core: IoLogger: don't set O_CLOEXEC on standard handles

nested_test was failing due to the recent change to centralize
O_CLOEXEC, since stdout and stderr were being marked as non-inheritable.
That meant child processes would start with no stdout/stderr, triggering
a race between Waker opening its pipes, and IoLogger dup2'ing its pipes
over the stdio handles.

Since the stdio handles were closed, Waker would receive one of them as
one end of its pipe, and consequently have it overwritten by IoLogger.

When IoLogger dups over the top of fd 2, it becomes possible for
Waker.on_read() to be called due to pipe's other end to be closed,
causing an OSError exception with errno EAGAIN to appear.

mitogen/core.py

@@ -656,11 +656,12 @@ class LogHandler(logging.Handler):
 
 
 class Side(object):
-    def __init__(self, stream, fd, keep_alive=True):
+    def __init__(self, stream, fd, cloexec=True, keep_alive=True):
         self.stream = stream
         self.fd = fd
         self.keep_alive = keep_alive
-        set_cloexec(fd)
+        if cloexec:
+            set_cloexec(fd)
         set_nonblock(fd)
 
     def __repr__(self):
@@ -1053,14 +1054,13 @@ class IoLogger(BasicStream):
         self._broker = broker
         self._name = name
         self._log = logging.getLogger(name)
-
         self._rsock, self._wsock = socket.socketpair()
         os.dup2(self._wsock.fileno(), dest_fd)
         set_cloexec(self._rsock.fileno())
         set_cloexec(self._wsock.fileno())
 
         self.receive_side = Side(self, self._rsock.fileno())
-        self.transmit_side = Side(self, dest_fd)
+        self.transmit_side = Side(self, dest_fd, cloexec=False)
         self._broker.start_receive(self)
 
     def __repr__(self):