Test getrandom_uninit

src/lib.rs

@@ -404,3 +404,10 @@ pub fn getrandom_uninit(dest: &mut [MaybeUninit<u8>]) -> Result<&mut [u8], Error
     // since it returned `Ok`.
     Ok(unsafe { slice_assume_init_mut(dest) })
 }
+
+// Don't run normal unit tests when testing custom getrandom
+#[cfg(all(
+    test,
+    not(all(target_family = "wasm", target_os = "unknown", feature = "custom"))
+))]
+pub(crate) mod tests;

src/tests.rs

@@ -0,0 +1,130 @@
+//! Common tests and testing utilities
+extern crate std;
+
+use crate::Error;
+use std::{mem::MaybeUninit, sync::mpsc, thread, vec, vec::Vec};
+
+#[cfg(feature = "test-in-browser")]
+wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
+
+fn num_diff_bits(s1: &[u8], s2: &[u8]) -> usize {
+    assert_eq!(s1.len(), s2.len());
+    s1.iter()
+        .zip(s2.iter())
+        .map(|(a, b)| (a ^ b).count_ones() as usize)
+        .sum()
+}
+
+// A function which fills a buffer with random bytes.
+type FillFn<B> = fn(&mut [B]) -> Result<(), Error>;
+
+// Helper trait for testing different `FillFn`s.
+pub(crate) trait Byte: Sized + 'static {
+    fn make_vec(len: usize, fill: FillFn<Self>) -> Vec<u8>;
+}
+impl Byte for u8 {
+    fn make_vec(len: usize, fill: FillFn<u8>) -> Vec<u8> {
+        let mut v = vec![0; len];
+        fill(&mut v).unwrap();
+        v
+    }
+}
+impl Byte for MaybeUninit<u8> {
+    fn make_vec(len: usize, fill: FillFn<MaybeUninit<u8>>) -> Vec<u8> {
+        // Using Vec::spare_capacity_mut more consistently gives us truly
+        // uninitialized memory regardless of optimization level.
+        let mut v = Vec::with_capacity(len);
+        fill(v.spare_capacity_mut()).unwrap();
+        unsafe { v.set_len(len) }
+        v
+    }
+}
+
+// For calls of size `len`, count the number of bits which differ between calls
+// and check that between 3 and 5 bits per byte differ. Probability of failure:
+// ~ 10^(-30) = 2 * CDF[BinomialDistribution[8*256, 0.5], 3*256]
+pub(crate) fn check_bits<B: Byte>(len: usize, fill: FillFn<B>) {
+    let mut num_bytes = 0;
+    let mut diff_bits = 0;
+    while num_bytes < 256 {
+        let v1 = B::make_vec(len, fill);
+        let v2 = B::make_vec(len, fill);
+
+        num_bytes += len;
+        diff_bits += num_diff_bits(&v1, &v2);
+    }
+
+    // When the custom feature is enabled, don't check RNG quality.
+    assert!(diff_bits > 3 * num_bytes);
+    assert!(diff_bits < 5 * num_bytes);
+}
+
+pub(crate) fn check_multithreading<B: Byte>(fill: FillFn<B>) {
+    let mut txs = vec![];
+    for _ in 0..20 {
+        let (tx, rx) = mpsc::channel();
+        txs.push(tx);
+
+        thread::spawn(move || {
+            // wait until all the tasks are ready to go.
+            rx.recv().unwrap();
+            for _ in 0..100 {
+                check_bits(1000, fill);
+                thread::yield_now();
+            }
+        });
+    }
+
+    // start all the tasks
+    for tx in txs.iter() {
+        tx.send(()).unwrap();
+    }
+}
+
+macro_rules! define_tests {
+    ($fill:expr) => {
+        #[cfg(all(target_family = "wasm", target_os = "unknown"))]
+        use wasm_bindgen_test::wasm_bindgen_test as test;
+
+        #[test]
+        fn fill_zero() {
+            $fill(&mut []).unwrap();
+        }
+        #[test]
+        fn fill_small() {
+            for len in 1..=64 {
+                crate::tests::check_bits(len, $fill);
+            }
+        }
+        #[test]
+        fn fill_large() {
+            crate::tests::check_bits(1_000, $fill);
+        }
+        #[test]
+        fn fill_huge() {
+            crate::tests::check_bits(1_000_000, $fill);
+        }
+        // On WASM, the thread API always fails/panics.
+        #[test]
+        #[cfg_attr(target_family = "wasm", ignore)]
+        fn multithreading() {
+            crate::tests::check_multithreading($fill)
+        }
+    };
+}
+pub(crate) use define_tests;
+
+define_tests!(crate::getrandom);
+mod uninit {
+    use super::*;
+
+    fn wrapper(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+        let dest_ptr = dest.as_ptr().cast::<u8>();
+        let res = crate::getrandom_uninit(dest)?;
+        // Ensure that the output points to the same bytes as the input.
+        assert_eq!(res.as_ptr(), dest_ptr);
+        assert_eq!(res.len(), dest.len());
+        Ok(())
+    }
+    super::define_tests!(wrapper);
+}