rand: don't block before random pool is initialized

src/libstd/collections/hash/map.rs

@@ -202,8 +202,10 @@ fn test_resize_policy() {
 /// The hashes are all keyed by the thread-local random number generator
 /// on creation by default. This means that the ordering of the keys is
 /// randomized, but makes the tables more resistant to
-/// denial-of-service attacks (Hash DoS). This behavior can be
-/// overridden with one of the constructors.
+/// denial-of-service attacks (Hash DoS). No guarantees are made to the
+/// quality of the random data. The implementation uses the best available
+/// random data from your platform at the time of creation. This behavior
+/// can be overridden with one of the constructors.
 ///
 /// It is required that the keys implement the `Eq` and `Hash` traits, although
 /// this can frequently be achieved by using `#[derive(PartialEq, Eq, Hash)]`.

src/libstd/sys/unix/rand.rs

@@ -41,8 +41,10 @@ mod imp {
         #[cfg(target_arch = "aarch64")]
         const NR_GETRANDOM: libc::c_long = 278;
 
+        const GRND_NONBLOCK: libc::c_uint = 0x0001;
+
         unsafe {
-            libc::syscall(NR_GETRANDOM, buf.as_mut_ptr(), buf.len(), 0)
+            libc::syscall(NR_GETRANDOM, buf.as_mut_ptr(), buf.len(), GRND_NONBLOCK)
         }
     }
 
@@ -63,6 +65,19 @@ mod imp {
                 let err = errno() as libc::c_int;
                 if err == libc::EINTR {
                     continue;
+                } else if err == libc::EAGAIN {
+                    // if getrandom() returns EAGAIN it would have blocked
+                    // because the non-blocking pool (urandom) has not
+                    // initialized in the kernel yet due to a lack of entropy
+                    // the fallback we do here is to avoid blocking applications
+                    // which could depend on this call without ever knowing
+                    // they do and don't have a work around. The PRNG of
+                    // /dev/urandom will still be used but not over a completely
+                    // full entropy pool
+                    let reader = File::open("/dev/urandom").expect("Unable to open /dev/urandom");
+                    let mut reader_rng = ReaderRng::new(reader);
+                    reader_rng.fill_bytes(& mut v[read..]);
+                    read += v.len() as usize;
                 } else {
                     panic!("unexpected getrandom error: {}", err);
                 }