add a test for the `getrandom` fallback

tests/ui/std/linux-getrandom-fallback.rs

diff --git a/tests/ui/std/linux-getrandom-fallback.rs b/tests/ui/std/linux-getrandom-fallback.rs
new file mode 100644
index 0000000..c50fbe8
--- /dev/null
+++ b/tests/ui/std/linux-getrandom-fallback.rs
@@ -0,0 +1,56 @@
+//@ only-linux
+//@ run-pass
+
+#![feature(random)]
+#![feature(rustc_private)]
+
+extern crate libc;
+
+use std::ffi::{c_void, c_uint, c_int};
+use std::random::{SystemRng, Rng};
+use std::cell::Cell;
+use std::panic::catch_unwind;
+
+thread_local! {
+    static GETRANDOM_ERROR: Cell<c_int> = const { Cell::new(libc::ENOMEM) };
+}
+
+// This interposes the symbol defined in the libc to return an error when we
+// want it to.
+#[unsafe(no_mangle)]
+fn getrandom(_buf: *const c_void, _size: usize, _flags: c_uint) -> isize {
+    let errno = GETRANDOM_ERROR.get();
+    unsafe { libc::__errno_location().write(errno) };
+    -1
+}
+
+fn main() {
+    // Step one:
+    // Test that the interposed symbol actually gets used by having it return an
+    // error that makes `SystemRng` panic.
+    GETRANDOM_ERROR.set(libc::ENOMEM);
+    catch_unwind(|| {
+        let mut buf = [0; 16];
+        SystemRng.fill_bytes(&mut buf);
+    }).expect_err("SystemRng should panic upon receiving ENOMEM from the interposed getrandom");
+
+    // Step two:
+    // Emulate a missing `getrandom` by returning `ENOSYS`. This should excercise
+    // the fallback code.
+    GETRANDOM_ERROR.set(libc::ENOSYS);
+    let mut buf = [0; 16];
+    SystemRng.fill_bytes(&mut buf);
+
+    // Smoke check that the buffer was actually filled. It is possible for this
+    // to spuriously fail, but the likelyhood of that happening is 1 in 2^256.
+    assert_ne!(buf, [0; 16]);
+
+    // And lastly, check that the random pool has been initialized by manually
+    // calling the getrandom syscall and checking that it does not block. This
+    // is unlikely to catch any issues since the randomness pool is nearly always
+    // initialized, but who knows.
+    let r = unsafe {
+        libc::syscall(libc::SYS_getrandom, buf.as_mut_ptr(), buf.len(), libc::GRND_NONBLOCK)
+    };
+    assert_ne!(r, -1);
+}