Fix race condition between signal handler and main thread.

runtime/bin/fdutils_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/ioctl.h>
 
 #include "bin/fdutils.h"
@@ skipping 24 matching lines @@
   }
   *is_blocking = (status & O_NONBLOCK) == 0;
   return true;
 }
 
 
 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
   ASSERT(available >= 0);
 #endif
   return static_cast<intptr_t>(available);
 }
 
 
 ssize_t FDUtils::ReadFromBlocking(int fd, void* buffer, size_t count) {
@@ skipping 44 matching lines @@
       ASSERT(errno != EWOULDBLOCK);
       return -1;
     } else {
       ASSERT(bytes_written > 0);
       remaining -= bytes_written;
       buffer_pos += bytes_written;
     }
   }
   return count;
 }