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//! `std::boxed::Box`-like API on top of a lock-free memory pool
//!
//! # Example usage
//!
//! ```
//! use heapless::{box_pool, pool::boxed::{Box, BoxBlock}};
//!
//! box_pool!(P: u128);
//!
//! // cannot allocate without first giving memory blocks to the pool
//! assert!(P.alloc(42).is_err());
//!
//! // (some `no_std` runtimes have safe APIs to create `&'static mut` references)
//! let block: &'static mut BoxBlock<u128> = unsafe {
//! static mut B: BoxBlock <u128>= BoxBlock::new();
//! &mut B
//! };
//!
//! // give block of memory to the pool
//! P.manage(block);
//!
//! // it's now possible to allocate
//! let mut boxed = P.alloc(1).unwrap();
//!
//! // mutation is possible
//! *boxed += 1;
//! assert_eq!(2, *boxed);
//!
//! // number of boxes is limited to the number of blocks managed by the pool
//! let res = P.alloc(3);
//! assert!(res.is_err());
//!
//! // give another memory block to the pool
//! P.manage(unsafe {
//! static mut B: BoxBlock<u128> = BoxBlock::new();
//! &mut B
//! });
//!
//! // cloning also consumes a memory block from the pool
//! let mut separate_box = boxed.clone();
//! *separate_box += 1;
//! assert_eq!(3, *separate_box);
//!
//! // after the clone it's not possible to allocate again
//! let res = P.alloc(4);
//! assert!(res.is_err());
//!
//! // `boxed`'s destructor returns the memory block to the pool
//! drop(boxed);
//!
//! // it's possible to allocate again
//! let res = P.alloc(5);
//!
//! assert!(res.is_ok());
//! ```
//!
//! # Array block initialization
//!
//! You can create a static variable that contains an array of memory blocks and give all the blocks
//! to the `BoxPool`. This requires an intermediate `const` value as shown below:
//!
//! ```
//! use heapless::{box_pool, pool::boxed::BoxBlock};
//!
//! box_pool!(P: u128);
//!
//! const POOL_CAPACITY: usize = 8;
//!
//! let blocks: &'static mut [BoxBlock<u128>] = {
//! const BLOCK: BoxBlock<u128> = BoxBlock::new(); // <=
//! static mut BLOCKS: [BoxBlock<u128>; POOL_CAPACITY] = [BLOCK; POOL_CAPACITY];
//! unsafe { &mut BLOCKS }
//! };
//!
//! for block in blocks {
//! P.manage(block);
//! }
//! ```
use core::{
fmt,
hash::{Hash, Hasher},
mem::{ManuallyDrop, MaybeUninit},
ops, ptr,
};
use stable_deref_trait::StableDeref;
use super::treiber::{NonNullPtr, Stack, UnionNode};
/// Creates a new `BoxPool` singleton with the given `$name` that manages the specified `$data_type`
///
/// For more extensive documentation see the [module level documentation](crate::pool::boxed)
#[macro_export]
macro_rules! box_pool {
($name:ident: $data_type:ty) => {
pub struct $name;
impl $crate::pool::boxed::BoxPool for $name {
type Data = $data_type;
fn singleton() -> &'static $crate::pool::boxed::BoxPoolImpl<$data_type> {
static $name: $crate::pool::boxed::BoxPoolImpl<$data_type> =
$crate::pool::boxed::BoxPoolImpl::new();
&$name
}
}
impl $name {
/// Inherent method version of `BoxPool::alloc`
#[allow(dead_code)]
pub fn alloc(
&self,
value: $data_type,
) -> Result<$crate::pool::boxed::Box<$name>, $data_type> {
<$name as $crate::pool::boxed::BoxPool>::alloc(value)
}
/// Inherent method version of `BoxPool::manage`
#[allow(dead_code)]
pub fn manage(&self, block: &'static mut $crate::pool::boxed::BoxBlock<$data_type>) {
<$name as $crate::pool::boxed::BoxPool>::manage(block)
}
}
};
}
/// A singleton that manages `pool::boxed::Box`-es
///
/// # Usage
///
/// Do not implement this trait yourself; instead use the `box_pool!` macro to create a type that
/// implements this trait.
///
/// # Semver guarantees
///
/// *Implementing* this trait is exempt from semver guarantees.
/// i.e. a new patch release is allowed to break downstream `BoxPool` implementations.
