stable_alloc_shim/

core_alloc.rs

use core::alloc::Layout;
use core::fmt;
use core::ptr;
use core::ptr::NonNull;

use crate::nonnull_as_mut_ptr;
use crate::nonnull_len;

/// The `AllocError` error indicates an allocation failure
/// that may be due to resource exhaustion or to
/// something wrong when combining the given input arguments with this
/// allocator.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub struct AllocError;

// (we need this for downstream impl of trait Error)
impl fmt::Display for AllocError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("memory allocation failed")
    }
}

/// An implementation of `Allocator` can allocate, grow, shrink, and deallocate arbitrary blocks of
/// data described via [`Layout`][].
///
/// `Allocator` is designed to be implemented on ZSTs, references, or smart pointers because having
/// an allocator like `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
/// allocated memory.
///
/// Unlike [`GlobalAlloc`][], zero-sized allocations are allowed in `Allocator`. If an underlying
/// allocator does not support this (like jemalloc) or return a null pointer (such as
/// `libc::malloc`), this must be caught by the implementation.
///
/// ### Currently allocated memory
///
/// Some of the methods require that a memory block be *currently allocated* via an allocator. This
/// means that:
///
/// * the starting address for that memory block was previously returned by [`allocate`], [`grow`], or
///   [`shrink`], and
///
/// * the memory block has not been subsequently deallocated, where blocks are either deallocated
///   directly by being passed to [`deallocate`] or were changed by being passed to [`grow`] or
///   [`shrink`] that returns `Ok`. If `grow` or `shrink` have returned `Err`, the passed pointer
///   remains valid.
///
/// [`allocate`]: Allocator::allocate
/// [`grow`]: Allocator::grow
/// [`shrink`]: Allocator::shrink
/// [`deallocate`]: Allocator::deallocate
///
/// ### Memory fitting
///
/// Some of the methods require that a layout *fit* a memory block. What it means for a layout to
/// "fit" a memory block means (or equivalently, for a memory block to "fit" a layout) is that the
/// following conditions must hold:
///
/// * The block must be allocated with the same alignment as [`layout.align()`], and
///
/// * The provided [`layout.size()`] must fall in the range `min ..= max`, where:
///   - `min` is the size of the layout most recently used to allocate the block, and
///   - `max` is the latest actual size returned from [`allocate`], [`grow`], or [`shrink`].
///
/// [`layout.align()`]: Layout::align
/// [`layout.size()`]: Layout::size
///
/// # Safety
///
/// * Memory blocks returned from an allocator must point to valid memory and retain their validity
///   until the instance and all of its clones are dropped,
///
/// * cloning or moving the allocator must not invalidate memory blocks returned from this
///   allocator. A cloned allocator must behave like the same allocator, and
///
/// * any pointer to a memory block which is [*currently allocated*] may be passed to any other
///   method of the allocator.
///
/// [*currently allocated*]: #currently-allocated-memory
pub unsafe trait Allocator {
    /// Attempts to allocate a block of memory.
    ///
    /// On success, returns a [`NonNull<[u8]>`][NonNull] meeting the size and alignment guarantees of `layout`.
    ///
    /// The returned block may have a larger size than specified by `layout.size()`, and may or may
    /// not have its contents initialized.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
    /// allocator's size or alignment constraints.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
    /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
    /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to
    /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>;

    /// Behaves like `allocate`, but also ensures that the returned memory is zero-initialized.
    ///
    /// # Errors
    ///
    /// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
    /// allocator's size or alignment constraints.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
    /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
    /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to
    /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        let ptr = self.allocate(layout)?;
        // SAFETY: `alloc` returns a valid memory block
        unsafe { nonnull_as_mut_ptr(ptr).write_bytes(0, nonnull_len(ptr)) }
        Ok(ptr)
    }

    /// Deallocates the memory referenced by `ptr`.
    ///
    /// # Safety
    ///
    /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator, and
    /// * `layout` must [*fit*] that block of memory.
    ///
    /// [*currently allocated*]: #currently-allocated-memory
    /// [*fit*]: #memory-fitting
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout);

    /// Attempts to extend the memory block.
    ///
    /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
    /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
    /// this, the allocator may extend the allocation referenced by `ptr` to fit the new layout.
    ///
    /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
    /// transferred to this allocator. The memory may or may not have been freed, and should be
    /// considered unusable.
    ///
    /// If this method returns `Err`, then ownership of the memory block has not been transferred to
    /// this allocator, and the contents of the memory block are unaltered.
    ///
    /// # Safety
    ///
    /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
    /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
    /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
    ///
    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
    ///
    /// [*currently allocated*]: #currently-allocated-memory
    /// [*fit*]: #memory-fitting
    ///
    /// # Errors
    ///
    /// Returns `Err` if the new layout does not meet the allocator's size and alignment
    /// constraints of the allocator, or if growing otherwise fails.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
    /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
    /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to
    /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        debug_assert!(
            new_layout.size() >= old_layout.size(),
            "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
        );

        let new_ptr = self.allocate(new_layout)?;

        // SAFETY: because `new_layout.size()` must be greater than or equal to
        // `old_layout.size()`, both the old and new memory allocation are valid for reads and
        // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
        // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
        // safe. The safety contract for `dealloc` must be upheld by the caller.
        unsafe {
            ptr::copy_nonoverlapping(ptr.as_ptr(), nonnull_as_mut_ptr(new_ptr), old_layout.size());
            self.deallocate(ptr, old_layout);
        }

