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/*
* Copyright (C) 2021-2022 taylor.fish <contact@taylor.fish>
*
* This file is part of fixed-typed-arena.
*
* fixed-typed-arena is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* fixed-typed-arena is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with fixed-typed-arena. If not, see <https://www.gnu.org/licenses/>.
*/
//! An arena that returns references with arbitrary lifetimes.
use super::chunk::ChunkRef;
use super::options::{ChunkSizePriv, SupportsPositionsPriv};
use super::ArenaOptions;
use alloc::alloc::handle_alloc_error;
use alloc::boxed::Box;
use alloc::sync::Arc;
use core::fmt::{Debug, Display};
use core::hint::unreachable_unchecked;
use core::marker::PhantomData;
use core::mem;
use core::ptr::{self, NonNull};
use integral_constant::Bool;
pub(crate) mod iter;
use iter::{IntoIter, Iter, IterMut, IterPtr, Position};
type Array<T, Options> =
<<Options as ArenaOptions<T>>::ChunkSize as ChunkSizePriv<T>>::Array;
type SupportsPositions<T, Options> =
<Options as ArenaOptions<T>>::SupportsPositions;
type ArenaRc<T, Options> =
<SupportsPositions<T, Options> as SupportsPositionsPriv>::Rc;
type ArenaChunk<T, Options> = ChunkRef<T, Array<T, Options>>;
/// Checks whether `old` and `new` point to the same allocation (see
/// [`Arc::ptr_eq`]), but allows `old` to be [`None`], even if `new` is
/// [`Some`].
fn valid_rc_update<T>(old: &Option<Arc<T>>, new: &Option<Arc<T>>) -> bool {
match (old, new) {
(Some(old), Some(new)) => Arc::ptr_eq(old, new),
(Some(_old), None) => false,
(None, _new) => true,
}
}
// Invariants:
//
// * Every chunk except for `tail` must be full (all items initialized).
// * If `tail_len` is less than `Self::CHUNK_SIZE`, `tail` is `Some`.
// * If `tail` is `Some`, it contains at least one item (`tail_len > 0`).
// * If `tail` is `Some`, the items in `tail` up to index `tail_len`
// (exclusive) are initialized.
//
/// Like [`Arena`], but returns references of any lifetime, including
/// `'static`.
///
/// This lets the arena be used without being borrowed, but it comes with the
/// tradeoff that the arena leaks memory unless the unsafe [`drop`](Self::drop)
/// method is called.
///
/// [`Arena`]: super::arena::Arena
pub struct ManuallyDropArena<T, Options: ArenaOptions<T> = super::Options> {
rc: Option<ArenaRc<T, Options>>,
head: Option<ArenaChunk<T, Options>>,
tail: Option<ArenaChunk<T, Options>>,
tail_len: usize,
len: usize,
/// Lets dropck know that `T` may be dropped.
phantom: PhantomData<Box<T>>,
}
impl<T, Options: ArenaOptions<T>> Default for ManuallyDropArena<T, Options> {
fn default() -> Self {
Self::new()
}
}
impl<T, Options: ArenaOptions<T>> ManuallyDropArena<T, Options> {
const CHUNK_SIZE: usize = ArenaChunk::<T, Options>::CAPACITY;
/// Creates a new [`ManuallyDropArena`].
pub fn new() -> Self {
Self {
rc: None,
head: None,
tail: None,
tail_len: Self::CHUNK_SIZE,
len: 0,
phantom: PhantomData,
}
}
fn ensure_free_space(&mut self) -> Result<(), impl Debug + Display> {
assert!(
Self::CHUNK_SIZE > 0,
"cannot allocate items when chunk size is 0",
);
if self.tail_len < Self::CHUNK_SIZE {
// `self.tail` cannot be `None`. The only time `self.tail` is
// `None` is after calling `Self::new`, which also sets
// `self.tail_len` to `Self::CHUNK_SIZE`.
return Ok(());
}
let chunk = if let Some(chunk) = ChunkRef::new(self.tail.take()) {
chunk
} else {
return Err("could not allocate chunk");
};
self.head.get_or_insert_with(|| chunk.clone());
self.tail = Some(chunk);
self.tail_len = 0;
Ok(())
}
fn alloc_ptr(&mut self, value: T) -> NonNull<T> {
self.try_alloc_ptr(value).unwrap_or_else(|| {
handle_alloc_error(ArenaChunk::<T, Options>::LAYOUT);
})
}
fn try_alloc_ptr(&mut self, value: T) -> Option<NonNull<T>> {
self.ensure_free_space().ok()?;
SupportsPositions::<T, Options>::init_rc(&mut self.rc);
let chunk = self.tail.as_mut().unwrap_or_else(|| {
// SAFETY: `Self::ensure_free_space` ensures that `self.tail`
// is not `None`.
unsafe { unreachable_unchecked() }
});
// SAFETY: `Self::ensure_free_space` ensures that `self.tail_len` is
// less than the chunk size.
let item = unsafe { chunk.get(self.tail_len) };
// SAFETY: `ChunkRef::get` returns valid, properly aligned pointers.
unsafe {
item.as_ptr().write(value);
}
self.tail_len += 1;
self.len += 1;
Some(item)
}
/// Drops the contents of the arena. The arena will leak memory when
/// dropped unless this method is called.
