); impl Drop for S1 { fn drop(&mut self) { let S1(ref tx) = *self; unsafe { let _ = K2.try_with(|s| *s.get() = Some(Foo(tx.clone()))); } } } let (tx, rx) = channel(); let _t = thread::spawn(move|| unsafe { let mut tx = Some(tx); K1.with(|s| *s.get() = Some(S1(tx.take().unwrap()))); }); rx.recv().unwrap(); } } #[cfg(test)] mod dynamic_tests { use cell::RefCell; use collections::HashMap; #[test] fn smoke() { fn square(i: i32) -> i32 { i * i } thread_local!(static FOO: i32 = square(3)); FOO.with(|f| { assert_eq!(*f, 9); }); } #[test] fn hashmap() { fn map() -> RefCell<HashMap<i32, i32>> { let mut m = HashMap::new(); m.insert(1, 2); RefCell::new(m) } thread_local!(static FOO: RefCell<HashMap<i32, i32>> = map()); FOO.with(|map| { assert_eq!(map.borrow()[&1], 2); }); } #[test] fn refcell_vec() { thread_local!(static FOO: RefCell<Vec<u32>> = RefCell::new(vec![1, 2, 3])); FOO.with(|vec| { assert_eq!(vec.borrow().len(), 3); vec.borrow_mut().push(4); assert_eq!(vec.borrow()[3], 4); }); } }