Fix race condition between signal handler and main thread.

runtime/bin/fdutils_macos.cc

Index: runtime/bin/fdutils_macos.cc
diff --git a/runtime/bin/fdutils_macos.cc b/runtime/bin/fdutils_macos.cc
index 0fe89db9fc18673e1ed5847198bb61d0a0629877..ade4bbe15e016d14ffc2d586dabf83769f3ef8dd 100644
--- a/runtime/bin/fdutils_macos.cc
+++ b/runtime/bin/fdutils_macos.cc
@@ -42,6 +42,7 @@ intptr_t FDUtils::AvailableBytes(intptr_t fd) {
   int available;  // ioctl for FIONREAD expects an 'int*' argument.
   int result = TEMP_FAILURE_RETRY(ioctl(fd, FIONREAD, &available));
   if (result < 0) {
+    perror("ioctl FIONREAD failed");
     return result;
   }
 #ifdef DEBUG