Add support for arc4random() to random_device.

Nuxi CloudABI (https://github.com/NuxiNL/cloudlibc) does not allow
processes to access the global filesystem namespace. This breaks
random_device, as it attempts to use /dev/{u,}random. This change adds
support for arc4random(), which is present on CloudABI.

In my opinion it would also make sense to use arc4random() on other
operating systems, such as *BSD and Mac OS X, but I'd rather leave that
to the maintainers of the respective platforms. Switching to
arc4random() does change the ABI.

This change also attempts to make some cleanups to the code. It adds a
single #define for every random interface, instead of testing against
operating systems explicitly.

As discussed, also validate the token argument to be equal to
"/dev/urandom" on all systems that only provide pseudo-random numbers.
This should cause little to no breakage, as "/dev/urandom" is also the
default argument value.

Reviewed by: jfb
Differential Revision: http://reviews.llvm.org/D8134


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@231764 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -110,12 +110,22 @@
 # endif
 #endif // __sun__
 
-#if defined(__native_client__)
+#if defined(__CloudABI__)
+  // Certain architectures provide arc4random(). Prefer using
+  // arc4random() over /dev/{u,}random to make it possible to obtain
+  // random data even when using sandboxing mechanisms such as chroots,
+  // Capsicum, etc.
+# define _LIBCPP_USING_ARC4_RANDOM
+#elif defined(__native_client__)
   // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
   // including accesses to the special files under /dev. C++11's
   // std::random_device is instead exposed through a NaCl syscall.
 # define _LIBCPP_USING_NACL_RANDOM
-#endif // defined(__native_client__)
+#elif defined(_WIN32)
+# define _LIBCPP_USING_WIN32_RANDOM
+#else
+# define _LIBCPP_USING_DEV_RANDOM
+#endif
 
 #if !defined(_LIBCPP_LITTLE_ENDIAN) || !defined(_LIBCPP_BIG_ENDIAN)
 # include <endian.h>

include/random

@@ -3475,9 +3475,9 @@ typedef shuffle_order_engine<minstd_rand0, 256>                         knuth_b;
 
 class _LIBCPP_TYPE_VIS random_device
 {
-#if !(defined(_WIN32) || defined(_LIBCPP_USING_NACL_RANDOM))
+#ifdef _LIBCPP_USING_DEV_RANDOM
     int __f_;
-#endif // !(defined(_WIN32) || defined(_LIBCPP_USING_NACL_RANDOM))
+#endif // defined(_LIBCPP_USING_DEV_RANDOM)
 public:
     // types
     typedef unsigned result_type;