        Ok(new_ptr)
    }

    /// Behaves like `grow`, but also ensures that the new contents are set to zero before being
    /// returned.
    ///
    /// The memory block will contain the following contents after a successful call to
    /// `grow_zeroed`:
    ///   * Bytes `0..old_layout.size()` are preserved from the original allocation.
    ///   * Bytes `old_layout.size()..old_size` will either be preserved or zeroed, depending on
    ///     the allocator implementation. `old_size` refers to the size of the memory block prior
    ///     to the `grow_zeroed` call, which may be larger than the size that was originally
    ///     requested when it was allocated.
    ///   * Bytes `old_size..new_size` are zeroed. `new_size` refers to the size of the memory
    ///     block returned by the `grow_zeroed` call.
    ///
    /// # Safety
    ///
    /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
    /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
    /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
    ///
    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
    ///
    /// [*currently allocated*]: #currently-allocated-memory
    /// [*fit*]: #memory-fitting
    ///
    /// # Errors
    ///
    /// Returns `Err` if the new layout does not meet the allocator's size and alignment
    /// constraints of the allocator, or if growing otherwise fails.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
    /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
    /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to
    /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        debug_assert!(
            new_layout.size() >= old_layout.size(),
            "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
        );

        let new_ptr = self.allocate_zeroed(new_layout)?;

        // SAFETY: because `new_layout.size()` must be greater than or equal to
        // `old_layout.size()`, both the old and new memory allocation are valid for reads and
        // writes for `old_layout.size()` bytes. Also, because the old allocation wasn't yet
        // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
        // safe. The safety contract for `dealloc` must be upheld by the caller.
        unsafe {
            ptr::copy_nonoverlapping(ptr.as_ptr(), nonnull_as_mut_ptr(new_ptr), old_layout.size());
            self.deallocate(ptr, old_layout);
        }

        Ok(new_ptr)
    }

    /// Attempts to shrink the memory block.
    ///
    /// Returns a new [`NonNull<[u8]>`][NonNull] containing a pointer and the actual size of the allocated
    /// memory. The pointer is suitable for holding data described by `new_layout`. To accomplish
    /// this, the allocator may shrink the allocation referenced by `ptr` to fit the new layout.
    ///
    /// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
    /// transferred to this allocator. The memory may or may not have been freed, and should be
    /// considered unusable.
    ///
    /// If this method returns `Err`, then ownership of the memory block has not been transferred to
    /// this allocator, and the contents of the memory block are unaltered.
    ///
    /// # Safety
    ///
    /// * `ptr` must denote a block of memory [*currently allocated*] via this allocator.
    /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
    /// * `new_layout.size()` must be smaller than or equal to `old_layout.size()`.
    ///
    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
    ///
    /// [*currently allocated*]: #currently-allocated-memory
    /// [*fit*]: #memory-fitting
    ///
    /// # Errors
    ///
    /// Returns `Err` if the new layout does not meet the allocator's size and alignment
    /// constraints of the allocator, or if shrinking otherwise fails.
    ///
    /// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
    /// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
    /// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
    ///
    /// Clients wishing to abort computation in response to an allocation error are encouraged to
    /// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
    ///
    /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        debug_assert!(
            new_layout.size() <= old_layout.size(),
            "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
        );

        let new_ptr = self.allocate(new_layout)?;

        // SAFETY: because `new_layout.size()` must be lower than or equal to
        // `old_layout.size()`, both the old and new memory allocation are valid for reads and
        // writes for `new_layout.size()` bytes. Also, because the old allocation wasn't yet
        // deallocated, it cannot overlap `new_ptr`. Thus, the call to `copy_nonoverlapping` is
        // safe. The safety contract for `dealloc` must be upheld by the caller.
        unsafe {
            ptr::copy_nonoverlapping(ptr.as_ptr(), nonnull_as_mut_ptr(new_ptr), new_layout.size());
            self.deallocate(ptr, old_layout);
        }

        Ok(new_ptr)
    }

    /// Creates a "by reference" adapter for this instance of `Allocator`.
    ///
    /// The returned adapter also implements `Allocator` and will simply borrow this.
    #[inline(always)]
    fn by_ref(&self) -> &Self
    where
        Self: Sized,
    {
        self
    }
}

unsafe impl<A> Allocator for &A
where
    A: Allocator + ?Sized,
{
    #[inline]
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        (**self).allocate(layout)
    }

    #[inline]
    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        (**self).allocate_zeroed(layout)
    }

    #[inline]
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        // SAFETY: the safety contract must be upheld by the caller
        unsafe { (**self).deallocate(ptr, layout) }
    }

    #[inline]
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        // SAFETY: the safety contract must be upheld by the caller
        unsafe { (**self).grow(ptr, old_layout, new_layout) }
    }

    #[inline]
    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        // SAFETY: the safety contract must be upheld by the caller
        unsafe { (**self).grow_zeroed(ptr, old_layout, new_layout) }
    }

    #[inline]
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        // SAFETY: the safety contract must be upheld by the caller
        unsafe { (**self).shrink(ptr, old_layout, new_layout) }
    }
}