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// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.
//! Types to support writing to a contiguous byte buffer.
//!
//! This is different from `bytes::BufMut` in that the buffer is required to be
//! contiguous, which allows for more efficient use with type erasure.
use std::mem::MaybeUninit;
/// Models a partially written, contiguous byte buffer.
pub trait Buffer {
/// Returns the unwritten portion of the buffer. The returned data may or
/// may not be initialized.
///
/// # Safety
/// The caller must ensure that no uninitialized bytes are written to the
/// slice.
///
/// An astute reader might note that the `Vec<u8>` implementation does not
/// require the unsafe bound on this function, as those bytes returned by
/// are truly `MaybeUninit`. However, based on the backing storage of [Buffer]
/// this is not always the case.
///
/// For example, a `Buffer` implementation on a `Cursor<&[u8]>` could be used
/// to _uninitialize_ a portion of the slice, by doing the following:
///
/// ```ignore
/// // some_cursor contains a Cursor based implementation of Buffer which is
/// // backed by storage that is always initialized.
/// let foo = some_cursor.unwritten();
/// foo[0].write(MaybeUninit::uninit()) // This is UB!! ⚠️
/// ```
///
/// Thus the caller must ensure that uninitialize bytes are _never_
/// written to the returned slice, and why this function is unsafe.
unsafe fn unwritten(&mut self) -> &mut [MaybeUninit<u8>];
/// Extends the initialized region of the buffer.
///
/// # Safety
/// The caller must ensure that the next `len` bytes have been initialized.
unsafe fn extend_written(&mut self, len: usize);
}
impl Buffer for Vec<u8> {
unsafe fn unwritten(&mut self) -> &mut [MaybeUninit<u8>] {
self.spare_capacity_mut()
}
unsafe fn extend_written(&mut self, len: usize) {
// SAFETY: The caller guarantees that `len` bytes have been written.
unsafe {
self.set_len(self.len() + len);
}
}
}
impl Buffer for Buf<'_> {
unsafe fn unwritten(&mut self) -> &mut [MaybeUninit<u8>] {
&mut self.buf[*self.filled..]
}
unsafe fn extend_written(&mut self, len: usize) {
*self.filled += len;
}
}
impl Buffer for std::io::Cursor<&mut [u8]> {
unsafe fn unwritten(&mut self) -> &mut [MaybeUninit<u8>] {
let slice = self.get_mut();
// SAFETY: the caller promises not to uninitialize any initialized data.
unsafe { std::slice::from_raw_parts_mut(slice.as_mut_ptr().cast(), slice.len()) }
}
unsafe fn extend_written(&mut self, len: usize) {
self.set_position(self.position() + len as u64);
}
}
/// An accessor for writing to a partially-initialized byte buffer.
pub struct Buf<'a> {
buf: &'a mut [MaybeUninit<u8>],
filled: &'a mut usize,
}
impl<'a> Buf<'a> {
/// Returns the remaining bytes that fit.
#[inline(always)]
pub fn remaining(&self) -> usize {
self.buf.len() - *self.filled
}
/// Returns the number of bytes that have been written.
#[inline(always)]
pub fn len(&self) -> usize {
*self.filled
}
/// Extends the initialized portion of the buffer with `b`. Panics if it
/// doesn't fit.
#[inline(always)]
pub fn push(&mut self, b: u8) {
self.buf[*self.filled] = MaybeUninit::new(b);
*self.filled += 1;
}
/// Extends the initialized portion of the buffer with `buf`. Panics if the
/// data does not fit.
#[inline(always)]
pub fn append(&mut self, buf: &[u8]) {
assert!(buf.len() <= self.remaining());
// SAFETY: copying into self.buf with bounds checked above.
unsafe {
self.buf
.as_mut_ptr()
.add(*self.filled)
.cast::<u8>()
.copy_from_nonoverlapping(buf.as_ptr(), buf.len());
}
*self.filled += buf.len();
}
/// Extends the initialized portion of the buffer with `len` bytes equal to
/// `val`. Panics if the data does not fit.
#[inline(always)]
pub fn fill(&mut self, val: u8, len: usize) {
self.buf[*self.filled..][..len].fill(MaybeUninit::new(val));
*self.filled += len;
}
/// Splits this buffer into two at `split_at` and calls `f` to fill out each
/// part.
///
/// If the left buffer is not filled in full but the right buffer is
/// partially initialized, then the remainder of the left buffer will be
/// zero-initialized.
#[track_caller]
pub fn write_split<R>(&mut self, split_at: usize, f: impl FnOnce(Buf<'_>, Buf<'_>) -> R) -> R {
let (left, right) = self.buf[*self.filled..].split_at_mut(split_at);
let mut left_filled = 0;
let mut right_filled = 0;
let r = f(
Buf {
buf: left,
filled: &mut left_filled,
},
Buf {
buf: right,
filled: &mut right_filled,
},
);
assert!(left_filled <= left.len());
assert!(right_filled <= right.len());
*self.filled += left_filled;
if right_filled > 0 {
let to_zero = left.len() - left_filled;
self.fill(0, to_zero);
*self.filled += right_filled;
}
r
}
}
/// Calls `f` with a [`Buf`], which provides safe methods for
/// extending the initialized portion of the buffer.
pub fn write_with<T, F, R>(buffer: &mut T, f: F) -> R
where
T: Buffer + ?Sized,
F: FnOnce(Buf<'_>) -> R,
{
let mut filled = 0;
// SAFETY: Buf will only write initialized bytes to the buffer.
let buf = unsafe { buffer.unwritten() };
let r = f(Buf {
buf,
filled: &mut filled,
});
// SAFETY: `filled` bytes are known to have been written.
unsafe {
buffer.extend_written(filled);
}
r
}
#[cfg(test)]
mod tests {
use super::write_with;
#[test]
#[should_panic]
fn test_append_vec_panic() {
let mut v = vec![1, 2, 3];
write_with(&mut v, |mut buf| {
buf.append(&vec![0; buf.remaining() + 1]);
});
}
#[test]
fn test_append_vec() {
let mut v = vec![1, 2, 3, 4];
v.reserve(3);
write_with(&mut v, |mut buf| {
buf.append(&[5, 6]);
buf.push(7);
});
assert_eq!(&v, &[1, 2, 3, 4, 5, 6, 7]);
}
}