PEP 446: update after Guido's review

pep-0446.txt

@@ -24,6 +24,10 @@ fixes also a race condition in multi-threaded applications on operating
 systems supporting atomic flags to create non-inheritable file
 descriptors.
 
+We are aware of the code breakage this is likely to cause, and doing it
+anyway for the good of mankind. (Details in the section "Backward
+Compatibility" below.)
+
 
 Rationale
 =========
@@ -79,6 +83,11 @@ the *dwFlags* field of the ``STARTUPINFO`` structure and the
 ``TRUE``. So when at least one standard stream is replaced, all
 inheritable handles are inherited by the child process.
 
+The default value of the *close_fds* parameter of ``subprocess`` process
+is ``True`` (``bInheritHandles=FALSE``) if *stdin*, *stdout* and
+*stderr* parameters are ``None``, ``False`` (``bInheritHandles=TRUE``)
+otherwise.
+
 See also:
 
 * `Handle Inheritance
@@ -430,12 +439,6 @@ MAXFD is the maximum number of file descriptors, even if there are only
 few open file descriptors. This maximum can be read using:
 ``os.sysconf("SC_OPEN_MAX")``.
 
-There is no portable nor reliable function to close all open file
-descriptors between ``fork()`` and ``execv()``. Another thread may
-create an inheritable file descriptors while we are closing existing
-file descriptors. Holding the CPython GIL reduces the risk of the race
-condition.
-
 The operation can be slow if MAXFD is large. For example, on a FreeBSD
 buildbot with ``MAXFD=655,000``, the operation took 300 ms: see
 `issue #11284: slow close file descriptors
@@ -445,10 +448,6 @@ On Linux, Python 3.3 gets the list of all open file descriptors from
 ``/proc/<PID>/fd/``, and so performances depends on the number of open
 file descriptors, not on MAXFD.
 
-FreeBSD, OpenBSD and Solaris provide a ``closefrom()`` function. It
-cannot be used by the ``subprocess`` module when the *pass_fds*
-parameter is a non-empty list of file descriptors.
-
 See also:
 
 * `Python issue #1663329 <http://bugs.python.org/issue1663329>`_:
@@ -485,6 +484,8 @@ descriptors non-inheritable by default:
  * ``socket.socket.fromfd()``
  * ``socket.socketpair()``
 
+``os.dup2()`` still creates inheritable by default, see below.
+
 When available, atomic flags are used to make file descriptors
 non-inheritable. The atomicity is not guaranteed because a fallback is
 required when atomic flags are not available.
@@ -518,14 +519,20 @@ New methods:
 Other Changes
 -------------
 
-* On UNIX, subprocess makes file descriptors of the *pass_fds* parameter
-  inheritable. The file descriptor is made inheritable in the child
-  process after the ``fork()`` and before ``execv()``, so the inheritable
-  flag of file descriptors is unchanged in the parent process.
+On UNIX, subprocess makes file descriptors of the *pass_fds* parameter
+inheritable. The file descriptor is made inheritable in the child
+process after the ``fork()`` and before ``execv()``, so the inheritable
+flag of file descriptors is unchanged in the parent process.
 
-* ``os.dup2()`` has a new optional *inheritable* parameter:
-  ``os.dup2(fd, fd2, inheritable=True)``. *fd2* is created inheritable
-  by default, but non-inheritable if *inheritable* is ``False``.
+``os.dup2()`` has a new optional *inheritable* parameter: ``os.dup2(fd,
+fd2, inheritable=True)``. *fd2* is created inheritable by default, but
+non-inheritable if *inheritable* is ``False``.
+
+``os.dup2()`` behaves differently than ``os.dup()`` because the most
+common use case of ``os.dup2()`` is to replace the file descriptors of
+the standard streams: ``stdin`` (``0``), ``stdout`` (``1``) and
+``stdout`` (``2``). Standard streams are expected to be inherited by
+child processes.
 
 Since Python should only create non-inheritable file descriptors, it is
 safe to use subprocess with the *close_fds* parameter set to ``False``.