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

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    // Copyright 2014 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.

    //! A "once initialization" primitive
    //!
    //! This primitive is meant to be used to run one-time initialization. An
    //! example use case would be for initializing an FFI library.

    // A "once" is a relatively simple primitive, and it's also typically provided
    // by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS
    // primitives, however, tend to have surprising restrictions, such as the Unix
    // one doesn't allow an argument to be passed to the function.
    //
    // As a result, we end up implementing it ourselves in the standard library.
    // This also gives us the opportunity to optimize the implementation a bit which
    // should help the fast path on call sites. Consequently, let's explain how this
    // primitive works now!
    //
    // So to recap, the guarantees of a Once are that it will call the
    // initialization closure at most once, and it will never return until the one
    // that's running has finished running. This means that we need some form of
    // blocking here while the custom callback is running at the very least.
    // Additionally, we add on the restriction of **poisoning**. Whenever an
    // initialization closure panics, the Once enters a "poisoned" state which means
    // that all future calls will immediately panic as well.
    //
    // So to implement this, one might first reach for a `StaticMutex`, but those
    // unfortunately need to be deallocated (e.g. call `destroy()`) to free memory
    // on all OSes (some of the BSDs allocate memory for mutexes). It also gets a
    // lot harder with poisoning to figure out when the mutex needs to be
    // deallocated because it's not after the closure finishes, but after the first
    // successful closure finishes.
    //
    // All in all, this is instead implemented with atomics and lock-free
    // operations! Whee! Each `Once` has one word of atomic state, and this state is
    // CAS'd on to determine what to do. There are four possible state of a `Once`:
    //
    // * Incomplete - no initialization has run yet, and no thread is currently
    //                using the Once.
    // * Poisoned - some thread has previously attempted to initialize the Once, but
    //              it panicked, so the Once is now poisoned. There are no other
    //              threads currently accessing this Once.
    // * Running - some thread is currently attempting to run initialization. It may
    //             succeed, so all future threads need to wait for it to finish.
    //             Note that this state is accompanied with a payload, described
    //             below.
    // * Complete - initialization has completed and all future calls should finish
    //              immediately.
    //
    // With 4 states we need 2 bits to encode this, and we use the remaining bits
    // in the word we have allocated as a queue of threads waiting for the thread
    // responsible for entering the RUNNING state. This queue is just a linked list
    // of Waiter nodes which is monotonically increasing in size. Each node is
    // allocated on the stack, and whenever the running closure finishes it will
    // consume the entire queue and notify all waiters they should try again.
    //
    // You'll find a few more details in the implementation, but that's the gist of
    // it!

    use fmt;
    use marker;
    use ptr;
    use sync::atomic::{AtomicUsize, AtomicBool, Ordering};
    use thread::{self, Thread};

    /// A synchronization primitive which can be used to run a one-time global
    /// initialization. Useful for one-time initialization for FFI or related
    /// functionality. This type can only be constructed with the [`ONCE_INIT`]
    /// value or the equivalent [`Once::new`] constructor.
    ///
    /// [`ONCE_INIT`]: constant.ONCE_INIT.html
    /// [`Once::new`]: struct.Once.html#method.new
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Once;
    ///
    /// static START: Once = Once::new();
    ///
    /// START.call_once(|| {
    ///     // run initialization here
    /// });
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Once {
        // This `state` word is actually an encoded version of just a pointer to a
        // `Waiter`, so we add the `PhantomData` appropriately.
        state: AtomicUsize,
        _marker: marker::PhantomData<*mut Waiter>,
    }

    // The `PhantomData` of a raw pointer removes these two auto traits, but we
    // enforce both below in the implementation so this should be safe to add.
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl Sync for Once {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl Send for Once {}

    /// State yielded to [`call_once_force`]’s closure parameter. The state can be
    /// used to query the poison status of the [`Once`].
    ///
    /// [`call_once_force`]: struct.Once.html#method.call_once_force
    /// [`Once`]: struct.Once.html
    #[unstable(feature = "once_poison", issue = "33577")]
    #[derive(Debug)]
    pub struct OnceState {
        poisoned: bool,
    }

    /// Initialization value for static [`Once`] values.
    ///
    /// [`Once`]: struct.Once.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Once, ONCE_INIT};
    ///
    /// static START: Once = ONCE_INIT;
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const ONCE_INIT: Once = Once::new();

