Fix multithreaded wasm crash (solves #164)

Cargo.toml

@@ -31,6 +31,7 @@ wasi = "0.10"
 stdweb = { version = "0.4.18", default-features = false, optional = true }
 [target.'cfg(all(target_arch = "wasm32", target_os = "unknown", not(cargo_web)))'.dependencies]
 wasm-bindgen = { version = "0.2.62", default-features = false, optional = true }
+js-sys = { version = "0.3", optional = true }
 [target.'cfg(all(target_arch = "wasm32", target_os = "unknown", not(cargo_web)))'.dev-dependencies]
 wasm-bindgen-test = "0.3.18"
 
@@ -40,7 +41,7 @@ std = []
 # Feature to enable fallback RDRAND-based implementation on x86/x86_64
 rdrand = []
 # Feature to enable JavaScript bindings on wasm32-unknown-unknown
-js = ["stdweb", "wasm-bindgen"]
+js = ["stdweb", "wasm-bindgen", "js-sys"]
 # Feature to enable custom RNG implementations
 custom = []
 # Unstable feature to support being a libstd dependency

src/wasm-bindgen.rs

@@ -10,11 +10,15 @@ use crate::Error;
 extern crate std;
 use std::thread_local;
 
+use js_sys::Uint8Array;
 use wasm_bindgen::prelude::*;
 
+// Maximum is 65536 bytes see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
+const BROWSER_CRYPTO_BUFFER_SIZE: usize = 256;
+
 enum RngSource {
     Node(NodeCrypto),
-    Browser(BrowserCrypto),
+    Browser(BrowserCrypto, Uint8Array),
 }
 
 // JsValues are always per-thread, so we initialize RngSource for each thread.
@@ -33,17 +37,18 @@ pub(crate) fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
                     return Err(Error::NODE_RANDOM_FILL_SYNC);
                 }
             }
-            RngSource::Browser(n) => {
-                // see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
-                //
-                // where it says:
-                //
-                // > A QuotaExceededError DOMException is thrown if the
-                // > requested length is greater than 65536 bytes.
-                for chunk in dest.chunks_mut(65536) {
-                    if n.get_random_values(chunk).is_err() {
+            RngSource::Browser(crypto, buf) => {
+                // getRandomValues does not work with all types of WASM memory,
+                // so we initially write to browser memory to avoid exceptions.
+                for chunk in dest.chunks_mut(BROWSER_CRYPTO_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 = buf.subarray(0, chunk.len() as u32);
+
+                    if crypto.get_random_values(&sub_buf).is_err() {
                         return Err(Error::WEB_GET_RANDOM_VALUES);
                     }
+                    sub_buf.copy_to(chunk);
                 }
             }
         };
@@ -63,7 +68,9 @@ fn getrandom_init() -> Result<RngSource, Error> {
             (_, crypto) if !crypto.is_undefined() => crypto,
             _ => return Err(Error::WEB_CRYPTO),
         };
-        return Ok(RngSource::Browser(crypto));
+
+        let buf = Uint8Array::new_with_length(BROWSER_CRYPTO_BUFFER_SIZE as u32);
+        return Ok(RngSource::Browser(crypto, buf));
     }
 
     let crypto = MODULE.require("crypto").map_err(|_| Error::NODE_CRYPTO)?;
@@ -84,7 +91,7 @@ extern "C" {
 
     type BrowserCrypto;
     #[wasm_bindgen(method, js_name = getRandomValues, catch)]
-    fn get_random_values(me: &BrowserCrypto, buf: &mut [u8]) -> Result<(), JsValue>;
+    fn get_random_values(me: &BrowserCrypto, buf: &Uint8Array) -> Result<(), JsValue>;
 
     #[wasm_bindgen(js_name = module)]
     static MODULE: NodeModule;