std: move `Once` implementations to `sys`

1 files changed, 0 insertions, 294 deletions

diff --git a/library/std/src/sys_common/once/queue.rs b/library/std/src/sys_common/once/queue.rs
deleted file mode 100644
index 730cdb768bd..00000000000
--- a/library/std/src/sys_common/once/queue.rs
+++ /dev/null
@@ -1,294 +0,0 @@
-// 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!
-//
-// Atomic orderings:
-// When running `Once` we deal with multiple atomics:
-// `Once.state_and_queue` and an unknown number of `Waiter.signaled`.
-// * `state_and_queue` is used (1) as a state flag, (2) for synchronizing the
-//   result of the `Once`, and (3) for synchronizing `Waiter` nodes.
-//     - At the end of the `call` function we have to make sure the result
-//       of the `Once` is acquired. So every load which can be the only one to
-//       load COMPLETED must have at least acquire ordering, which means all
-//       three of them.
-//     - `WaiterQueue::drop` is the only place that may store COMPLETED, and
-//       must do so with release ordering to make the result available.
-//     - `wait` inserts `Waiter` nodes as a pointer in `state_and_queue`, and
-//       needs to make the nodes available with release ordering. The load in
-//       its `compare_exchange` can be relaxed because it only has to compare
-//       the atomic, not to read other data.
-//     - `WaiterQueue::drop` must see the `Waiter` nodes, so it must load
-//       `state_and_queue` with acquire ordering.
-//     - There is just one store where `state_and_queue` is used only as a
-//       state flag, without having to synchronize data: switching the state
-//       from INCOMPLETE to RUNNING in `call`. This store can be Relaxed,
-//       but the read has to be Acquire because of the requirements mentioned
-//       above.
-// * `Waiter.signaled` is both used as a flag, and to protect a field with
-//   interior mutability in `Waiter`. `Waiter.thread` is changed in
-//   `WaiterQueue::drop` which then sets `signaled` with release ordering.
-//   After `wait` loads `signaled` with acquire ordering and sees it is true,
-//   it needs to see the changes to drop the `Waiter` struct correctly.
-// * There is one place where the two atomics `Once.state_and_queue` and
-//   `Waiter.signaled` come together, and might be reordered by the compiler or
-//   processor. Because both use acquire ordering such a reordering is not
-//   allowed, so no need for `SeqCst`.
-
-use crate::cell::Cell;
-use crate::fmt;
-use crate::ptr;
-use crate::sync as public;
-use crate::sync::atomic::{AtomicBool, AtomicPtr, Ordering};
-use crate::sync::once::ExclusiveState;
-use crate::thread::{self, Thread};
-
-type Masked = ();
-
-pub struct Once {
-    state_and_queue: AtomicPtr<Masked>,
-}
-
-pub struct OnceState {
-    poisoned: bool,
-    set_state_on_drop_to: Cell<*mut Masked>,
-}
-
-// Four states that a Once can be in, encoded into the lower bits of
-// `state_and_queue` 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, used while in the
-// RUNNING state.
-// Note: `Waiter` can't hold a mutable pointer to the next thread, because then
-// `wait` would both hand out a mutable reference to its `Waiter` node, and keep
-// a shared reference to check `signaled`. Instead we hold shared references and
-// use interior mutability.
-#[repr(align(4))] // Ensure the two lower bits are free to use as state bits.
-struct Waiter {
-    thread: Cell<Option<Thread>>,
-    signaled: AtomicBool,
-    next: *const Waiter,
-}
-
-// Head of a linked list of waiters.
-// Every node is a struct on the stack of a waiting thread.
-// Will wake up the waiters when it gets dropped, i.e. also on panic.
-struct WaiterQueue<'a> {
-    state_and_queue: &'a AtomicPtr<Masked>,
-    set_state_on_drop_to: *mut Masked,
-}
-
-impl Once {
-    #[inline]
-    #[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]
-    pub const fn new() -> Once {
-        Once { state_and_queue: AtomicPtr::new(ptr::without_provenance_mut(INCOMPLETE)) }
-    }
-
-    #[inline]
-    pub fn is_completed(&self) -> bool {
-        // An `Acquire` load is enough because that makes all the initialization
-        // operations visible to us, and, this being a fast path, weaker
-        // ordering helps with performance. This `Acquire` synchronizes with
-        // `Release` operations on the slow path.
-        self.state_and_queue.load(Ordering::Acquire).addr() == COMPLETE
-    }
-
-    #[inline]
-    pub(crate) fn state(&mut self) -> ExclusiveState {
-        match self.state_and_queue.get_mut().addr() {
-            INCOMPLETE => ExclusiveState::Incomplete,
-            POISONED => ExclusiveState::Poisoned,
-            COMPLETE => ExclusiveState::Complete,
-            _ => unreachable!("invalid Once state"),
-        }
-    }
-
-    // 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]
-    #[track_caller]
-    pub fn call(&self, ignore_poisoning: bool, init: &mut dyn FnMut(&public::OnceState)) {
-        let mut state_and_queue = self.state_and_queue.load(Ordering::Acquire);
-        loop {
-            match state_and_queue.addr() {
-                COMPLETE => break,
-                POISONED if !ignore_poisoning => {
-                    // Panic to propagate the poison.
