Sync getentropy() checks with use-builtin-arc4random checks

Without this, we actually fail to build a library that includes the
bultin getentropy when compiling for 10.11 on 10.12.

1 files changed, 34 insertions, 1 deletions

diff --git a/m4/check-os-options.m4 b/m4/check-os-options.m4
index f2ff57f..1a7b940 100644
--- a/m4/check-os-options.m4
+++ b/m4/check-os-options.m4
@@ -17,10 +17,43 @@ case $host_os in
         *darwin*)
                 HOST_OS=darwin
                 HOST_ABI=macosx
+                #
+                # Don't use arc4random on systems before 10.12 because of
                 # weak seed on failure to open /dev/random, based on latest
                 # public source:
                 # http://www.opensource.apple.com/source/Libc/Libc-997.90.3/gen/FreeBSD/arc4random.c
-                USE_BUILTIN_ARC4RANDOM=yes
+                #
+                # We use the presence of getentropy() to detect 10.12. The
+                # following check take into account that:
+                #
+                #   - iOS <= 10.1 fails because of missing getentropy and
+                #     hence they miss sys/random.h
+                #
+                #   - in macOS 10.12 getentropy is not tagged as introduced in
+                #     10.12 so we cannot use it for target < 10.12
+                #
+                AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#include <AvailabilityMacros.h>
+#include <unistd.h>
+#include <sys/random.h>  /* Systems without getentropy() should die here */
+
+/* Based on: https://gitweb.torproject.org/tor.git/commit/?id=16fcbd21 */
+#ifndef MAC_OS_X_VERSION_10_12
+#  define MAC_OS_X_VERSION_10_12 101200
+#endif
+#if defined(MAC_OS_X_VERSION_MIN_REQUIRED)
+#  if MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12
+#    error "Running on Mac OSX 10.11 or earlier"
+#  endif
+#endif
+                       ]], [[
+char buf[1]; getentropy(buf, 1);
+                                           ]])],
+                       [ USE_BUILTIN_ARC4RANDOM=no ],
+                       [ USE_BUILTIN_ARC4RANDOM=yes ]
+                )
+                AC_MSG_CHECKING([whether to use builtin arc4random])
+                AC_MSG_RESULT([$USE_BUILTIN_ARC4RANDOM])
                 # Not available on iOS
                 AC_CHECK_HEADER([arpa/telnet.h], [], [BUILD_NC=no])
                 ;;