denoland-deno/serde_v8/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;

use super::transl8::FromV8;

/// 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,
      }),
    }
  }

  pub fn from_view(
    scope: &mut v8::HandleScope,
    view: v8::Local<v8::ArrayBufferView>,
  ) -> Result<Self, v8::DataError> {
    let buffer = view.buffer(scope).ok_or(v8::DataError::NoData {
      expected: "view to have a buffer",
    })?;
    Self::from_buffer(buffer, view.byte_offset(), view.byte_length())
  }
}

impl FromV8 for ZeroCopyBuf {
  fn from_v8(
    scope: &mut v8::HandleScope,
    value: v8::Local<v8::Value>,
  ) -> Result<Self, crate::Error> {
    if value.is_array_buffer() {
      value
        .try_into()
        .and_then(|b| Self::from_buffer(b, 0, b.byte_length()))
    } else {
      value
        .try_into()
        .and_then(|view| Self::from_view(scope, view))
    }
    .map_err(|_| crate::Error::ExpectedBuffer)
  }
}

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
}