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//! Implements dynamically typed arenas, where any type of item can be allocated.
#![deny(missing_docs)]
use std::alloc::Layout;
use std::cell::RefCell;
use std::marker::PhantomData;
use std::mem;
use std::os::raw::c_void;
use std::ptr::{self, NonNull};
use bumpalo::Bump;
/// Marker trait that indicates whether or a `DynamicArena` may be sent across threads
pub trait SendAbility: Sized {
/// Create an arena corresponding to this type of thread-safety
fn create_arena<'a>() -> DynamicArena<'a, Self>;
}
/// Marker type that indicates you expect everything in the `DynamicArena` to be `Send`
///
/// Although this prevents you from allocating non-`Send` types in the arena,
/// it allows you to `Send` the dynamic itself arena across threads.
/// We can't safely implement `Send` for `DynamicArena` without this bound,
/// since you could otherwise place a `Rc` in the arena, send it across threads,
/// and then proceed to drop the arena and mutate the reference count.
pub struct Sendable {
_marker: (),
}
impl SendAbility for Sendable {
#[inline]
fn create_arena<'a>() -> DynamicArena<'a, Self> {
DynamicArena::new_send()
}
}
/// Marker type that indiates everything in the `DynamicArena` isn't nesicarrily `Send`.
///
/// This prevents you from `Send`ing the arena itself across threads,
/// as described in the `Sendable` docs.
pub struct NonSend {
_marker: std::rc::Rc<()>,
}
impl SendAbility for NonSend {
#[inline]
fn create_arena<'a>() -> DynamicArena<'a, Self> {
DynamicArena::new_bounded()
}
}
struct DynamicArenaItem {
drop: unsafe fn(*mut c_void),
value: *mut c_void,
}
impl Drop for DynamicArenaItem {
#[inline]
fn drop(&mut self) {
unsafe { (self.drop)(self.value) }
}
}
unsafe impl Send for DynamicArenaItem {}
/// An alias for an arena allocator which requires that everything is `Send + 'a`.
pub type DynamicSendArena<'a> = DynamicArena<'a, Sendable>;
/// An arena allocator where any type of object can be allocated.
///
/// Unlike typed arenas, any `Sized` object can be allocated here and they
/// don't all have to have the same statically known type in advance.
/// Usually you don't have to worry about the arena's lifetime,
/// since it should be static by default (see).
///
/// ## Performance
/// Although this is _slightly_ slower than a `typed_arena::Arena`,
/// it can be much more memory and time efficient than having a ton of seperate typed arenas.
/// The only point where dynamic dispatch actually gets involved is when the arena is dropped,
/// since we have to dynamically dispatch the drop functions instead of statically dispatching them.
///
/// ## Safety
/// In order to prevent use after free in a `DynamicArena`, all pointers in the allocated items
/// need to be valid for the lifetime `'a` to ensure all references outlive the arena itself.
/// Unfortunately, this statically prevents all self referential structs with `alloc`,
/// since they can't be known ahead of time to be safe and outlive the arena itself,
/// and we can't perform [dropchk](https://doc.rust-lang.org/nightly/nomicon/dropck.html)
/// on a dynamically typed arena (only statically typed ones).
///
/// The alternatives to this are `alloc_unchecked`, which bypasses the lifetime and safety,
/// and `alloc_copy`, which bypasses the lifetime by ensuring `T: Copy`.
/// This is safe, since a copyable item can never have a custom drop,
/// and the drop function could never possibly trigger use after free.
///
/// This means you can use self-referential structs with a `DynamicArena` as long as they implement `Copy`.
/// One way to make your types implement copy and support self-refrential structs,
/// is by replacing owned objects with their borrowed counterparts and then allocating them in the arena.
/// For example
/// ````
/// struct OwnedSelfReferential<'a> {
/// next: Option<&'a OwnedSelfReferential<'a>>,
/// text: String,
/// array: Vec<u32>
/// }
/// ````
/// can't be used with either `alloc` (since it's self referential),
/// nor `alloc_copy` (since `String` and `Vec` need to be dropped).
/// However by replacing `String` with `&'a str` and `&'a [u32]`,
/// we can make the structure `Copy` and enable use with `alloc_copy`.
///
/// Then, when someone needs to arena-allocate the struct they can use
/// the same arena to allocate the `String` and `Vec<u32>` first,
/// before they proceed to allocate the copyable struct.
pub struct DynamicArena<'a, S = NonSend> {
/// The underlying arena, where we request that they allocate arbitrary bytes.
handle: Bump,
/// The list of untyped values we've allocated in the arena,
/// and whose drop functions need to be invoked.
///
/// The drop functions are dynamically dispatched,
/// and each item could invoke completely different code for completely different types.
/// This is only needed for types that need to be dropped (as determined by `mem::needs_drop`),
/// and types that need need to be dropped don't need to be added.
items: RefCell<Vec<DynamicArenaItem>>,
/// This is the magic `PhantomData` combination to have proper lifetime invariance.