-                    panic!("Once instance has previously been poisoned");
-                }
-                POISONED | INCOMPLETE => {
-                    // Try to register this thread as the one RUNNING.
-                    let exchange_result = self.state_and_queue.compare_exchange(
-                        state_and_queue,
-                        ptr::without_provenance_mut(RUNNING),
-                        Ordering::Acquire,
-                        Ordering::Acquire,
-                    );
-                    if let Err(old) = exchange_result {
-                        state_and_queue = old;
-                        continue;
-                    }
-                    // `waiter_queue` will manage other waiting threads, and
-                    // wake them up on drop.
-                    let mut waiter_queue = WaiterQueue {
-                        state_and_queue: &self.state_and_queue,
-                        set_state_on_drop_to: ptr::without_provenance_mut(POISONED),
-                    };
-                    // Run the initialization function, letting it know if we're
-                    // poisoned or not.
-                    let init_state = public::OnceState {
-                        inner: OnceState {
-                            poisoned: state_and_queue.addr() == POISONED,
-                            set_state_on_drop_to: Cell::new(ptr::without_provenance_mut(COMPLETE)),
-                        },
-                    };
-                    init(&init_state);
-                    waiter_queue.set_state_on_drop_to = init_state.inner.set_state_on_drop_to.get();
-                    break;
-                }
-                _ => {
-                    // All other values must be RUNNING with possibly a
-                    // pointer to the waiter queue in the more significant bits.
-                    assert!(state_and_queue.addr() & STATE_MASK == RUNNING);
-                    wait(&self.state_and_queue, state_and_queue);
-                    state_and_queue = self.state_and_queue.load(Ordering::Acquire);
-                }
-            }
-        }
-    }
-}
-
-fn wait(state_and_queue: &AtomicPtr<Masked>, mut current_state: *mut Masked) {
-    // Note: the following code was carefully written to avoid creating a
-    // mutable reference to `node` that gets aliased.
-    loop {
-        // Don't queue this thread if the status is no longer running,
-        // otherwise we will not be woken up.
-        if current_state.addr() & STATE_MASK != RUNNING {
-            return;
-        }
-
-        // Create the node for our current thread.
-        let node = Waiter {
-            thread: Cell::new(Some(thread::current())),
-            signaled: AtomicBool::new(false),
-            next: current_state.with_addr(current_state.addr() & !STATE_MASK) as *const Waiter,
-        };
-        let me = core::ptr::addr_of!(node) as *const Masked as *mut Masked;
-
-        // Try to slide in the node at the head of the linked list, making sure
-        // that another thread didn't just replace the head of the linked list.
-        let exchange_result = state_and_queue.compare_exchange(
-            current_state,
-            me.with_addr(me.addr() | RUNNING),
-            Ordering::Release,
-            Ordering::Relaxed,
-        );
-        if let Err(old) = exchange_result {
-            current_state = old;
-            continue;
-        }
-
-        // We have enqueued ourselves, now lets wait.
-        // It is important not to return before being signaled, otherwise we
-        // would drop our `Waiter` node and leave a hole in the linked list
-        // (and a dangling reference). Guard against spurious wakeups by
-        // reparking ourselves until we are signaled.
-        while !node.signaled.load(Ordering::Acquire) {
-            // If the managing thread happens to signal and unpark us before we
-            // can park ourselves, the result could be this thread never gets
-            // unparked. Luckily `park` comes with the guarantee that if it got
-            // an `unpark` just before on an unparked thread it does not park.
-            thread::park();
-        }
-        break;
-    }
-}
-
-#[stable(feature = "std_debug", since = "1.16.0")]
-impl fmt::Debug for Once {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("Once").finish_non_exhaustive()
-    }
-}
-
-impl Drop for WaiterQueue<'_> {
-    fn drop(&mut self) {
-        // Swap out our state with however we finished.
-        let state_and_queue =
-            self.state_and_queue.swap(self.set_state_on_drop_to, Ordering::AcqRel);
-
-        // We should only ever see an old state which was RUNNING.
-        assert_eq!(state_and_queue.addr() & STATE_MASK, RUNNING);
-
-        // Walk the entire linked list of waiters and wake them up (in lifo
-        // order, last to register is first to wake up).
-        unsafe {
-            // Right after setting `node.signaled = true` the other thread may
-            // free `node` if there happens to be has a spurious wakeup.
-            // So we have to take out the `thread` field and copy the pointer to
-            // `next` first.
-            let mut queue =
-                state_and_queue.with_addr(state_and_queue.addr() & !STATE_MASK) as *const Waiter;
-            while !queue.is_null() {
-                let next = (*queue).next;
-                let thread = (*queue).thread.take().unwrap();
-                (*queue).signaled.store(true, Ordering::Release);
-                // ^- FIXME (maybe): This is another case of issue #55005
-                // `store()` has a potentially dangling ref to `signaled`.
-                queue = next;
-                thread.unpark();
-            }
-        }
-    }
-}
-
-impl OnceState {
-    #[inline]
-    pub fn is_poisoned(&self) -> bool {
-        self.poisoned
-    }
-
-    #[inline]
-    pub fn poison(&self) {
-        self.set_state_on_drop_to.set(ptr::without_provenance_mut(POISONED));
-    }
-}