crates/getrandom/src/backends/wasm_js.rs

//! Implementation for WASM based on Web and Node.js
use crate::Error;
use core::mem::MaybeUninit;

pub use crate::util::{inner_u32, inner_u64};

#[cfg(not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none"))))]
compile_error!("`wasm_js` backend can be enabled only for OS-less WASM targets!");

use wasm_bindgen::{prelude::wasm_bindgen, JsValue};

// Maximum buffer size allowed in `Crypto.getRandomValuesSize` is 65536 bytes.
// See https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
const MAX_BUFFER_SIZE: usize = 65536;

#[cfg(not(target_feature = "atomics"))]
#[inline]
pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
    for chunk in dest.chunks_mut(MAX_BUFFER_SIZE) {
        if get_random_values(chunk).is_err() {
            return Err(Error::WEB_CRYPTO);
        }
    }
    Ok(())
}

#[cfg(target_feature = "atomics")]
pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
    // getRandomValues does not work with all types of WASM memory,
    // so we initially write to browser memory to avoid exceptions.
    let buf_len = usize::min(dest.len(), MAX_BUFFER_SIZE);
    let buf_len_u32 = buf_len
        .try_into()
        .expect("buffer length is bounded by MAX_BUFFER_SIZE");
    let buf = js_sys::Uint8Array::new_with_length(buf_len_u32);
    for chunk in dest.chunks_mut(buf_len) {
        let chunk_len = chunk
            .len()
            .try_into()
            .expect("chunk length is bounded by MAX_BUFFER_SIZE");
        // The chunk can be smaller than buf's length, so we call to
        // JS to create a smaller view of buf without allocation.
        let sub_buf = if chunk_len == buf_len_u32 {
            &buf
        } else {
            &buf.subarray(0, chunk_len)
        };

        if get_random_values(sub_buf).is_err() {
            return Err(Error::WEB_CRYPTO);
        }

        sub_buf.copy_to_uninit(chunk);
    }
    Ok(())
}

#[wasm_bindgen]
extern "C" {
    // Crypto.getRandomValues()
    #[cfg(not(target_feature = "atomics"))]
    #[wasm_bindgen(js_namespace = ["globalThis", "crypto"], js_name = getRandomValues, catch)]
    fn get_random_values(buf: &mut [MaybeUninit<u8>]) -> Result<(), JsValue>;
    #[cfg(target_feature = "atomics")]
    #[wasm_bindgen(js_namespace = ["globalThis", "crypto"], js_name = getRandomValues, catch)]
    fn get_random_values(buf: &js_sys::Uint8Array) -> Result<(), JsValue>;
}

impl Error {
    /// The environment does not support the Web Crypto API.
    pub(crate) const WEB_CRYPTO: Error = Self::new_internal(10);
}