[m-rev.] for post-commit review: add an arc4random() based system RNG implementation

[Julien Fischer]

I assume no-one wants to run Mercury on old (circa 2014) versions of
NetBSD or OpenBSD.  The macOS stuff is a little ugly, but it will have
to be that way until we drop support for 10.{9,10,11}.

------------------------------------------

Add an arc4random() based system RNG implementation.

library/random.system_rng.m:
     As above.

     Use the arc4random() based implementation on FreeBSD >= 12.0
     and macOS >= 10.12, falling back to the urandom approach on earlier
     versions.

     Always use the arc4random() approach on NetBSD and OpenBSD,
     both of which have used ChaCha20 in their implementation
     of arc4random() since 2014.

Julien.

diff --git a/library/random.system_rng.m b/library/random.system_rng.m
index 04d91627d..af736e12d 100644
--- a/library/random.system_rng.m
+++ b/library/random.system_rng.m
@@ -110,6 +110,12 @@
  #if defined(MR_HAVE_SYS_STAT_H)
      #include <sys/stat.h>
  #endif
+#if defined(MR_HAVE_SYS_PARAM_H)
+    #include <sys/param.h>
+#endif
+#if defined(MR_MAC_OSX)
+    #include <AvailabilityMacros.h>
+#endif

  #include <errno.h>
  #include <stdint.h>
@@ -120,12 +126,16 @@
  //
  // Only one of the following must be defined.
  //
-// ML_SYSRAND_IMPL_ARC4RANDOM (NYI)
+// ML_SYSRAND_IMPL_ARC4RANDOM
  //    the system RNG is implemented by calling the arc4random() family of
  //    functions. Note: this for when arc4random() is provided by libc (as on
  //    macOS and the BSDs), not for when it is provided as a separate library
  //    (e.g. libbsd on Linux).
  //
+//    This should only be enabled on systems where arc4random() uses a secure
+//    PRNG, such as ChaCha20; it should _not_ be enabled on systems where
+//    arc4random() still uses RC4.
+//
  // ML_SYSRAND_IMPL_RAND_S (NYI)
  //    the system RNG is implemented by calling the rand_s() function
  //    (Windows only).
@@ -142,6 +152,23 @@

  #if defined(__linux__) || defined(MR_SOLARIS) || defined(_AIX)
      #define ML_SYSRAND_IMPL_URANDOM
+#elif defined(__OpenBSD__) || defined(__NetBSD__)
+    #define ML_SYSRAND_IMPL_ARC4RANDOM
+#elif __FreeBSD__ >= 12
+    // arc4random() on FreeBSD used RC4 until version 12.
+    #define ML_SYSRAND_IMPL_ARC4RANDOM
+#elif defined(__FreeBSD__)
+    #define ML_SYSRAND_IMPL_URANDOM
+#elif defined(MR_MAC_OSX)
+   // arc4random() on macOS used RC4 until version 10.12.
+   // XXX this will be unnecessary when we stop supporting versions
+   // of macOS before 10.12.
+   #if defined(MAC_OS_X_VERSION_10_12) && \
+        MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_12
+      #define ML_SYSRAND_IMPL_ARC4RANDOM
+   #else
+      #define ML_SYSRAND_IMPL_URANDOM
+   #endif
  #else
      #define ML_SYSRAND_IMPL_NONE
  #endif
@@ -158,7 +185,7 @@
  // When succeeded is MR_TRUE, returns a handle through which the system
  // RNG can be accessed; err_msg will point to the empty string in this case.
  // When succeeded is MR_FALSE, the value return is not a valid handle and
-// err_msg will point ot a string (on the Mercury heap) describing why a handle
+// err_msg will point to a string (on the Mercury heap) describing why a handle
  // for the system RNG could not be acquired.
  //
  extern ML_SystemRandomHandle ML_random_open(MR_Bool *succeeded,
@@ -623,6 +650,31 @@ ML_random_generate_bytes(ML_SystemRandomHandle handle,
      return MR_TRUE;
  }

+#elif defined(ML_SYSRAND_IMPL_ARC4RANDOM)
+
+ML_SystemRandomHandle
+ML_random_open(MR_Bool *succeeded, MR_String *err_msg)
+{
+    *succeeded = MR_TRUE;
+    *err_msg = MR_make_string_const(\"\");
+    return 0;
+}
+
+MR_Bool
+ML_random_close(ML_SystemRandomHandle handle, MR_String *err_msg)
+{
+    *err_msg = MR_make_string_const(\"\");
+    return MR_TRUE;
+}
+
+MR_Bool
+ML_random_generate_bytes(ML_SystemRandomHandle handle,
+    unsigned char *buffer, size_t len, MR_String *err_msg)
+{
+    arc4random_buf(buffer, len);
+    return MR_TRUE;
+}
+
  #else // ML_SYSRAND_IMPL_NONE

  ML_SystemRandomHandle