Use GRND_INSECURE instead of /dev/urandom when possible

From reading the source code, it appears like the desired semantic of
std::unix::rand is to always provide some bytes and never block. For
that reason GRND_NONBLOCK is checked before calling getrandom(0), so
that getrandom(0) won't block. If it would block, then the function
falls back to using /dev/urandom, which for the time being doesn't
block. There are some drawbacks to using /dev/urandom, however, and so
getrandom(GRND_INSECURE) was created as a replacement for this exact
circumstance.

getrandom(GRND_INSECURE) is the same as /dev/urandom, except:

- It won't leave a warning in dmesg if used at early boot time, which is
  a common occurance (and the reason why I found this issue);

- It won't introduce a tiny delay at early boot on newer kernels when
  /dev/urandom tries to opportunistically create jitter entropy;

- It only requires 1 syscall, rather than 3.

Other than that, it returns the same "quality" of randomness as
/dev/urandom, and never blocks.

It's only available on kernels ≥5.6, so we try to use it, cache the
result of that attempt, and fall back to to the previous code if it
didn't work.

library/std/src/sys/unix/rand.rs

@@ -30,6 +30,9 @@ mod imp {
 
     #[cfg(any(target_os = "linux", target_os = "android"))]
     fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
+        use crate::sync::atomic::{AtomicBool, Ordering};
+        use crate::sys::os::errno;
+
         // A weak symbol allows interposition, e.g. for perf measurements that want to
         // disable randomness for consistency. Otherwise, we'll try a raw syscall.
         // (`getrandom` was added in glibc 2.25, musl 1.1.20, android API level 28)
@@ -41,6 +44,18 @@ mod imp {
             ) -> libc::ssize_t
         }
 
+        // This provides the best quality random numbers available at the given moment
+        // without ever blocking, and is preferable to falling back to /dev/urandom.
+        static GRND_INSECURE_AVAILABLE: AtomicBool = AtomicBool::new(true);
+        if GRND_INSECURE_AVAILABLE.load(Ordering::Relaxed) {
+            let ret = unsafe { getrandom(buf.as_mut_ptr().cast(), buf.len(), libc::GRND_INSECURE) };
+            if ret == -1 && errno() as libc::c_int == libc::EINVAL {
+                GRND_INSECURE_AVAILABLE.store(false, Ordering::Relaxed);
+            } else {
+                return ret;
+            }
+        }
+
         unsafe { getrandom(buf.as_mut_ptr().cast(), buf.len(), libc::GRND_NONBLOCK) }
     }