Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 347 insertions, 0 deletions

diff --git a/third_party/rust/atomic_refcell/src/lib.rs b/third_party/rust/atomic_refcell/src/lib.rs
new file mode 100644
index 0000000000..21ffe49746
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+++ b/third_party/rust/atomic_refcell/src/lib.rs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Implements a container type providing RefCell-like semantics for objects
+//! shared across threads.
+//!
+//! RwLock is traditionally considered to be the |Sync| analogue of RefCell.
+//! However, for consumers that can guarantee that they will never mutably
+//! borrow the contents concurrently with immutable borrows, an RwLock is
+//! overkill, and has key disadvantages:
+//! * Performance: Even the fastest existing implementation of RwLock (that of
+//!   parking_lot) performs at least two atomic operations during immutable
+//!   borrows. This makes mutable borrows significantly cheaper than immutable
+//!   borrows, leading to weird incentives when writing performance-critical
+//!   code.
+//! * Features: Implementing AtomicRefCell on top of RwLock makes it impossible
+//!   to implement useful things like AtomicRef{,Mut}::map.
+//!
+//! As such, we re-implement RefCell semantics from scratch with a single atomic
+//! reference count. The primary complication of this scheme relates to keeping
+//! things in a consistent state when one thread performs an illegal borrow and
+//! panics. Since an AtomicRefCell can be accessed by multiple threads, and since
+//! panics are recoverable, we need to ensure that an illegal (panicking) access by
+//! one thread does not lead to undefined behavior on other, still-running threads.
+//!
+//! So we represent things as follows:
+//! * Any value with the high bit set (so half the total refcount space) indicates
+//!   a mutable borrow.
+//! * Mutable borrows perform an atomic compare-and-swap, swapping in the high bit
+//!   if the current value is zero. If the current value is non-zero, the thread
+//!   panics and the value is left undisturbed.
+//! * Immutable borrows perform an atomic increment. If the new value has the high
+//!   bit set, the thread panics. The incremented refcount is left as-is, since it
+//!   still represents a valid mutable borrow. When the mutable borrow is released,
+//!   the refcount is set unconditionally to zero, clearing any stray increments by
+//!   panicked threads.
+//!
+//! There are a few additional purely-academic complications to handle overflow,
+//! which are documented in the implementation.
+//!
+//! The rest of this module is mostly derived by copy-pasting the implementation of
+//! RefCell and fixing things up as appropriate. Certain non-threadsafe methods
+//! have been removed. We segment the concurrency logic from the rest of the code to
+//! keep the tricky parts small and easy to audit.
+
+#![allow(unsafe_code)]
+#![deny(missing_docs)]
+
+use std::cell::UnsafeCell;
+use std::cmp;
+use std::fmt;
+use std::fmt::Debug;
+use std::ops::{Deref, DerefMut};
+use std::sync::atomic;
+use std::sync::atomic::AtomicUsize;
+
+/// A threadsafe analogue to RefCell.
+pub struct AtomicRefCell<T: ?Sized> {
+    borrow: AtomicUsize,
+    value: UnsafeCell<T>,
+}
+
+impl<T> AtomicRefCell<T> {
+    /// Creates a new `AtomicRefCell` containing `value`.
+    #[inline]
+    pub fn new(value: T) -> AtomicRefCell<T> {
+        AtomicRefCell {
+            borrow: AtomicUsize::new(0),
+            value: UnsafeCell::new(value),
+        }
+    }
+
+    /// Consumes the `AtomicRefCell`, returning the wrapped value.
+    #[inline]
+    pub fn into_inner(self) -> T {
+        debug_assert!(self.borrow.load(atomic::Ordering::Acquire) == 0);
+        unsafe { self.value.into_inner() }
+    }
+}
+
+impl<T: ?Sized> AtomicRefCell<T> {
+    /// Immutably borrows the wrapped value.
+    #[inline]
+    pub fn borrow(&self) -> AtomicRef<T> {
+        AtomicRef {
+            value: unsafe { &*self.value.get() },
+            borrow: AtomicBorrowRef::new(&self.borrow),
+        }
+    }
+
+    /// Mutably borrows the wrapped value.