///
/// Otherwise the lifetime would be 'variant',
/// and borrow checking wouldn't properly enforce the bound for `alloc`.
/// This is enforced by the `compile-fail/invalid-drop-counted.rs`
marker: PhantomData<*mut &'a ()>,
send: PhantomData<S>,
}
impl DynamicArena<'static, NonSend> {
/// Create a new dynamic arena whose allocated items must outlive the `'static` lifetime,
/// and whose items aren't required to be `Send`.
///
/// Usually this is what you want,
/// since it's only a bound for the allocated items.
#[inline]
pub fn new() -> Self {
DynamicArena::new_bounded()
}
}
impl<'a, S> DynamicArena<'a, S> {
/// Create an arena with pre-allocated capacity for the specified number of items
/// and bytes.
///
/// NOTE: The "item" capacity excludes `Copy` references that
/// don't need to be dropped.
pub fn with_capacity(item_capacity: usize, byte_capacity: usize) -> Self {
DynamicArena {
handle: Bump::with_capacity(byte_capacity),
items: RefCell::new(Vec::with_capacity(item_capacity)),
marker: PhantomData,
send: PhantomData,
}
}
/// Allocate the specified value in this arena,
/// returning a reference which will be valid for the lifetime of the entire arena.
///
/// The bound on the item requires that `T: Copy`
/// to ensure there's no drop function that needs to be invoked.
#[inline]
#[allow(clippy::mut_from_ref)]
pub fn alloc_copy<T: Copy + Send>(&self, value: T) -> &mut T {
unsafe { self.alloc_unchecked(value) }
}
/// Allocate the specified value in this arena,
/// without calling its `Drop` function.
///
/// Since this doesn't call the 'Drop' impleentation,
/// this function leaks the underlying memory.
///
/// ## Safety
/// Technically, this function is safe to use.
/// However, it leaks memory unconditionally (without calling Drop).
#[inline]
#[allow(clippy::mut_from_ref)]
pub unsafe fn alloc_unchecked<T>(&self, value: T) -> &mut T {
let ptr = self.alloc_layout(Layout::new::<T>()).as_ptr().cast::<T>();
ptr.write(value);
&mut *ptr
}
/// Allocate space for an object with the specified layout
///
/// The returned pointer points at uninitialized memory
///
/// ## Safety
/// Technically, only the use of the memory is unsafe.
///
/// It would theoretically be possible to mark this function safe,
/// just like [Bump::alloc_layout].
#[inline]
pub unsafe fn alloc_layout(&self, layout: Layout) -> NonNull<u8> {
self.handle.alloc_layout(layout)
}
/// Dynamically drop the specified value,
/// invoking the drop function when the arena is dropped.
///
/// ## Safety
/// This assumes it's safe to drop the value at the same time the arena is dropped.
/// Not only are you assuming that [ptr::drop_in_place] would be safe,
/// you're also assuming that the drop function won't reference any dangling pointers,
/// and that [dropchk](https://doc.rust-lang.org/nomicon/dropck.html) would be successful.
///
/// Normally these invariants are statically checked by the `alloc` method,
/// which ensures that the memory is owned and all pointers
/// would be valid for the lifetime of the entire arena.
#[inline]
pub unsafe fn dynamic_drop<T>(&self, value: *mut T) {
if mem::needs_drop::<T>() {
self.items.borrow_mut().push(DynamicArenaItem {
drop: mem::transmute::<unsafe fn(*mut T), unsafe fn(*mut c_void)>(
ptr::drop_in_place::<T>,
),
value: value as *mut c_void,
})
}
}
/// Retrieve the underlying [bump allocator](bumpalo::Bump) for this arena
#[inline]
pub fn as_bumpalo(&self) -> &'_ bumpalo::Bump {
&self.handle
}
}
impl<'a> DynamicArena<'a, Sendable> {
/// Create a new empty arena, bounded by the inferred lifetime for this type `'a`
///
/// Since this arena has been marked `Sendable`,
/// all items in the arena need to implement `Send`.
pub fn new_send() -> Self {
DynamicArena {
handle: Bump::new(),
items: RefCell::new(Vec::new()),
marker: PhantomData,
send: PhantomData,
}
}
/// Allocate the specified value in this arena,
/// returning a reference which will be valid for the lifetime of the entire arena.
///
/// The bound on this item requires that `T: 'a`
/// to ensure the drop function is safe to invoke.
/// Additionally, since the arena is `Sendable`,
/// the bound on the item also requires that `T: Send`.
