util: Specific GetOSRandom for Linux/FreeBSD/OpenBSD

These are available in sandboxes without access to files or
devices. Also [they are safer and more straightforward](https://en.wikipedia.org/wiki/Entropy-supplying_system_calls)
to use than `/dev/urandom` as reading from a file has quite a few edge
cases:

- Linux: `getrandom(buf, buflen, 0)`. [getrandom(2)](http://man7.org/linux/man-pages/man2/getrandom.2.html)
  was introduced in version 3.17 of the Linux kernel.
- OpenBSD: `getentropy(buf, buflen)`. The [getentropy(2)](http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2)
  function appeared in OpenBSD 5.6.
- FreeBSD and NetBSD: `sysctl(KERN_ARND)`. Not sure when this was added
  but it has existed for quite a while.

Alternatives:

- Linux has sysctl `CTL_KERN` / `KERN_RANDOM` / `RANDOM_UUID`
  which gives 16 bytes of randomness. This may be available
  on older kernels, however [sysctl is deprecated on Linux](https://lwn.net/Articles/605392/)
  and even removed in some distros so we shouldn't use it.

Add tests for `GetOSRand()`:

- Test that no error happens (otherwise `RandFailure()` which aborts)
- Test that all 32 bytes are overwritten (initialize with zeros, try multiple times)

Discussion:

- When to use these? Currently they are always used when available.
  Another option would be to use them only when `/dev/urandom` is not
  available. But this would mean these code paths receive less testing,
  and I'm not sure there is any reason to prefer `/dev/urandom`.

Closes: #9676

src/test/random_tests.cpp

@@ -0,0 +1,46 @@
+// Copyright (c) 2017 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "random.h"
+
+#include "test/test_bitcoin.h"
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(random_tests, BasicTestingSetup)
+
+static const ssize_t MAX_TRIES = 1024;
+
+BOOST_AUTO_TEST_CASE(osrandom_tests)
+{
+    /* This does not measure the quality of randomness, but it does test that
+     * OSRandom() overwrites all 32 bytes of the output given a maximum
+     * number of tries.
+     */
+    uint8_t data[NUM_OS_RANDOM_BYTES];
+    bool overwritten[NUM_OS_RANDOM_BYTES] = {}; /* Tracks which bytes have been overwritten at least once */
+    int num_overwritten;
+    int tries = 0;
+    /* Loop until all bytes have been overwritten at least once */
+    do {
+        memset(data, 0, NUM_OS_RANDOM_BYTES);
+        GetOSRand(data);
+        for (int x=0; x < NUM_OS_RANDOM_BYTES; ++x) {
+            overwritten[x] |= (data[x] != 0);
+        }
+
+        num_overwritten = 0;
+        for (int x=0; x < NUM_OS_RANDOM_BYTES; ++x) {
+            if (overwritten[x]) {
+                num_overwritten += 1;
+            }
+        }
+
+        tries += 1;
+    } while (num_overwritten < NUM_OS_RANDOM_BYTES && tries < MAX_TRIES);
+    BOOST_CHECK(num_overwritten == NUM_OS_RANDOM_BYTES); /* If this failed, bailed out after too many tries */
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+