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use std::{
mem,
ops::Deref,
sync::atomic::{AtomicUsize, Ordering},
};
use crate::RefOverflow;
/// An immutable reference to data in a `Cell`.
///
/// Access the value via `std::ops::Deref` (e.g. `*val`)
#[derive(Debug)]
pub struct CellRef<'a, T>
where
T: ?Sized + 'a,
{
pub(crate) flag: &'a AtomicUsize,
pub(crate) value: &'a T,
}
/// Cast max `isize` as `usize`, so we don't have to do it in multiple places.
pub(crate) const REF_LIMIT_MAX: usize = isize::MAX as usize;
impl<'a, T> CellRef<'a, T>
where
T: ?Sized,
{
/// Returns a clone of this `CellRef`.
///
/// This method allows handling of reference overflows, but:
///
/// * Having 2 billion (32-bit system) / 9 quintillion (64-bit system)
/// references to an object is not a realistic scenario in most
/// applications.
///
/// * Applications that hold `CellRef`s with an ever-increasing reference
/// count are not supported by this library.
///
/// Reaching `isize::MAX` may be possible with
/// `std::mem::forget(CellRef::clone(&r))`.
pub fn try_clone(&self) -> Result<Self, RefOverflow> {
let previous_value = self.flag.fetch_add(1, Ordering::Relaxed);
let overflow = previous_value >= REF_LIMIT_MAX;
if unlikely(overflow) {
self.flag.fetch_sub(1, Ordering::Relaxed);
Err(RefOverflow)
} else {
Ok(CellRef {
flag: self.flag,
value: self.value,
})
}
}
/// Makes a new `CellRef` for a component of the borrowed data which
/// preserves the existing borrow.
///
/// The `Cell` is already immutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `CellRef::map(...)`. A method would interfere with methods of the
/// same name on the contents of a `CellRef` used through `Deref`.
/// Further this preserves the borrow of the value and hence does the
/// proper cleanup when it's dropped.
///
/// # Examples
///
/// This can be used to avoid pointer indirection when a boxed item is
/// stored in the `Cell`.
///
/// ```rust
/// use rt_ref::{Cell, CellRef};
///
/// let cb = Cell::new(Box::new(5));
///
/// // Borrowing the cell causes the `CellRef` to store a reference to the `Box`, which is a
/// // pointer to the value on the heap, not the actual value.
/// let boxed_ref: CellRef<'_, Box<usize>> = cb.borrow();
/// assert_eq!(**boxed_ref, 5); // Notice the double deref to get the actual value.
///
/// // By using `map` we can let `CellRef` store a reference directly to the value on the heap.
/// let pure_ref: CellRef<'_, usize> = CellRef::map(boxed_ref, Box::as_ref);
///
/// assert_eq!(*pure_ref, 5);
/// ```
///
/// We can also use `map` to get a reference to a sub-part of the borrowed
/// value.
///
/// ```rust
/// # use rt_ref::{Cell, CellRef};
///
/// let c = Cell::new((5, 'b'));
/// let b1: CellRef<'_, (u32, char)> = c.borrow();
/// let b2: CellRef<'_, u32> = CellRef::map(b1, |t| &t.0);
/// assert_eq!(*b2, 5);
/// ```
pub fn map<U, F>(self, f: F) -> CellRef<'a, U>
where
F: FnOnce(&T) -> &U,
U: ?Sized,
{
let flag = unsafe { &*(self.flag as *const _) };
let value = unsafe { &*(self.value as *const _) };
mem::forget(self);
CellRef {
flag,
value: f(value),
}
}
}
impl<'a, T> Deref for CellRef<'a, T>
where
T: ?Sized,
{
type Target = T;
fn deref(&self) -> &T {
self.value
}
}
impl<'a, T> Drop for CellRef<'a, T>
where
T: ?Sized,
{
fn drop(&mut self) {
self.flag.fetch_sub(1, Ordering::Release);
}
}
impl<'a, T> Clone for CellRef<'a, T>
where
T: ?Sized,
{
/// Returns a clone of this `CellRef`.
///
/// # Panics
///
/// Panics if the number of references is `isize::MAX`:
///
/// * Having 2 billion / 9 quintillion references to an object is not a
/// realistic scenario in most applications.
/// * Applications that hold `CellRef`s with an ever-increasing reference
/// count are not supported by this library.
///
/// Reaching `isize::MAX` may be possible with
/// `std::mem::forget(CellRef::clone(&r))`.
fn clone(&self) -> Self {
self.try_clone()
.unwrap_or_else(|e| panic!("Failed to clone `CellRef`: {e}"))
}
}
/// Trick to mimic `std::intrinsics::unlikely` on stable Rust.
#[cold]
#[inline(always)]
fn cold() {}
#[inline(always)]
fn unlikely(cond: bool) -> bool {
if cond {
cold();
}
cond
}
#[cfg(test)]
mod tests {
use std::{
error::Error,
sync::atomic::{AtomicUsize, Ordering},
};
use crate::RefOverflow;
use super::{CellRef, REF_LIMIT_MAX};
#[test]
fn try_clone_returns_ok_when_ref_count_less_than_isize_max() {
let flag = &AtomicUsize::new(1);
let value = &1u32;
let cell_ref = CellRef { flag, value };
assert_eq!(1, cell_ref.flag.load(Ordering::SeqCst));
let try_clone_result = cell_ref.try_clone();
let cloned = try_clone_result.expect("try_clone_result to be ok");
assert_eq!(2, cloned.flag.load(Ordering::SeqCst));
}
#[test]
fn try_clone_returns_err_when_ref_count_equals_isize_max() {
let flag = &AtomicUsize::new(REF_LIMIT_MAX);
let value = &1u32;
let cell_ref = CellRef { flag, value };
assert_eq!(REF_LIMIT_MAX, cell_ref.flag.load(Ordering::SeqCst));
let try_clone_result = cell_ref.try_clone();
let e = try_clone_result.expect_err("try_clone_result to be err");
assert_eq!(RefOverflow, e);
assert!(e.source().is_none());
// Ensure that the overflow is not persisted
assert_eq!(REF_LIMIT_MAX, cell_ref.flag.load(Ordering::SeqCst));
}
#[test]
fn clone_returns_cell_ref_when_ref_count_less_than_isize_max() {
let flag = &AtomicUsize::new(1);
let value = &1u32;
let cell_ref = CellRef { flag, value };
assert_eq!(1, cell_ref.flag.load(Ordering::SeqCst));
let cloned = cell_ref.clone();
assert_eq!(2, cell_ref.flag.load(Ordering::SeqCst));
assert_eq!(2, cloned.flag.load(Ordering::SeqCst));
}
#[test]
#[should_panic(expected = "Failed to clone `CellRef`: Ref count exceeded `isize::MAX`")]
fn clone_panics_when_ref_count_equals_isize_max() {
let flag = &AtomicUsize::new(REF_LIMIT_MAX);
let value = &1u32;
let cell_ref = CellRef { flag, value };
assert_eq!(REF_LIMIT_MAX, cell_ref.flag.load(Ordering::SeqCst));
let _clone = cell_ref.clone();
}
}