+    #[inline]
+    pub fn borrow_mut(&self) -> AtomicRefMut<T> {
+        AtomicRefMut {
+            value: unsafe { &mut *self.value.get() },
+            borrow: AtomicBorrowRefMut::new(&self.borrow),
+        }
+    }
+
+    /// Returns a raw pointer to the underlying data in this cell.
+    ///
+    /// External synchronization is needed to avoid data races when dereferencing
+    /// the pointer.
+    #[inline]
+    pub fn as_ptr(&self) -> *mut T {
+        self.value.get()
+    }
+}
+
+//
+// Core synchronization logic. Keep this section small and easy to audit.
+//
+
+const HIGH_BIT: usize = !(::std::usize::MAX >> 1);
+const MAX_FAILED_BORROWS: usize = HIGH_BIT + (HIGH_BIT >> 1);
+
+struct AtomicBorrowRef<'b> {
+    borrow: &'b AtomicUsize,
+}
+
+impl<'b> AtomicBorrowRef<'b> {
+    #[inline]
+    fn new(borrow: &'b AtomicUsize) -> Self {
+        let new = borrow.fetch_add(1, atomic::Ordering::Acquire) + 1;
+
+        // If the new count has the high bit set, panic. The specifics of how
+        // we panic is interesting for soundness, but irrelevant for real programs.
+        if new & HIGH_BIT != 0 {
+            Self::do_panic(borrow, new);
+        }
+
+        AtomicBorrowRef { borrow: borrow }
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn do_panic(borrow: &'b AtomicUsize, new: usize) {
+        if new == HIGH_BIT {
+            // We overflowed into the reserved upper half of the refcount
+            // space. Before panicking, decrement the refcount to leave things
+            // in a consistent immutable-borrow state.
+            //
+            // This can basically only happen if somebody forget()s AtomicRefs
+            // in a tight loop.
+            borrow.fetch_sub(1, atomic::Ordering::Release);
+            panic!("too many immutable borrows");
+        } else if new >= MAX_FAILED_BORROWS {
+            // During the mutable borrow, an absurd number of threads have
+            // incremented the refcount and panicked. To avoid hypothetically
+            // wrapping the refcount, we abort the process once a certain
+            // threshold is reached.
+            //
+            // This requires billions of threads to have panicked already, and
+            // so will never happen in a real program.
+            println!("Too many failed borrows");
+            ::std::process::exit(1);
+        } else {
+            // This is the normal case, and the only one which should happen
+            // in a real program.
+            panic!("already mutably borrowed");
+        }
+    }
+}
+
+impl<'b> Drop for AtomicBorrowRef<'b> {
+    #[inline]
+    fn drop(&mut self) {
+        let old = self.borrow.fetch_sub(1, atomic::Ordering::Release);
+        // This assertion is technically incorrect in the case where another
+        // thread hits the hypothetical overflow case, since we might observe
+        // the refcount before it fixes it up (and panics). But that never will
+        // never happen in a real program, and this is a debug_assert! anyway.
+        debug_assert!(old & HIGH_BIT == 0);
+    }
+}
+
+struct AtomicBorrowRefMut<'b> {
+    borrow: &'b AtomicUsize,
+}
+
+impl<'b> Drop for AtomicBorrowRefMut<'b> {
+    #[inline]
+    fn drop(&mut self) {
+        self.borrow.store(0, atomic::Ordering::Release);
+    }
+}
+
+impl<'b> AtomicBorrowRefMut<'b> {
+    #[inline]
+    fn new(borrow: &'b AtomicUsize) -> AtomicBorrowRefMut<'b> {
+        // Use compare-and-swap to avoid corrupting the immutable borrow count
+        // on illegal mutable borrows.
+        let old = match borrow.compare_exchange(0, HIGH_BIT, atomic::Ordering::Acquire, atomic::Ordering::Relaxed) {
+            Ok(x) => x,
+            Err(x) => x,
+        };
+        assert!(old == 0, "already {} borrowed", if old & HIGH_BIT == 0 { "immutably" } else { "mutably" });
+        AtomicBorrowRefMut {
+            borrow: borrow
+        }
+    }
+}
+
+unsafe impl<T: ?Sized + Send + Sync> Send for AtomicRefCell<T> {}
+unsafe impl<T: ?Sized + Send + Sync> Sync for AtomicRefCell<T> {}
+
+//
+// End of core synchronization logic. No tricky thread stuff allowed below
+// this point.
