local.rs.html -- source
Description: Source to the Rust file `src/libstd/thread/local.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Thread local storage

    #![unstable(feature = "thread_local_internals", issue = "0")]

    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.
    ///
    /// 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
    /// let t = thread::spawn(move|| {
    ///     FOO.with(|f| {
    ///         assert_eq!(*f.borrow(), 1);
    ///         *f.borrow_mut() = 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(|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.
    ///
    /// [`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 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(thread_local_internals)]
    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(thread_local_internals, cfg_target_thread_local, thread_local)]
    #[allow_internal_unsafe]
    macro_rules! __thread_local_inner {
        (@key $t:ty, $init:expr) => {
            {
                #[inline]
                fn __init() -> $t { $init }

                unsafe fn __getit() -> $crate::option::Option<&'static $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(__init)
                }

                unsafe {
                    $crate::thread::LocalKey::new(__getit)
                }
            }
        };
        ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $init:expr) => {
            $(#[$attr])* $vis const $name: $crate::thread::LocalKey<$t> =
                $crate::__thread_local_inner!(@key $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")]
    #[derive(Clone, Copy, Eq, PartialEq)]
    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)
        }
    }

    #[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 = "0"
        )]
        pub const unsafe fn new(inner: unsafe fn() -> 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`](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 thread_local = (self.inner)().ok_or(AccessError { _private: () })?;
                Ok(f(thread_local))
            }
        }
    }

    mod lazy {
        use crate::cell::UnsafeCell;
        use crate::hint;
        use crate::mem;

        pub struct LazyKeyInner<T> {
            inner: UnsafeCell<Option<T>>,
        }

        impl<T> LazyKeyInner<T> {
            pub const fn new() -> LazyKeyInner<T> {
                LazyKeyInner { inner: UnsafeCell::new(None) }
            }

            pub unsafe fn get(&self) -> Option<&'static T> {
                (*self.inner.get()).as_ref()
            }

            pub unsafe fn initialize<F: FnOnce() -> T>(&self, init: F) -> &'static T {
                // Execute the initialization up front, *then* move it into our slot,
                // just in case initialization fails.
                let value = init();
                let ptr = self.inner.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(),
                }
            }

            #[allow(unused)]
            pub unsafe fn take(&mut self) -> Option<T> {
                (*self.inner.get()).take()
            }
        }
    }

    /// 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 super::lazy::LazyKeyInner;
        use crate::fmt;

        pub struct Key<T> {
            inner: LazyKeyInner<T>,
        }

        unsafe impl<T> 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: LazyKeyInner::new() }
            }

            pub unsafe fn get(&self, init: fn() -> T) -> Option<&'static T> {
                let value = match self.inner.get() {
                    Some(ref value) => value,
                    None => self.inner.initialize(init),
                };
                Some(value)
            }
        }
    }

    #[doc(hidden)]
    #[cfg(target_thread_local)]
    pub mod fast {
        use super::lazy::LazyKeyInner;
        use crate::cell::Cell;
        use crate::fmt;
        use crate::mem;
        use crate::sys::fast_thread_local::register_dtor;

        #[derive(Copy, Clone)]
        enum DtorState {
            Unregistered,
            Registered,
            RunningOrHasRun,
        }

        // This data structure has been carefully constructed so that the fast path
        // only contains one branch on x86. That optimization is necessary to avoid
        // duplicated tls lookups on OSX.
        //
        // LLVM issue: https://bugs.llvm.org/show_bug.cgi?id=41722
        pub struct Key<T> {
            // If `LazyKeyInner::get` returns `None`, that indicates either:
            //   * The value has never been initialized
            //   * The value is being recursively initialized
            //   * The value has already been destroyed or is being destroyed
            // To determine which kind of `None`, check `dtor_state`.
            //
            // This is very optimizer friendly for the fast path - initialized but
            // not yet dropped.
            inner: LazyKeyInner<T>,

            // Metadata to keep track of the state of the destructor. Remember that
            // this variable is thread-local, not global.
            dtor_state: Cell<DtorState>,
        }