///
/// *Using* the trait, e.g. in generic code, does fall under semver guarantees.
pub trait BoxPool: Sized {
/// The data type managed by the memory pool
type Data: 'static;
/// `box_pool!` implementation detail
#[doc(hidden)]
fn singleton() -> &'static BoxPoolImpl<Self::Data>;
/// Allocate a new `Box` initialized to the given `value`
///
/// `manage` should be called at least once before calling `alloc`
///
/// # Errors
///
/// The `Err`or variant is returned when the memory pool has run out of memory blocks
fn alloc(value: Self::Data) -> Result<Box<Self>, Self::Data> {
Ok(Box {
node_ptr: Self::singleton().alloc(value)?,
})
}
/// Add a statically allocated memory block to the memory pool
fn manage(block: &'static mut BoxBlock<Self::Data>) {
Self::singleton().manage(block)
}
}
/// Like `std::boxed::Box` but managed by memory pool `P` rather than `#[global_allocator]`
pub struct Box<P>
where
P: BoxPool,
{
node_ptr: NonNullPtr<UnionNode<MaybeUninit<P::Data>>>,
}
impl<A> Clone for Box<A>
where
A: BoxPool,
A::Data: Clone,
{
fn clone(&self) -> Self {
A::alloc((**self).clone()).ok().expect("OOM")
}
}
impl<A> fmt::Debug for Box<A>
where
A: BoxPool,
A::Data: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
A::Data::fmt(self, f)
}
}
impl<P> ops::Deref for Box<P>
where
P: BoxPool,
{
type Target = P::Data;
fn deref(&self) -> &Self::Target {
unsafe { &*self.node_ptr.as_ptr().cast::<P::Data>() }
}
}
impl<P> ops::DerefMut for Box<P>
where
P: BoxPool,
{
fn deref_mut(&mut self) -> &mut Self::Target {
unsafe { &mut *self.node_ptr.as_ptr().cast::<P::Data>() }
}
}
unsafe impl<P> StableDeref for Box<P> where P: BoxPool {}
impl<A> fmt::Display for Box<A>
where
A: BoxPool,
A::Data: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
A::Data::fmt(self, f)
}
}
impl<P> Drop for Box<P>
where
P: BoxPool,
{
fn drop(&mut self) {
let node = self.node_ptr;
unsafe { ptr::drop_in_place(node.as_ptr().cast::<P::Data>()) }
unsafe { P::singleton().stack.push(node) }
}
}
impl<A> Eq for Box<A>
where
A: BoxPool,
A::Data: Eq,
{
}
impl<A> Hash for Box<A>
where
A: BoxPool,
A::Data: Hash,
{
fn hash<H>(&self, state: &mut H)
where
H: Hasher,
{
(**self).hash(state)
}
}
impl<A> Ord for Box<A>
where
A: BoxPool,
A::Data: Ord,
{
fn cmp(&self, other: &Self) -> core::cmp::Ordering {
A::Data::cmp(self, other)
}
}
impl<A, B> PartialEq<Box<B>> for Box<A>
where
A: BoxPool,
B: BoxPool,
A::Data: PartialEq<B::Data>,
{
fn eq(&self, other: &Box<B>) -> bool {
A::Data::eq(self, other)
}
}
impl<A, B> PartialOrd<Box<B>> for Box<A>
where
A: BoxPool,
B: BoxPool,
A::Data: PartialOrd<B::Data>,
{
fn partial_cmp(&self, other: &Box<B>) -> Option<core::cmp::Ordering> {
A::Data::partial_cmp(self, other)
}
}
unsafe impl<P> Send for Box<P>
where
P: BoxPool,
P::Data: Send,
{
}
unsafe impl<P> Sync for Box<P>
where
P: BoxPool,
P::Data: Sync,
{
}
/// `box_pool!` implementation detail
// newtype to avoid having to make field types public
#[doc(hidden)]
pub struct BoxPoolImpl<T> {
stack: Stack<UnionNode<MaybeUninit<T>>>,
}
impl<T> BoxPoolImpl<T> {
pub const fn new() -> Self {
Self {
stack: Stack::new(),
}
}
fn alloc(&self, value: T) -> Result<NonNullPtr<UnionNode<MaybeUninit<T>>>, T> {
if let Some(node_ptr) = self.stack.try_pop() {
unsafe { node_ptr.as_ptr().cast::<T>().write(value) }
Ok(node_ptr)
} else {
Err(value)
}
}
fn manage(&self, block: &'static mut BoxBlock<T>) {
let node: &'static mut _ = &mut block.node;
unsafe { self.stack.push(NonNullPtr::from_static_mut_ref(node)) }
}
}
unsafe impl<T> Sync for BoxPoolImpl<T> {}
/// A chunk of memory that a `BoxPool` singleton can manage
pub struct BoxBlock<T> {
node: UnionNode<MaybeUninit<T>>,
}
impl<T> BoxBlock<T> {
/// Creates a new memory block