    // Four states that a Once can be in, encoded into the lower bits of `state` in
    // the Once structure.
    const INCOMPLETE: usize = 0x0;
    const POISONED: usize = 0x1;
    const RUNNING: usize = 0x2;
    const COMPLETE: usize = 0x3;

    // Mask to learn about the state. All other bits are the queue of waiters if
    // this is in the RUNNING state.
    const STATE_MASK: usize = 0x3;

    // Representation of a node in the linked list of waiters in the RUNNING state.
    struct Waiter {
        thread: Option<Thread>,
        signaled: AtomicBool,
        next: *mut Waiter,
    }

    // Helper struct used to clean up after a closure call with a `Drop`
    // implementation to also run on panic.
    struct Finish<'a> {
        panicked: bool,
        me: &'a Once,
    }

    impl Once {
        /// Creates a new `Once` value.
        #[stable(feature = "once_new", since = "1.2.0")]
        pub const fn new() -> Once {
            Once {
                state: AtomicUsize::new(INCOMPLETE),
                _marker: marker::PhantomData,
            }
        }

        /// Performs an initialization routine once and only once. The given closure
        /// will be executed if this is the first time `call_once` has been called,
        /// and otherwise the routine will *not* be invoked.
        ///
        /// This method will block the calling thread if another initialization
        /// routine is currently running.
        ///
        /// When this function returns, it is guaranteed that some initialization
        /// has run and completed (it may not be the closure specified). It is also
        /// guaranteed that any memory writes performed by the executed closure can
        /// be reliably observed by other threads at this point (there is a
        /// happens-before relation between the closure and code executing after the
        /// return).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Once;
        ///
        /// static mut VAL: usize = 0;
        /// static INIT: Once = Once::new();
        ///
        /// // Accessing a `static mut` is unsafe much of the time, but if we do so
        /// // in a synchronized fashion (e.g. write once or read all) then we're
        /// // good to go!
        /// //
        /// // This function will only call `expensive_computation` once, and will
        /// // otherwise always return the value returned from the first invocation.
        /// fn get_cached_val() -> usize {
        ///     unsafe {
        ///         INIT.call_once(|| {
        ///             VAL = expensive_computation();
        ///         });
        ///         VAL
        ///     }
        /// }
        ///
        /// fn expensive_computation() -> usize {
        ///     // ...
        /// # 2
        /// }
        /// ```
        ///
        /// # Panics
        ///
        /// The closure `f` will only be executed once if this is called
        /// concurrently amongst many threads. If that closure panics, however, then
        /// it will *poison* this `Once` instance, causing all future invocations of
        /// `call_once` to also panic.
        ///
        /// This is similar to [poisoning with mutexes][poison].
        ///
        /// [poison]: struct.Mutex.html#poisoning
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn call_once<F>(&self, f: F) where F: FnOnce() {
            // Fast path, just see if we've completed initialization.
            // An `Acquire` load is enough because that makes all the initialization
            // operations visible to us. The cold path uses SeqCst consistently
            // because the performance difference really does not matter there,
            // and SeqCst minimizes the chances of something going wrong.
            if self.state.load(Ordering::Acquire) == COMPLETE {
                return
            }

            let mut f = Some(f);
            self.call_inner(false, &mut |_| f.take().unwrap()());
        }

        /// Performs the same function as [`call_once`] except ignores poisoning.
        ///
        /// Unlike [`call_once`], if this `Once` has been poisoned (i.e. a previous
        /// call to `call_once` or `call_once_force` caused a panic), calling
        /// `call_once_force` will still invoke the closure `f` and will _not_
        /// result in an immediate panic. If `f` panics, the `Once` will remain
        /// in a poison state. If `f` does _not_ panic, the `Once` will no
        /// longer be in a poison state and all future calls to `call_once` or
        /// `call_one_force` will no-op.
        ///
        /// The closure `f` is yielded a [`OnceState`] structure which can be used
        /// to query the poison status of the `Once`.
        ///
        /// [`call_once`]: struct.Once.html#method.call_once
        /// [`OnceState`]: struct.OnceState.html
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_poison)]
        ///
        /// use std::sync::Once;
        /// use std::thread;
        ///
        /// static INIT: Once = Once::new();
        ///
        /// // poison the once
        /// let handle = thread::spawn(|| {
        ///     INIT.call_once(|| panic!());
        /// });
        /// assert!(handle.join().is_err());
        ///
        /// // poisoning propagates
        /// let handle = thread::spawn(|| {
        ///     INIT.call_once(|| {});
        /// });
        /// assert!(handle.join().is_err());
        ///
        /// // call_once_force will still run and reset the poisoned state
        /// INIT.call_once_force(|state| {
        ///     assert!(state.poisoned());
        /// });
        ///
        /// // once any success happens, we stop propagating the poison
        /// INIT.call_once(|| {});
        /// ```
        #[unstable(feature = "once_poison", issue = "33577")]
        pub fn call_once_force<F>(&self, f: F) where F: FnOnce(&OnceState) {
            // same as above, just with a different parameter to `call_inner`.
            // An `Acquire` load is enough because that makes all the initialization
            // operations visible to us. The cold path uses SeqCst consistently
            // because the performance difference really does not matter there,
            // and SeqCst minimizes the chances of something going wrong.
            if self.state.load(Ordering::Acquire) == COMPLETE {
                return
            }

