Account for SharedArrayBuffer not being available

src/backends/wasm_js.rs

@@ -6,10 +6,7 @@
 extern crate std;
 
 use crate::Error;
-use core::{
-    mem::MaybeUninit,
-    sync::atomic::{AtomicU8, Ordering},
-};
+use core::mem::MaybeUninit;
 #[cfg(feature = "std")]
 use std::thread_local;
 
@@ -18,42 +15,18 @@ 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 js_sys::{SharedArrayBuffer, Uint8Array, WebAssembly::Memory};
-use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
+use js_sys::{JsString, Uint8Array};
+use wasm_bindgen::{prelude::wasm_bindgen, JsValue};
 
 // Size of our temporary Uint8Array buffer used with WebCrypto methods
 // Maximum is 65536 bytes see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
 const CRYPTO_BUFFER_SIZE: u16 = 256;
 
-const MEMORY_KIND_UNINIT: u8 = 0;
-const MEMORY_KIND_NOT_SHARED: u8 = 1;
-const MEMORY_KIND_SHARED: u8 = 2;
-
-static MEMORY_KIND: AtomicU8 = AtomicU8::new(0);
-
 pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     CRYPTO.with(|crypto| {
         let crypto = crypto.as_ref().ok_or(Error::WEB_CRYPTO)?;
 
-        let shared = loop {
-            break match MEMORY_KIND.load(Ordering::Relaxed) {
-                MEMORY_KIND_NOT_SHARED => false,
-                MEMORY_KIND_SHARED => true,
-                MEMORY_KIND_UNINIT => {
-                    let memory: Memory = wasm_bindgen::memory().unchecked_into();
-                    let val = if memory.buffer().is_instance_of::<SharedArrayBuffer>() {
-                        MEMORY_KIND_SHARED
-                    } else {
-                        MEMORY_KIND_NOT_SHARED
-                    };
-                    MEMORY_KIND.store(val, Ordering::Relaxed);
-                    continue;
-                }
-                _ => unreachable!(),
-            };
-        };
-
-        if shared {
+        if is_sab() {
             // getRandomValues does not work with all types of WASM memory,
             // so we initially write to browser memory to avoid exceptions.
             let buf = Uint8Array::new_with_length(CRYPTO_BUFFER_SIZE.into());
@@ -88,7 +61,62 @@ pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     })
 }
 
+#[cfg(not(target_feature = "atomics"))]
+fn is_sab() -> bool {
+    use core::sync::atomic::{AtomicU8, Ordering};
+
+    use js_sys::WebAssembly::Memory;
+    use js_sys::{Object, SharedArrayBuffer};
+    use wasm_bindgen::JsCast;
+
+    const MEMORY_KIND_UNINIT: u8 = 0;
+    const MEMORY_KIND_NOT_SHARED: u8 = 1;
+    const MEMORY_KIND_SHARED: u8 = 2;
+
+    static MEMORY_KIND: AtomicU8 = AtomicU8::new(0);
+
+    loop {
+        break match MEMORY_KIND.load(Ordering::Relaxed) {
+            MEMORY_KIND_NOT_SHARED => false,
+            MEMORY_KIND_SHARED => true,
+            MEMORY_KIND_UNINIT => {
+                let buffer = wasm_bindgen::memory().unchecked_into::<Memory>().buffer();
+
+                // `SharedArrayBuffer` is only available with COOP & COEP. But even without its
+                // possible to create a shared `WebAssembly.Memory`, so we check for that via
+                // the constructor name.
+                //
+                // Keep in mind that `crossOriginIsolated` is not available on Node.js, in
+                // which case we can still use `instanceof` because `SharedArrayBuffer` is
+                // always available.
+                let shared = match CROSS_ORIGIN_ISOLATED.with(Option::clone) {
+                    Some(true) | None => buffer.is_instance_of::<SharedArrayBuffer>(),
+                    Some(false) => {
+                        let constructor_name = Object::from(buffer).constructor().name();
+                        SHARED_ARRAY_BUFFER_NAME.with(|name| &constructor_name == name)
+                    }
+                };
+
+                let val = if shared {
+                    MEMORY_KIND_SHARED
+                } else {
+                    MEMORY_KIND_NOT_SHARED
+                };
+                MEMORY_KIND.store(val, Ordering::Relaxed);
+                continue;
+            }
+            _ => unreachable!(),
+        };
+    }
+}
+
+#[cfg(target_feature = "atomics")]
+fn is_sab() -> bool {
+    true
+}
+
 #[wasm_bindgen]
+#[rustfmt::skip]
 extern "C" {
     // Web Crypto API: Crypto interface (https://www.w3.org/TR/WebCryptoAPI/)
     type Crypto;
@@ -100,4 +128,9 @@ extern "C" {
     fn get_random_values(this: &Crypto, buf: &Uint8Array) -> Result<(), JsValue>;
     #[wasm_bindgen(method, js_name = getRandomValues, catch)]
     fn get_random_values_ref(this: &Crypto, buf: &mut [u8]) -> Result<(), JsValue>;
+    // Returns the [`crossOriginIsolated`](https://developer.mozilla.org/en-US/docs/Web/API/crossOriginIsolated) global property.
+    #[wasm_bindgen(thread_local_v2, js_namespace = globalThis, js_name = crossOriginIsolated)]
+    static CROSS_ORIGIN_ISOLATED: Option<bool>;
+    #[wasm_bindgen(thread_local_v2, static_string)]
+    static SHARED_ARRAY_BUFFER_NAME: JsString = "SharedArrayBuffer";
 }