///
/// # Safety
///
/// You must ensure that no references to items (or parts of items) in the
/// arena exist when calling this method, except possibly for references
/// within the items themselves.
///
/// However, if there are references to other items (or parts of items)
/// within the items themselves, at least one of the following must be
/// true:
///
/// * `T` does not have a custom [`Drop`] impl.
/// * `T`'s [`Drop`] impl does not directly or indirectly access any data
/// via the references to other items or parts of items. (This is
/// essentially the requirement imposed by [`#[may_dangle]`][dropck].)
///
/// Additionally, there must be no instances of [`Iter`] or [`IterMut`]
/// for this arena.
///
/// [dropck]: https://doc.rust-lang.org/nomicon/dropck.html
pub unsafe fn drop(&mut self) {
let mut head = if let Some(head) = self.head.take() {
head
} else {
return;
};
self.tail = None;
let tail_len = mem::replace(&mut self.tail_len, Self::CHUNK_SIZE);
self.len = 0;
self.rc = None;
// Drop the items in all chunks except the last.
while let Some(next) = head.next() {
// SAFETY: All chunks except for the tail are guaranteed to
// be full (all items initialized). We know this isn't the
// tail chunk because `head.next()` is not `None`.
unsafe {
head.drop_all();
}
// SAFETY: No clones of this `ChunkRef` exist. `self.head`
// and `self.tail` are both `None`, and the chunks form a
// singly linked list. Caller guarantees no iterators exist.
unsafe {
head.dealloc();
}
head = next;
}
// `head` is now the tail chunk; drop its items.
for i in 0..tail_len {
// SAFETY: The items in the tail chunk (when not `None`) at
// indices up to `self.tail_len` are always initialized.
unsafe {
head.drop_item(i);
}
}
// SAFETY: No clones of this `ChunkRef` exist for the same
// reasons as the other chunks above.
unsafe {
head.dealloc();
}
}
/// Alias of [`Self::drop`]. Can be used to prevent name collisions when
/// this arena is stored in a [`Deref`](core::ops::Deref) type:
///
/// ```
/// # use fixed_typed_arena::ManuallyDropArena;
/// let mut arena = Box::new(ManuallyDropArena::<u8, 8>::new());
/// //unsafe { arena.drop() }; // Compile error: resolves to `Drop::drop`
/// unsafe { arena.manually_drop() }; // Works as expected
/// ```
///
/// # Safety
///
/// Same requirements as [`Self::drop`].
pub unsafe fn manually_drop(&mut self) {
// SAFETY: Checked by caller.
unsafe {
self.drop();
}
}
/// Returns the total number of items that have been allocated.
pub fn len(&self) -> usize {
self.len
}
/// Checks whether the arena is empty.
pub fn is_empty(&self) -> bool {
self.len == 0
}
/// Allocates a new item in the arena and initializes it with `value`.
/// Returns a reference to the allocated item. The reference can have any
/// lifetime, including `'static`, as long as `T` outlives that lifetime.
///
/// This method calls [`handle_alloc_error`] if memory allocation fails;
/// for a version that returns [`None`], see [`Self::try_alloc`].
///
/// [`handle_alloc_error`]: alloc::alloc::handle_alloc_error
pub fn alloc<'a>(&mut self, value: T) -> &'a mut T
where
Options: 'a + ArenaOptions<T, Mutable = Bool<true>>,
{
// SAFETY: `Self::alloc_ptr` returns initialized, properly aligned
// pointers, and we can return a reference with an arbitrary lifetime
// because the arena must be manually dropped.
unsafe { self.alloc_ptr(value).as_mut() }
}
/// Like [`Self::alloc`], but returns [`None`] if memory allocation fails.
pub fn try_alloc<'a>(&mut self, value: T) -> Option<&'a mut T>
where
Options: 'a + ArenaOptions<T, Mutable = Bool<true>>,
{
// SAFETY: See `Self::alloc`.
Some(unsafe { self.try_alloc_ptr(value)?.as_mut() })
}
/// Allocates a new item in the arena and initializes it with `value`.
/// Returns a shared/immutable reference to the allocated item. The
/// reference can have any lifetime, including `'static`, as long as `T`
/// outlives that lifetime.
///
/// This method calls [`handle_alloc_error`] if memory allocation fails;
/// for a version that returns [`None`], see [`Self::try_alloc`].
///
/// [`handle_alloc_error`]: alloc::alloc::handle_alloc_error
pub fn alloc_shared<'a>(&mut self, value: T) -> &'a T
where
Options: 'a,
{
// SAFETY: See `Self::alloc`.
unsafe { self.alloc_ptr(value).as_ref() }
}
/// Like [`Self::alloc_shared`], but returns [`None`] if memory allocation
/// fails.
pub fn try_alloc_shared<'a>(&mut self, value: T) -> Option<&'a T>
where
Options: 'a,
{
// SAFETY: See `Self::alloc`.