            let mut f = Some(f);
            self.call_inner(true, &mut |p| {
                f.take().unwrap()(&OnceState { poisoned: p })
            });
        }

        // This is a non-generic function to reduce the monomorphization cost of
        // using `call_once` (this isn't exactly a trivial or small implementation).
        //
        // Additionally, this is tagged with `#[cold]` as it should indeed be cold
        // and it helps let LLVM know that calls to this function should be off the
        // fast path. Essentially, this should help generate more straight line code
        // in LLVM.
        //
        // Finally, this takes an `FnMut` instead of a `FnOnce` because there's
        // currently no way to take an `FnOnce` and call it via virtual dispatch
        // without some allocation overhead.
        #[cold]
        fn call_inner(&self,
                      ignore_poisoning: bool,
                      init: &mut dyn FnMut(bool)) {
            let mut state = self.state.load(Ordering::SeqCst);

            'outer: loop {
                match state {
                    // If we're complete, then there's nothing to do, we just
                    // jettison out as we shouldn't run the closure.
                    COMPLETE => return,

                    // If we're poisoned and we're not in a mode to ignore
                    // poisoning, then we panic here to propagate the poison.
                    POISONED if !ignore_poisoning => {
                        panic!("Once instance has previously been poisoned");
                    }

                    // Otherwise if we see a poisoned or otherwise incomplete state
                    // we will attempt to move ourselves into the RUNNING state. If
                    // we succeed, then the queue of waiters starts at null (all 0
                    // bits).
                    POISONED |
                    INCOMPLETE => {
                        let old = self.state.compare_and_swap(state, RUNNING,
                                                              Ordering::SeqCst);
                        if old != state {
                            state = old;
                            continue
                        }

                        // Run the initialization routine, letting it know if we're
                        // poisoned or not. The `Finish` struct is then dropped, and
                        // the `Drop` implementation here is responsible for waking
                        // up other waiters both in the normal return and panicking
                        // case.
                        let mut complete = Finish {
                            panicked: true,
                            me: self,
                        };
                        init(state == POISONED);
                        complete.panicked = false;
                        return
                    }

                    // All other values we find should correspond to the RUNNING
                    // state with an encoded waiter list in the more significant
                    // bits. We attempt to enqueue ourselves by moving us to the
                    // head of the list and bail out if we ever see a state that's
                    // not RUNNING.
                    _ => {
                        assert!(state & STATE_MASK == RUNNING);
                        let mut node = Waiter {
                            thread: Some(thread::current()),
                            signaled: AtomicBool::new(false),
                            next: ptr::null_mut(),
                        };
                        let me = &mut node as *mut Waiter as usize;
                        assert!(me & STATE_MASK == 0);

                        while state & STATE_MASK == RUNNING {
                            node.next = (state & !STATE_MASK) as *mut Waiter;
                            let old = self.state.compare_and_swap(state,
                                                                  me | RUNNING,
                                                                  Ordering::SeqCst);
                            if old != state {
                                state = old;
                                continue
                            }

                            // Once we've enqueued ourselves, wait in a loop.
                            // Afterwards reload the state and continue with what we
                            // were doing from before.
                            while !node.signaled.load(Ordering::SeqCst) {
                                thread::park();
                            }
                            state = self.state.load(Ordering::SeqCst);
                            continue 'outer
                        }
                    }
                }
            }
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for Once {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Once { .. }")
        }
    }

    impl<'a> Drop for Finish<'a> {
        fn drop(&mut self) {
            // Swap out our state with however we finished. We should only ever see
            // an old state which was RUNNING.
            let queue = if self.panicked {
                self.me.state.swap(POISONED, Ordering::SeqCst)
            } else {
                self.me.state.swap(COMPLETE, Ordering::SeqCst)
            };
            assert_eq!(queue & STATE_MASK, RUNNING);