+//
+
+impl<T: Clone> Clone for AtomicRefCell<T> {
+    #[inline]
+    fn clone(&self) -> AtomicRefCell<T> {
+        AtomicRefCell::new(self.borrow().clone())
+    }
+}
+
+impl<T: Default> Default for AtomicRefCell<T> {
+    #[inline]
+    fn default() -> AtomicRefCell<T> {
+        AtomicRefCell::new(Default::default())
+    }
+}
+
+impl<T: ?Sized + PartialEq> PartialEq for AtomicRefCell<T> {
+    #[inline]
+    fn eq(&self, other: &AtomicRefCell<T>) -> bool {
+        *self.borrow() == *other.borrow()
+    }
+}
+
+impl<T: ?Sized + Eq> Eq for AtomicRefCell<T> {}
+
+impl<T: ?Sized + PartialOrd> PartialOrd for AtomicRefCell<T> {
+    #[inline]
+    fn partial_cmp(&self, other: &AtomicRefCell<T>) -> Option<cmp::Ordering> {
+        self.borrow().partial_cmp(&*other.borrow())
+    }
+}
+
+impl<T: ?Sized + Ord> Ord for AtomicRefCell<T> {
+    #[inline]
+    fn cmp(&self, other: &AtomicRefCell<T>) -> cmp::Ordering {
+        self.borrow().cmp(&*other.borrow())
+    }
+}
+
+impl<T> From<T> for AtomicRefCell<T> {
+    fn from(t: T) -> AtomicRefCell<T> {
+        AtomicRefCell::new(t)
+    }
+}
+
+impl<'b> Clone for AtomicBorrowRef<'b> {
+    #[inline]
+    fn clone(&self) -> AtomicBorrowRef<'b> {
+        AtomicBorrowRef::new(self.borrow)
+    }
+}
+
+/// A wrapper type for an immutably borrowed value from an `AtomicRefCell<T>`.
+pub struct AtomicRef<'b, T: ?Sized + 'b> {
+    value: &'b T,
+    borrow: AtomicBorrowRef<'b>,
+}
+
+
+impl<'b, T: ?Sized> Deref for AtomicRef<'b, T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        self.value
+    }
+}
+
+impl<'b, T: ?Sized> AtomicRef<'b, T> {
+    /// Copies an `AtomicRef`.
+    #[inline]
+    pub fn clone(orig: &AtomicRef<'b, T>) -> AtomicRef<'b, T> {
+        AtomicRef {
+            value: orig.value,
+            borrow: orig.borrow.clone(),
+        }
+    }
+
+    /// Make a new `AtomicRef` for a component of the borrowed data.
+    #[inline]
+    pub fn map<U: ?Sized, F>(orig: AtomicRef<'b, T>, f: F) -> AtomicRef<'b, U>
+        where F: FnOnce(&T) -> &U
+    {
+        AtomicRef {
+            value: f(orig.value),
+            borrow: orig.borrow,
+        }
+    }
+}
+
+impl<'b, T: ?Sized> AtomicRefMut<'b, T> {
+    /// Make a new `AtomicRefMut` for a component of the borrowed data, e.g. an enum
+    /// variant.
+    #[inline]
+    pub fn map<U: ?Sized, F>(orig: AtomicRefMut<'b, T>, f: F) -> AtomicRefMut<'b, U>
+        where F: FnOnce(&mut T) -> &mut U
+    {
+        AtomicRefMut {
+            value: f(orig.value),
+            borrow: orig.borrow,
+        }
+    }
+}
+
+/// A wrapper type for a mutably borrowed value from an `AtomicRefCell<T>`.
+pub struct AtomicRefMut<'b, T: ?Sized + 'b> {
+    value: &'b mut T,
+    borrow: AtomicBorrowRefMut<'b>,
+}
+
+impl<'b, T: ?Sized> Deref for AtomicRefMut<'b, T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        self.value
+    }
+}
+
+impl<'b, T: ?Sized> DerefMut for AtomicRefMut<'b, T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        self.value
+    }
+}
+
+impl<'b, T: ?Sized + Debug + 'b> Debug for AtomicRef<'b, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(f)
+     }
+}
+
+impl<'b, T: ?Sized + Debug + 'b> Debug for AtomicRefMut<'b, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(f)
+    }
+}