denoland-deno/serde_v8/src/magic/zero_copy_buf.rs

// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.

use std::cell::Cell;
use std::ops::Deref;
use std::ops::DerefMut;

/// A ZeroCopyBuf encapsulates a slice that's been borrowed from a JavaScript
/// ArrayBuffer object. JavaScript objects can normally be garbage collected,
/// but the existence of a ZeroCopyBuf inhibits this until it is dropped. It
/// behaves much like an Arc<[u8]>.
///
/// # Cloning
/// Cloning a ZeroCopyBuf does not clone the contents of the buffer,
/// it creates a new reference to that buffer.
///
/// To actually clone the contents of the buffer do
/// `let copy = Vec::from(&*zero_copy_buf);`
#[derive(Clone)]
pub struct ZeroCopyBuf {
  backing_store: v8::SharedRef<v8::BackingStore>,
  byte_offset: usize,
  byte_length: usize,
}

unsafe impl Send for ZeroCopyBuf {}

impl ZeroCopyBuf {
  pub fn from_buffer(
    buffer: v8::Local<v8::ArrayBuffer>,
    byte_offset: usize,
    byte_length: usize,
  ) -> Result<Self, v8::DataError> {
    let backing_store = buffer.get_backing_store();
    match backing_store.is_shared() {
      true => Err(v8::DataError::BadType {
        actual: "shared ArrayBufferView",
        expected: "non-shared ArrayBufferView",
      }),
      false => Ok(Self {
        backing_store,
        byte_offset,
        byte_length,
      }),
    }
  }
}

impl<'s> TryFrom<v8::Local<'s, v8::ArrayBuffer>> for ZeroCopyBuf {
  type Error = v8::DataError;
  fn try_from(buffer: v8::Local<v8::ArrayBuffer>) -> Result<Self, Self::Error> {
    Self::from_buffer(buffer, 0, buffer.byte_length())
  }
}

// TODO(@AaronO): consider streamlining this as "ScopedValue" ?
type ScopedView<'a, 'b, 's> = (
  &'s mut v8::HandleScope<'a>,
  v8::Local<'b, v8::ArrayBufferView>,
);
impl<'a, 'b, 's> TryFrom<ScopedView<'a, 'b, 's>> for ZeroCopyBuf {
  type Error = v8::DataError;
  fn try_from(
    scoped_view: ScopedView<'a, 'b, 's>,
  ) -> Result<Self, Self::Error> {
    let (scope, view) = scoped_view;
    let buffer = view.buffer(scope).unwrap();
    Self::from_buffer(buffer, view.byte_offset(), view.byte_length())
  }
}

impl Deref for ZeroCopyBuf {
  type Target = [u8];
  fn deref(&self) -> &[u8] {
    unsafe {
      get_backing_store_slice(
        &self.backing_store,
        self.byte_offset,
        self.byte_length,
      )
    }
  }
}

impl DerefMut for ZeroCopyBuf {
  fn deref_mut(&mut self) -> &mut [u8] {
    unsafe {
      get_backing_store_slice_mut(
        &self.backing_store,
        self.byte_offset,
        self.byte_length,
      )
    }
  }
}

impl AsRef<[u8]> for ZeroCopyBuf {
  fn as_ref(&self) -> &[u8] {
    &*self
  }
}

impl AsMut<[u8]> for ZeroCopyBuf {
  fn as_mut(&mut self) -> &mut [u8] {
    &mut *self
  }
}

unsafe fn get_backing_store_slice(
  backing_store: &v8::SharedRef<v8::BackingStore>,
  byte_offset: usize,
  byte_length: usize,
) -> &[u8] {
  let cells: *const [Cell<u8>] =
    &backing_store[byte_offset..byte_offset + byte_length];
  let bytes = cells as *const [u8];
  &*bytes
}

#[allow(clippy::mut_from_ref)]
unsafe fn get_backing_store_slice_mut(
  backing_store: &v8::SharedRef<v8::BackingStore>,
  byte_offset: usize,
  byte_length: usize,
) -> &mut [u8] {
  let cells: *const [Cell<u8>] =
    &backing_store[byte_offset..byte_offset + byte_length];
  let bytes = cells as *const _ as *mut [u8];
  &mut *bytes
}