            // Decode the RUNNING to a list of waiters, then walk that entire list
            // and wake them up. Note that it is crucial that after we store `true`
            // in the node it can be free'd! As a result we load the `thread` to
            // signal ahead of time and then unpark it after the store.
            unsafe {
                let mut queue = (queue & !STATE_MASK) as *mut Waiter;
                while !queue.is_null() {
                    let next = (*queue).next;
                    let thread = (*queue).thread.take().unwrap();
                    (*queue).signaled.store(true, Ordering::SeqCst);
                    thread.unpark();
                    queue = next;
                }
            }
        }
    }

    impl OnceState {
        /// Returns whether the associated [`Once`] was poisoned prior to the
        /// invocation of the closure passed to [`call_once_force`].
        ///
        /// [`call_once_force`]: struct.Once.html#method.call_once_force
        /// [`Once`]: struct.Once.html
        ///
        /// # Examples
        ///
        /// A poisoned `Once`:
        ///
        /// ```
        /// #![feature(once_poison)]
        ///
        /// use std::sync::Once;
        /// use std::thread;
        ///
        /// static INIT: Once = Once::new();
        ///
        /// // poison the once
        /// let handle = thread::spawn(|| {
        ///     INIT.call_once(|| panic!());
        /// });
        /// assert!(handle.join().is_err());
        ///
        /// INIT.call_once_force(|state| {
        ///     assert!(state.poisoned());
        /// });
        /// ```
        ///
        /// An unpoisoned `Once`:
        ///
        /// ```
        /// #![feature(once_poison)]
        ///
        /// use std::sync::Once;
        ///
        /// static INIT: Once = Once::new();
        ///
        /// INIT.call_once_force(|state| {
        ///     assert!(!state.poisoned());
        /// });
        #[unstable(feature = "once_poison", issue = "33577")]
        pub fn poisoned(&self) -> bool {
            self.poisoned
        }
    }

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use panic;
        use sync::mpsc::channel;
        use thread;
        use super::Once;

        #[test]
        fn smoke_once() {
            static O: Once = Once::new();
            let mut a = 0;
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
        }

        #[test]
        fn stampede_once() {
            static O: Once = Once::new();
            static mut RUN: bool = false;

            let (tx, rx) = channel();
            for _ in 0..10 {
                let tx = tx.clone();
                thread::spawn(move|| {
                    for _ in 0..4 { thread::yield_now() }
                    unsafe {
                        O.call_once(|| {
                            assert!(!RUN);
                            RUN = true;
                        });
                        assert!(RUN);
                    }
                    tx.send(()).unwrap();
                });
            }

            unsafe {
                O.call_once(|| {
                    assert!(!RUN);
                    RUN = true;
                });
                assert!(RUN);
            }

            for _ in 0..10 {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn poison_bad() {
            static O: Once = Once::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.call_once(|| panic!());
            });
            assert!(t.is_err());

            // poisoning propagates
            let t = panic::catch_unwind(|| {
                O.call_once(|| {});
            });
            assert!(t.is_err());

            // we can subvert poisoning, however
            let mut called = false;
            O.call_once_force(|p| {
                called = true;
                assert!(p.poisoned())
            });
            assert!(called);

            // once any success happens, we stop propagating the poison
            O.call_once(|| {});
        }

        #[test]
        fn wait_for_force_to_finish() {
            static O: Once = Once::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.call_once(|| panic!());
            });
            assert!(t.is_err());

            // make sure someone's waiting inside the once via a force
            let (tx1, rx1) = channel();
            let (tx2, rx2) = channel();
            let t1 = thread::spawn(move || {
                O.call_once_force(|p| {
                    assert!(p.poisoned());
                    tx1.send(()).unwrap();
                    rx2.recv().unwrap();
                });
            });

            rx1.recv().unwrap();

            // put another waiter on the once
            let t2 = thread::spawn(|| {
                let mut called = false;
                O.call_once(|| {
                    called = true;
                });
                assert!(!called);
            });

            tx2.send(()).unwrap();

            assert!(t1.join().is_ok());
            assert!(t2.join().is_ok());

        }
    }

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