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

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    //! OS-based thread local storage
    //!
    //! This module provides an implementation of OS-based thread local storage,
    //! using the native OS-provided facilities (think `TlsAlloc` or
    //! `pthread_setspecific`). The interface of this differs from the other types
    //! of thread-local-storage provided in this crate in that OS-based TLS can only
    //! get/set pointers,
    //!
    //! This module also provides two flavors of TLS. One is intended for static
    //! initialization, and does not contain a `Drop` implementation to deallocate
    //! the OS-TLS key. The other is a type which does implement `Drop` and hence
    //! has a safe interface.
    //!
    //! # Usage
    //!
    //! This module should likely not be used directly unless other primitives are
    //! being built on. types such as `thread_local::spawn::Key` are likely much
    //! more useful in practice than this OS-based version which likely requires
    //! unsafe code to interoperate with.
    //!
    //! # Examples
    //!
    //! Using a dynamically allocated TLS key. Note that this key can be shared
    //! among many threads via an `Arc`.
    //!
    //! ```ignore (cannot-doctest-private-modules)
    //! let key = Key::new(None);
    //! assert!(key.get().is_null());
    //! key.set(1 as *mut u8);
    //! assert!(!key.get().is_null());
    //!
    //! drop(key); // deallocate this TLS slot.
    //! ```
    //!
    //! Sometimes a statically allocated key is either required or easier to work
    //! with, however.
    //!
    //! ```ignore (cannot-doctest-private-modules)
    //! static KEY: StaticKey = INIT;
    //!
    //! unsafe {
    //!     assert!(KEY.get().is_null());
    //!     KEY.set(1 as *mut u8);
    //! }
    //! ```

    #![allow(non_camel_case_types)]
    #![unstable(feature = "thread_local_internals", issue = "0")]
    #![allow(dead_code)] // sys isn't exported yet

    use crate::ptr;
    use crate::sync::atomic::{self, AtomicUsize, Ordering};
    use crate::sys::thread_local as imp;
    use crate::sys_common::mutex::Mutex;

    /// A type for TLS keys that are statically allocated.
    ///
    /// This type is entirely `unsafe` to use as it does not protect against
    /// use-after-deallocation or use-during-deallocation.
    ///
    /// The actual OS-TLS key is lazily allocated when this is used for the first
    /// time. The key is also deallocated when the Rust runtime exits or `destroy`
    /// is called, whichever comes first.
    ///
    /// # Examples
    ///
    /// ```ignore (cannot-doctest-private-modules)
    /// use tls::os::{StaticKey, INIT};
    ///
    /// static KEY: StaticKey = INIT;
    ///
    /// unsafe {
    ///     assert!(KEY.get().is_null());
    ///     KEY.set(1 as *mut u8);
    /// }
    /// ```
    pub struct StaticKey {
        /// Inner static TLS key (internals).
        key: AtomicUsize,
        /// Destructor for the TLS value.
        ///
        /// See `Key::new` for information about when the destructor runs and how
        /// it runs.
        dtor: Option<unsafe extern fn(*mut u8)>,
    }

    /// A type for a safely managed OS-based TLS slot.
    ///
    /// This type allocates an OS TLS key when it is initialized and will deallocate
    /// the key when it falls out of scope. When compared with `StaticKey`, this
    /// type is entirely safe to use.
    ///
    /// Implementations will likely, however, contain unsafe code as this type only
    /// operates on `*mut u8`, a raw pointer.
    ///
    /// # Examples
    ///
    /// ```ignore (cannot-doctest-private-modules)
    /// use tls::os::Key;
    ///
    /// let key = Key::new(None);
    /// assert!(key.get().is_null());
    /// key.set(1 as *mut u8);
    /// assert!(!key.get().is_null());
    ///
    /// drop(key); // deallocate this TLS slot.
    /// ```
    pub struct Key {
        key: imp::Key,
    }

    /// Constant initialization value for static TLS keys.
    ///
    /// This value specifies no destructor by default.
    pub const INIT: StaticKey = StaticKey::new(None);

    impl StaticKey {
        pub const fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> StaticKey {
            StaticKey {
                key: atomic::AtomicUsize::new(0),
                dtor,
            }
        }

        /// Gets the value associated with this TLS key
        ///
        /// This will lazily allocate a TLS key from the OS if one has not already
        /// been allocated.
        #[inline]
        pub unsafe fn get(&self) -> *mut u8 { imp::get(self.key()) }

        /// Sets this TLS key to a new value.
        ///
        /// This will lazily allocate a TLS key from the OS if one has not already
        /// been allocated.
        #[inline]
        pub unsafe fn set(&self, val: *mut u8) { imp::set(self.key(), val) }