        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: LazyKeyInner::new(), dtor_state: Cell::new(DtorState::Unregistered) }
            }

            pub unsafe fn get<F: FnOnce() -> T>(&self, init: F) -> Option<&'static T> {
                match self.inner.get() {
                    Some(val) => Some(val),
                    None => self.try_initialize(init),
                }
            }

            // `try_initialize` is only called once per fast thread local variable,
            // except in corner cases where thread_local dtors reference other
            // thread_local's, or it is being recursively initialized.
            //
            // Macos: Inlining this function can cause two `tlv_get_addr` calls to
            // be performed for every call to `Key::get`. The #[cold] hint makes
            // that less likely.
            // LLVM issue: https://bugs.llvm.org/show_bug.cgi?id=41722
            #[cold]
            unsafe fn try_initialize<F: FnOnce() -> T>(&self, init: F) -> Option<&'static T> {
                if !mem::needs_drop::<T>() || self.try_register_dtor() {
                    Some(self.inner.initialize(init))
                } else {
                    None
                }
            }

            // `try_register_dtor` is only called once per fast thread local
            // variable, except in corner cases where thread_local dtors reference
            // other thread_local's, or it is being recursively initialized.
            unsafe fn try_register_dtor(&self) -> bool {
                match self.dtor_state.get() {
                    DtorState::Unregistered => {
                        // dtor registration happens before initialization.
                        register_dtor(self as *const _ as *mut u8, destroy_value::<T>);
                        self.dtor_state.set(DtorState::Registered);
                        true
                    }
                    DtorState::Registered => {
                        // recursively initialized
                        true
                    }
                    DtorState::RunningOrHasRun => false,
                }
            }
        }

        unsafe extern "C" fn destroy_value<T>(ptr: *mut u8) {
            let ptr = ptr as *mut Key<T>;

            // Right before we run the user destructor be sure to set the
            // `Option<T>` to `None`, and `dtor_state` to `RunningOrHasRun`. This
            // causes future calls to `get` to run `try_initialize_drop` again,
            // which will now fail, and return `None`.
            let value = (*ptr).inner.take();
            (*ptr).dtor_state.set(DtorState::RunningOrHasRun);
            drop(value);
        }
    }

    #[doc(hidden)]
    pub mod os {
        use super::lazy::LazyKeyInner;
        use crate::cell::Cell;
        use crate::fmt;
        use crate::marker;
        use crate::ptr;
        use crate::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> Sync for Key<T> {}

        struct Value<T: 'static> {
            inner: LazyKeyInner<T>,
            key: &'static Key<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, init: fn() -> T) -> Option<&'static T> {
                let ptr = self.os.get() as *mut Value<T>;
                if ptr as usize > 1 {
                    if let Some(ref value) = (*ptr).inner.get() {
                        return Some(value);
                    }
                }
                self.try_initialize(init)
            }

            // `try_initialize` is only called once per os thread local variable,
            // except in corner cases where thread_local dtors reference other
            // thread_local's, or it is being recursively initialized.
            unsafe fn try_initialize(&'static self, init: fn() -> T) -> Option<&'static T> {
                let ptr = self.os.get() as *mut Value<T>;
                if ptr as usize == 1 {
                    // destructor is running
                    return None;
                }

                let ptr = if ptr.is_null() {
                    // If the lookup returned null, we haven't initialized our own
                    // local copy, so do that now.
                    let ptr: Box<Value<T>> = box Value { inner: LazyKeyInner::new(), key: self };
                    let ptr = Box::into_raw(ptr);
                    self.os.set(ptr as *mut u8);
                    ptr
                } else {
                    // recursive initialization
                    ptr
                };

                Some((*ptr).inner.initialize(init))
            }
        }

        unsafe extern "C" 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 crate::cell::{Cell, UnsafeCell};
        use crate::sync::mpsc::{channel, Sender};
        use crate::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()
            .expect("thread panicked");
        }

        #[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()
            .expect("thread panicked");
        }

        #[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()
            .expect("thread panicked");
        }

        // Note that this test will deadlock if TLS destructors aren't run (this
        // requires the destructor to be run to pass the test).
        #[test]
        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 crate::cell::RefCell;
        use crate::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);
            });
        }
    }