Use getrandom for seeding the RNG on WASM targets (#60)

Author: notgull

Cargo.toml

@@ -17,13 +17,13 @@ exclude = ["/.*"]
 [features]
 default = ["std"]
 alloc = []
-std = ["alloc", "instant"]
+std = ["alloc"]
+js = ["std", "getrandom"]
 
-[target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dependencies]
-instant = { version = "0.1", optional = true }
+[target.'cfg(all(any(target_arch = "wasm32", target_arch = "wasm64"), target_os = "unknown"))'.dependencies]
+getrandom = { version = "0.2", features = ["js"], optional = true }
 
-[target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dev-dependencies]
-instant = { version = "0.1", features = ["wasm-bindgen"] }
+[target.'cfg(all(any(target_arch = "wasm32", target_arch = "wasm64"), target_os = "unknown"))'.dev-dependencies]
 wasm-bindgen-test = "0.3"
 getrandom = { version = "0.2", features = ["js"] }
 

src/global_rng.rs

@@ -5,10 +5,8 @@ use crate::Rng;
 use std::cell::Cell;
 use std::ops::RangeBounds;
 
-#[cfg(all(target_arch = "wasm32", not(target_os = "wasi")))]
-use instant::Instant;
-#[cfg(not(all(target_arch = "wasm32", not(target_os = "wasi"))))]
-use std::time::Instant;
+// Chosen by fair roll of the dice.
+const DEFAULT_RNG_SEED: u64 = 0xef6f79ed30ba75a;
 
 impl Default for Rng {
     /// Initialize the `Rng` from the system's random number generator.
@@ -29,17 +27,7 @@ impl Rng {
 }
 
 thread_local! {
-    static RNG: Cell<Rng> = Cell::new(Rng({
-        use std::collections::hash_map::DefaultHasher;
-        use std::hash::{Hash, Hasher};
-        use std::thread;
-
-        let mut hasher = DefaultHasher::new();
-        Instant::now().hash(&mut hasher);
-        thread::current().id().hash(&mut hasher);
-        let hash = hasher.finish();
-        (hash << 1) | 1
-    }));
+    static RNG: Cell<Rng> = Cell::new(Rng(random_seed().unwrap_or(DEFAULT_RNG_SEED)));
 }
 
 /// Run an operation with the current thread-local generator.
@@ -190,3 +178,41 @@ pub fn f64() -> f64 {
 pub fn choose_multiple<T: Iterator>(source: T, amount: usize) -> Vec<T::Item> {
     with_rng(|rng| rng.choose_multiple(source, amount))
 }
+
+#[cfg(not(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown"
+)))]
+fn random_seed() -> Option<u64> {
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::{Hash, Hasher};
+    use std::thread;
+    use std::time::Instant;
+
+    let mut hasher = DefaultHasher::new();
+    Instant::now().hash(&mut hasher);
+    thread::current().id().hash(&mut hasher);
+    let hash = hasher.finish();
+    Some((hash << 1) | 1)
+}
+
+#[cfg(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown",
+    feature = "js"
+))]
+fn random_seed() -> Option<u64> {
+    // TODO(notgull): Failures should be logged somewhere.
+    let mut seed = [0u8; 8];
+    getrandom::getrandom(&mut seed).ok()?;
+    Some(u64::from_ne_bytes(seed))
+}
+
+#[cfg(all(
+    any(target_arch = "wasm32", target_arch = "wasm64"),
+    target_os = "unknown",
+    not(feature = "js")
+))]
+fn random_seed() -> Option<u64> {
+    None
+}

src/lib.rs

@@ -78,6 +78,21 @@
 //! - `std` (enabled by default): Enables the `std` library. This is required for the global
 //!   generator and global entropy. Without this feature, [`Rng`] can only be instantiated using
 //!   the [`with_seed`](Rng::with_seed) method.
+//! - `js`: Assumes that WebAssembly targets are being run in a JavaScript environment. See the
+//!   [WebAssembly Notes](#webassembly-notes) section for more information.
+//!
+//! # WebAssembly Notes
+//!
+//! For non-WASI WASM targets, there is additional sublety to consider when utilizing the global RNG.
+//! By default, `std` targets will use entropy sources in the standard library to seed the global RNG.
+//! However, these sources are not available by default on WASM targets outside of WASI.
+//!
+//! If the `js` feature is enabled, this crate will assume that it is running in a JavaScript
+//! environment. At this point, the [`getrandom`] crate will be used in order to access the available
+//! entropy sources and seed the global RNG. If the `js` feature is not enabled, the global RNG will
+//! use a predefined seed.
+//!
+//! [`getrandom`]: https://crates.io/crates/getrandom
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(docsrs, feature(doc_cfg))]