Work around sigaltstack() bug on Mac OS X when unregistering a signal stack

src/native_client/src/trusted/service_runtime/posix/nacl_signal.c

Index: src/native_client/src/trusted/service_runtime/posix/nacl_signal.c
diff --git a/src/native_client/src/trusted/service_runtime/posix/nacl_signal.c b/src/native_client/src/trusted/service_runtime/posix/nacl_signal.c
index 691329d336dfcd719e9d81b8fdcfffee38158498..2e68d5d7c741cbe51917d43188fbf74f7e5ea69b 100644
--- a/src/native_client/src/trusted/service_runtime/posix/nacl_signal.c
+++ b/src/native_client/src/trusted/service_runtime/posix/nacl_signal.c
@@ -146,7 +146,18 @@ void NaClSignalStackUnregister(void) {
    * fault, although that is unlikely.
    */
   stack_t st;
+#if NACL_OSX
+  /*
+   * This is a workaround for a bug in Mac OS X's libc, in which new
+   * versions of the sigaltstack() wrapper return ENOMEM if ss_size is
+   * less than MINSIGSTKSZ, even when ss_size should be ignored
+   * because we are unregistering the signal stack.
+   * See http://code.google.com/p/nativeclient/issues/detail?id=1053
+   */
+  st.ss_size = MINSIGSTKSZ;
+#else
   st.ss_size = 0;
+#endif
   st.ss_sp = NULL;
   st.ss_flags = SS_DISABLE;
   if (sigaltstack(&st, NULL) != 0) {