Some(unsafe { self.try_alloc_ptr(value)?.as_ref() })
}
fn end(&self) -> *const T {
self.tail.as_ref().map_or(ptr::null(), |c| {
// SAFETY: `self.tail_len` is necessarily less than or equal to
// the chunk capacity.
unsafe { c.get(self.tail_len) }.as_ptr()
})
}
fn iter_ptr<const DROP: bool>(&self) -> IterPtr<T, Options, DROP> {
IterPtr {
chunk: self.head.clone(),
index: 0,
end: self.end(),
rc: self.rc.clone(),
phantom: PhantomData,
}
}
/// Returns an iterator over the items in this arena.
pub fn iter<'a>(&self) -> Iter<'a, T, Options>
where
Options: ArenaOptions<T, Mutable = Bool<false>>,
{
unsafe { self.iter_unchecked() }
}
/// Returns an iterator over the items in this arena.
///
/// # Safety
///
/// There must be no mutable references (or references derived from mutable
/// references) to items (or parts of items) in this arena or instances of
/// [`IterMut`] for this arena.
pub unsafe fn iter_unchecked<'a>(&self) -> Iter<'a, T, Options> {
Iter {
inner: self.iter_ptr::<false>(),
phantom: PhantomData,
}
}
/// Returns a mutable iterator over the items in this arena.
///
/// # Safety
///
/// There must be no references to items (or parts of items) in this arena
/// or instances of [`Iter`] or [`IterMut`] for this arena.
pub unsafe fn iter_mut_unchecked<'a>(
&mut self,
) -> IterMut<'a, T, Options> {
IterMut {
inner: self.iter_ptr::<false>(),
phantom: PhantomData,
}
}
/// Returns an owning iterator over the items in this arena.
///
/// # Safety
///
/// There must be no references to items (or parts of items) in this arena
/// or instances of [`Iter`] or [`IterMut`] for this arena.
pub unsafe fn into_iter_unchecked(self) -> IntoIter<T, Options> {
IntoIter(self.iter_ptr())
}
}
impl<T, Options> ManuallyDropArena<T, Options>
where
Options: ArenaOptions<T, SupportsPositions = Bool<true>>,
{
fn iter_ptr_at(&self, position: &Position) -> IterPtr<T, Options> {
assert!(
valid_rc_update(&position.rc, &self.rc),
"`position` is not part of this arena",
);
// SAFETY: Checking the pointer equality of `self.rc` and `position.rc`
// above ensures `position` belongs to this arena. Note that if
// `position.rc` is `None`, it may have come from a different arena,
// but this is okay, because in this case `position` does not contain a
// pointer that we dereference.
let chunk = position.chunk.map(|p| unsafe { ChunkRef::from_ptr(p) });
IterPtr {
chunk: chunk.or_else(|| self.head.clone()),
index: position.index,
end: self.end(),
rc: self.rc.clone(),
phantom: PhantomData,
}
}
/// Returns an iterator starting at the specified position.
///
/// # Panics
///
/// May panic if `position` does not refer to a position in this arena.
pub fn iter_at<'a>(&self, position: &Position) -> Iter<'a, T, Options>
where
Options: ArenaOptions<T, Mutable = Bool<false>>,
{
unsafe { self.iter_at_unchecked(position) }
}
/// Returns an iterator starting at the specified position.
///
/// # Panics
///
/// May panic if `position` does not refer to a position in this arena.
///
/// # Safety
///
/// Same requirements as [`Self::iter_unchecked`].
pub unsafe fn iter_at_unchecked<'a>(
&self,
position: &Position,
) -> Iter<'a, T, Options> {
Iter {
inner: self.iter_ptr_at(position),
phantom: PhantomData,
}
}
/// Returns a mutable iterator starting at the specified position.
///
/// # Panics
///
/// May panic if `position` does not refer to a position in this arena.
///
/// # Safety
///
/// Same requirements as [`Self::iter_mut_unchecked`].
pub unsafe fn iter_mut_at_unchecked<'a>(
&mut self,
position: &Position,
) -> IterMut<'a, T, Options> {
IterMut {
inner: self.iter_ptr_at(position),
phantom: PhantomData,
}
}
}
// SAFETY: `ManuallyDropArena` owns its items and provides access to them using
// standard borrow rules, so it can be `Sync` as long as `T` is `Sync`.
unsafe impl<T, Options> Sync for ManuallyDropArena<T, Options>
where
T: Sync,
Options: ArenaOptions<T>,
{
}
// SAFETY: `ManuallyDropArena` owns its items, so it can be `Send` as long
// as `T` is both `Send` and `Sync`. `T` must be `Sync` because the life of
// the iterators `ManuallyDropArena` provides (e.g., from `Self::iter`) is
// unbounded, so a caller could obtain such an iterator and then move the arena
// to another thread while still possessing the iterator.
unsafe impl<T, Options> Send for ManuallyDropArena<T, Options>
where
T: Send + Sync,
Options: ArenaOptions<T>,
{
}