issue #155: core: prevent set_cloexec() use on standard handles

mitogen/core.py

@@ -185,6 +185,7 @@ def is_blacklisted_import(importer, fullname):
 
 def set_cloexec(fd):
     flags = fcntl.fcntl(fd, fcntl.F_GETFD)
+    assert fd > 2
     fcntl.fcntl(fd, fcntl.F_SETFD, flags | fcntl.FD_CLOEXEC)