pub const fn new() -> Self {
Self {
node: UnionNode {
data: ManuallyDrop::new(MaybeUninit::uninit()),
},
}
}
}
#[cfg(test)]
mod tests {
use core::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::thread;
use super::*;
#[test]
fn cannot_alloc_if_empty() {
box_pool!(P: i32);
assert_eq!(Err(42), P.alloc(42));
}
#[test]
fn can_alloc_if_pool_manages_one_block() {
box_pool!(P: i32);
let block = unsafe {
static mut B: BoxBlock<i32> = BoxBlock::new();
&mut B
};
P.manage(block);
assert_eq!(42, *P.alloc(42).unwrap());
}
#[test]
fn alloc_drop_alloc() {
box_pool!(P: i32);
let block = unsafe {
static mut B: BoxBlock<i32> = BoxBlock::new();
&mut B
};
P.manage(block);
let boxed = P.alloc(1).unwrap();
drop(boxed);
assert_eq!(2, *P.alloc(2).unwrap());
}
#[test]
fn runs_destructor_exactly_once_on_drop() {
static COUNT: AtomicUsize = AtomicUsize::new(0);
pub struct S;
impl Drop for S {
fn drop(&mut self) {
COUNT.fetch_add(1, Ordering::Relaxed);
}
}
box_pool!(P: S);
let block = unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
};
P.manage(block);
let boxed = P.alloc(S).ok().unwrap();
assert_eq!(0, COUNT.load(Ordering::Relaxed));
drop(boxed);
assert_eq!(1, COUNT.load(Ordering::Relaxed));
}
#[test]
fn zst_is_well_aligned() {
#[repr(align(4096))]
pub struct Zst4096;
box_pool!(P: Zst4096);
let block = unsafe {
static mut B: BoxBlock<Zst4096> = BoxBlock::new();
&mut B
};
P.manage(block);
let boxed = P.alloc(Zst4096).ok().unwrap();
let raw = &*boxed as *const Zst4096;
assert_eq!(0, raw as usize % 4096);
}
#[allow(clippy::redundant_clone)]
#[test]
fn can_clone_if_pool_is_not_exhausted() {
static STRUCT_CLONE_WAS_CALLED: AtomicBool = AtomicBool::new(false);
pub struct S;
impl Clone for S {
fn clone(&self) -> Self {
STRUCT_CLONE_WAS_CALLED.store(true, Ordering::Relaxed);
Self
}
}
box_pool!(P: S);
P.manage(unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
});
P.manage(unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
});
let first = P.alloc(S).ok().unwrap();
let _second = first.clone();
assert!(STRUCT_CLONE_WAS_CALLED.load(Ordering::Relaxed));
let is_oom = P.alloc(S).is_err();
assert!(is_oom);
}
#[allow(clippy::redundant_clone)]
#[test]
fn clone_panics_if_pool_exhausted() {
static STRUCT_CLONE_WAS_CALLED: AtomicBool = AtomicBool::new(false);
pub struct S;
impl Clone for S {
fn clone(&self) -> Self {
STRUCT_CLONE_WAS_CALLED.store(true, Ordering::Relaxed);
Self
}
}
box_pool!(P: S);
P.manage(unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
});
let first = P.alloc(S).ok().unwrap();
let thread = thread::spawn(move || {
let _second = first.clone();
});
let thread_panicked = thread.join().is_err();
assert!(thread_panicked);
// we diverge from `alloc::Box<T>` in that we call `T::clone` first and then request
// memory from the allocator whereas `alloc::Box<T>` does it the other way around
// assert!(!STRUCT_CLONE_WAS_CALLED.load(Ordering::Relaxed));
}
#[allow(clippy::redundant_clone)]
#[test]
fn panicking_clone_does_not_leak_memory() {
static STRUCT_CLONE_WAS_CALLED: AtomicBool = AtomicBool::new(false);
pub struct S;
impl Clone for S {
fn clone(&self) -> Self {
STRUCT_CLONE_WAS_CALLED.store(true, Ordering::Relaxed);
panic!()
}
}
box_pool!(P: S);
P.manage(unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
});
P.manage(unsafe {
static mut B: BoxBlock<S> = BoxBlock::new();
&mut B
});
let boxed = P.alloc(S).ok().unwrap();
let thread = thread::spawn(move || {
let _boxed = boxed.clone();
});
let thread_panicked = thread.join().is_err();
assert!(thread_panicked);
assert!(STRUCT_CLONE_WAS_CALLED.load(Ordering::Relaxed));
let once = P.alloc(S);
let twice = P.alloc(S);
assert!(once.is_ok());
assert!(twice.is_ok());
}
}