#[inline]
#[allow(clippy::mut_from_ref)]
pub fn alloc<T: Send + 'a>(&self, value: T) -> &mut T {
unsafe {
let target = self.alloc_unchecked(value);
self.dynamic_drop(target);
target
}
}
}
impl<'a> DynamicArena<'a, NonSend> {
/// Create a new empty arena, bounded by the inferred lifetime for this type `'a`
///
/// Since this arena has been marked `NonSend`,
/// the items in the arena don't necessarily need to implement `Send`.
pub fn new_bounded() -> Self {
DynamicArena {
handle: Bump::new(),
items: RefCell::new(Vec::new()),
marker: PhantomData,
send: PhantomData,
}
}
/// Allocate the specified value in this arena,
/// returning a reference which will be valid for the lifetime of the entire arena.
///
/// The bound on this item requires that `T: 'a`
/// to ensure the drop function is safe to invoke.
#[inline]
#[allow(clippy::mut_from_ref)]
pub fn alloc<T: 'a>(&self, value: T) -> &mut T {
unsafe {
let target = self.alloc_unchecked(value);
self.dynamic_drop(target);
target
}
}
}
impl<'a, S: SendAbility> Default for DynamicArena<'a, S> {
#[inline]
fn default() -> Self {
S::create_arena()
}
}
unsafe impl<'a, S: SendAbility + Send> Send for DynamicArena<'a, S> {}
impl<'a, S> Drop for DynamicArena<'a, S> {
#[inline]
fn drop(&mut self) {
// Items must be dropped before the arena
self.items.get_mut().clear();
}
}
#[cfg(test)]
mod test {
use super::*;
use std::cell::Cell;
const EXPECTED_DROP_COUNT: u32 = 4787;
const EXPECTED_DEPTHS: &[u32] = &[5, 27, 43];
struct DropCounted<'a>(&'a Cell<u32>);
impl<'a> Drop for DropCounted<'a> {
fn drop(&mut self) {
let old_count = self.0.get();
self.0.set(old_count + 1);
}
}
#[derive(Copy, Clone)]
pub struct SelfReferential<'a>(u32, Option<&'a SelfReferential<'a>>);
impl<'a> SelfReferential<'a> {
#[inline]
pub fn with_depth(arena: &'a DynamicArena, depth: u32) -> &'a Self {
arena.alloc_copy(match depth {
0 => SelfReferential(depth, None),
_ => SelfReferential(depth, Some(SelfReferential::with_depth(arena, depth - 1))),
})
}
#[inline]
pub fn depth(&self) -> u32 {
match self.1 {
Some(inner) => inner.depth() + 1,
None => 0,
}
}
}
#[test]
fn test_send() {
let arena = DynamicArena::new_send();
verify_copyable(do_copyable(&arena));
::std::thread::spawn(move || {
verify_copyable(do_copyable(&arena));
});
}
#[test]
fn copyable() {
let arena = DynamicArena::new();
for _ in 0..5 {
verify_copyable(do_copyable(&arena));
}
}
#[test]
fn self_referential() {
let arena = DynamicArena::new();
for _ in 0..5 {
verify_self_referential(do_self_referential(&arena));
}
}
#[test]
fn drop_counted() {
let cell = Box::new(Cell::new(0));
let arena = DynamicArena::new_bounded();
{
do_drop_counted(&arena, &cell);
assert_eq!(cell.get(), 0);
}
drop(arena);
assert_eq!(cell.get(), EXPECTED_DROP_COUNT);
}
#[test]
fn mixed() {
let cell = Cell::new(0);
let arena = DynamicArena::new_bounded();
{
do_drop_counted(&arena, &cell);
for _ in 0..5 {
verify_copyable(do_copyable(&arena));
verify_self_referential(do_self_referential(&arena));
}
assert_eq!(cell.get(), 0);
}
drop(arena);
assert_eq!(cell.get(), EXPECTED_DROP_COUNT);
}
fn do_copyable<'a, S>(arena: &'a DynamicArena<S>) -> Vec<&'a u32> {
let mut results = Vec::new();
for i in 0..10 {
results.push(&*arena.alloc_copy(i * 3));
}
results
}
fn verify_copyable(results: Vec<&u32>) {
for (actual, expected) in results.iter().zip(0..10) {
assert_eq!(**actual, expected * 3);
}
}
fn do_drop_counted<'a, 'd: 'a>(arena: &'a DynamicArena<'d>, counter: &'d Cell<u32>) {
for _ in 0..EXPECTED_DROP_COUNT {
arena.alloc(DropCounted(counter));
}
}
fn do_self_referential<'a>(arena: &'a DynamicArena) -> Vec<&'a SelfReferential<'a>> {
let mut result = Vec::new();
for &depth in EXPECTED_DEPTHS {
result.push(SelfReferential::with_depth(arena, depth));
}
result
}
fn verify_self_referential<'a>(results: Vec<&'a SelfReferential<'a>>) {
for (&actual, &depth) in results.iter().zip(EXPECTED_DEPTHS.iter()) {
assert_eq!(actual.depth(), depth);
}
}
}