        #[inline]
        unsafe fn key(&self) -> imp::Key {
            match self.key.load(Ordering::Relaxed) {
                0 => self.lazy_init() as imp::Key,
                n => n as imp::Key
            }
        }

        unsafe fn lazy_init(&self) -> usize {
            // Currently the Windows implementation of TLS is pretty hairy, and
            // it greatly simplifies creation if we just synchronize everything.
            //
            // Additionally a 0-index of a tls key hasn't been seen on windows, so
            // we just simplify the whole branch.
            if imp::requires_synchronized_create() {
                // We never call `INIT_LOCK.init()`, so it is UB to attempt to
                // acquire this mutex reentrantly!
                static INIT_LOCK: Mutex = Mutex::new();
                let _guard = INIT_LOCK.lock();
                let mut key = self.key.load(Ordering::SeqCst);
                if key == 0 {
                    key = imp::create(self.dtor) as usize;
                    self.key.store(key, Ordering::SeqCst);
                }
                rtassert!(key != 0);
                return key
            }

            // POSIX allows the key created here to be 0, but the compare_and_swap
            // below relies on using 0 as a sentinel value to check who won the
            // race to set the shared TLS key. As far as I know, there is no
            // guaranteed value that cannot be returned as a posix_key_create key,
            // so there is no value we can initialize the inner key with to
            // prove that it has not yet been set. As such, we'll continue using a
            // value of 0, but with some gyrations to make sure we have a non-0
            // value returned from the creation routine.
            // FIXME: this is clearly a hack, and should be cleaned up.
            let key1 = imp::create(self.dtor);
            let key = if key1 != 0 {
                key1
            } else {
                let key2 = imp::create(self.dtor);
                imp::destroy(key1);
                key2
            };
            rtassert!(key != 0);
            match self.key.compare_and_swap(0, key as usize, Ordering::SeqCst) {
                // The CAS succeeded, so we've created the actual key
                0 => key as usize,
                // If someone beat us to the punch, use their key instead
                n => { imp::destroy(key); n }
            }
        }
    }

    impl Key {
        /// Creates a new managed OS TLS key.
        ///
        /// This key will be deallocated when the key falls out of scope.
        ///
        /// The argument provided is an optionally-specified destructor for the
        /// value of this TLS key. When a thread exits and the value for this key
        /// is non-null the destructor will be invoked. The TLS value will be reset
        /// to null before the destructor is invoked.
        ///
        /// Note that the destructor will not be run when the `Key` goes out of
        /// scope.
        #[inline]
        pub fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
            Key { key: unsafe { imp::create(dtor) } }
        }

        /// See StaticKey::get
        #[inline]
        pub fn get(&self) -> *mut u8 {
            unsafe { imp::get(self.key) }
        }

        /// See StaticKey::set
        #[inline]
        pub fn set(&self, val: *mut u8) {
            unsafe { imp::set(self.key, val) }
        }
    }

    impl Drop for Key {
        fn drop(&mut self) {
            // Right now Windows doesn't support TLS key destruction, but this also
            // isn't used anywhere other than tests, so just leak the TLS key.
            // unsafe { imp::destroy(self.key) }
        }
    }

    pub unsafe fn register_dtor_fallback(t: *mut u8,
                                         dtor: unsafe extern fn(*mut u8)) {
        // The fallback implementation uses a vanilla OS-based TLS key to track
        // the list of destructors that need to be run for this thread. The key
        // then has its own destructor which runs all the other destructors.
        //
        // The destructor for DTORS is a little special in that it has a `while`
        // loop to continuously drain the list of registered destructors. It
        // *should* be the case that this loop always terminates because we
        // provide the guarantee that a TLS key cannot be set after it is
        // flagged for destruction.

        static DTORS: StaticKey = StaticKey::new(Some(run_dtors));
        type List = Vec<(*mut u8, unsafe extern fn(*mut u8))>;
        if DTORS.get().is_null() {
            let v: Box<List> = box Vec::new();
            DTORS.set(Box::into_raw(v) as *mut u8);
        }
        let list: &mut List = &mut *(DTORS.get() as *mut List);
        list.push((t, dtor));

        unsafe extern fn run_dtors(mut ptr: *mut u8) {
            while !ptr.is_null() {
                let list: Box<List> = Box::from_raw(ptr as *mut List);
                for (ptr, dtor) in list.into_iter() {
                    dtor(ptr);
                }
                ptr = DTORS.get();
                DTORS.set(ptr::null_mut());
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::{Key, StaticKey};

        fn assert_sync<T: Sync>() {}
        fn assert_send<T: Send>() {}

        #[test]
        fn smoke() {
            assert_sync::<Key>();
            assert_send::<Key>();

            let k1 = Key::new(None);
            let k2 = Key::new(None);
            assert!(k1.get().is_null());
            assert!(k2.get().is_null());
            k1.set(1 as *mut _);
            k2.set(2 as *mut _);
            assert_eq!(k1.get() as usize, 1);
            assert_eq!(k2.get() as usize, 2);
        }

        #[test]
        fn statik() {
            static K1: StaticKey = StaticKey::new(None);
            static K2: StaticKey = StaticKey::new(None);

            unsafe {
                assert!(K1.get().is_null());
                assert!(K2.get().is_null());
                K1.set(1 as *mut _);
                K2.set(2 as *mut _);
                assert_eq!(K1.get() as usize, 1);
                assert_eq!(K2.get() as usize, 2);
            }
        }
    }