python-peps/pep-0475.txt

PEP: 475
Title: Retry system calls failing with EINTR
Version: $Revision$
Last-Modified: $Date$
Author: Charles-François Natali <cf.natali@gmail.com>, Victor Stinner <victor.stinner@gmail.com>
BDFL-Delegate: Antoine Pitrou <solipsis@pitrou.net>
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 29-July-2014
Python-Version: 3.5
Resolution: https://mail.python.org/pipermail/python-dev/2015-February/138018.html


Abstract
========

System call wrappers provided in the standard library should be retried
automatically when they fail with ``EINTR``, to relieve application code
from the burden of doing so.

By system calls, we mean the functions exposed by the standard C library
pertaining to I/O or handling of other system resources.


Rationale
=========

Interrupted system calls
------------------------

On POSIX systems, signals are common.  Code calling system calls must be
prepared to handle them.  Examples of signals:

* The most common signal is ``SIGINT``, the signal sent when CTRL+c is
  pressed. By default, Python raises a ``KeyboardInterrupt`` exception
  when this signal is received.
* When running subprocesses, the ``SIGCHLD`` signal is sent when a
  child process exits.
* Resizing the terminal sends the ``SIGWINCH`` signal to the
  applications running in the terminal.
* Putting the application in background (ex: press CTRL-z and then
  type the ``bg`` command) sends the ``SIGCONT`` signal.

Writing a C signal handler is difficult: only "async-signal-safe"
functions can be called (for example, ``printf()`` and ``malloc()``
are not async-signal safe), and there are issues with reentrancy.
Therefore, when a signal is received by a process during the execution
of a system call, the system call can fail with the ``EINTR`` error to
give the program an opportunity to handle the signal without the
restriction on signal-safe functions.

This behaviour is system-dependent: on certain systems, using the
``SA_RESTART`` flag, some system calls are retried automatically instead
of failing with ``EINTR``.  Regardless, Python's ``signal.signal()``
function clears the ``SA_RESTART`` flag when setting the signal handler:
all system calls will probably fail with ``EINTR`` in Python.

Since receiving a signal is a non-exceptional occurrence, robust POSIX code
must be prepared to handle ``EINTR`` (which, in most cases, means
retry in a loop in the hope that the call eventually succeeds).
Without special support from Python, this can make application code
much more verbose than it needs to be.


Status in Python 3.4
--------------------

In Python 3.4, handling the ``InterruptedError`` exception (``EINTR``'s
dedicated exception class) is duplicated at every call site on a case by
case basis.  Only a few Python modules actually handle this exception,
and fixes usually took several years to cover a whole module.  Example of
code retrying ``file.read()`` on ``InterruptedError``::

    while True:
        try:
            data = file.read(size)
            break
        except InterruptedError:
            continue

List of Python modules in the standard library which handle
``InterruptedError``:

* ``asyncio``
* ``asyncore``
* ``io``, ``_pyio``
* ``multiprocessing``
* ``selectors``
* ``socket``
* ``socketserver``
* ``subprocess``

Other programming languages like Perl, Java and Go retry system calls
failing with ``EINTR`` at a lower level, so that libraries and applications
needn't bother.


Use Case 1: Don't Bother With Signals
-------------------------------------

In most cases, you don't want to be interrupted by signals and you
don't expect to get ``InterruptedError`` exceptions.  For example, do
you really want to write such complex code for a "Hello World"
example?

::

    while True:
        try:
            print("Hello World")
            break
        except InterruptedError:
            continue

``InterruptedError`` can happen in unexpected places. For example,
``os.close()`` and ``FileIO.close()`` may raise ``InterruptedError``:
see the article `close() and EINTR
<http://alobbs.com/post/54503240599/close-and-eintr>`_.

The `Python issues related to EINTR`_ section below gives examples of
bugs caused by ``EINTR``.

The expectation in this use case is that Python hides the
``InterruptedError`` and retries system calls automatically.


Use Case 2: Be notified of signals as soon as possible
------------------------------------------------------

Sometimes yet, you expect some signals and you want to handle them as
soon as possible.  For example, you may want to immediately quit a
program using the ``CTRL+c`` keyboard shortcut.

Besides, some signals are not interesting and should not disrupt the
application.  There are two options to interrupt an application on
only *some* signals:

* Set up a custom signal signal handler which raises an exception, such as
  ``KeyboardInterrupt`` for ``SIGINT``.
* Use a I/O multiplexing function like ``select()`` together with Python's
  signal wakeup file descriptor: see the function ``signal.set_wakeup_fd()``.

The expectation in this use case is for the Python signal handler to be
executed timely, and the system call to fail if the handler raised an
exception -- otherwise restart.


Proposal
========

This PEP proposes to handle EINTR and retries at the lowest level, i.e.
in the wrappers provided by the stdlib (as opposed to higher-level
libraries and applications).

Specifically, when a system call fails with ``EINTR``, its Python wrapper
must call the given signal handler (using ``PyErr_CheckSignals()``).
If the signal handler raises an exception, the Python wrapper bails out
and fails with the exception.

If the signal handler returns successfully, the Python wrapper retries the
system call automatically.  If the system call involves a timeout parameter,
the timeout is recomputed.

Modified functions
------------------

Example of standard library functions that need to be modified to comply
with this PEP:

* ``open()``, ``os.open()``
* ``os.read()``, ``io.FileIO.read()``, ``io.FileIO.readinto()``
* ``os.write()``, ``io.FileIO.write()``
* ``os.waitpid()``
* ``socket.accept()``
* ``socket.connect()``
* ``socket.recv()``, ``socket.recv_into()``
* ``socket.recv_from()``
* ``socket.send()``
* ``socket.sendto()``
* ``time.sleep()``
* ``select.select()``
* ``select.poll()``
* ``select.epoll.poll()``
* ``select.devpoll.poll()``
* ``select.kqueue.control()``
* ``selectors.SelectSelector.select()`` and other selector classes

(note: the ``selector`` module already retries on ``InterruptedError``, but it
doesn't recompute the timeout yet)

``os.close`` and ``close()`` methods are a special case: they will ignore
EINTR instead of retrying.  The reason is complex but involves behaviour
under Linux and the fact that `the file descriptor may really be closed
<http://lwn.net/Articles/576478/>`_ even if EINTR is returned.


InterruptedError handling
-------------------------

Since interrupted system calls are automatically retried, the
``InterruptedError`` exception should not occur anymore when calling those
system calls.  Therefore, manual handling of ``InterruptedError`` as
described in `Status in Python 3.4`_ can be removed, which will simplify
standard library code.


Backward compatibility
======================

Applications relying on the fact that system calls are interrupted
with ``InterruptedError`` will hang.  The authors of this PEP don't
think that such applications exist, since they would be exposed to
other issues such as race conditions (there is an opportunity for deadlock
if the signal comes before the system call).  Besides, such code would
be non-portable.

In any case, those applications must be fixed to handle signals differently,
to have a reliable behaviour on all platforms and all Python versions.
A possible strategy is to set up a signal handler raising a well-defined
exception, or use a wakeup file descriptor.

For applications using event loops, ``signal.set_wakeup_fd()`` is the
recommanded option to handle signals.  Python's low-level signal handler
will write signal numbers into the file descriptor and the event loop
will be awaken to read them.  The event loop can handle those signals
without the restriction of signal handlers (for example, the loop can
be woken up in any thread, not just the main thread).


Appendix
========

Wakeup file descriptor
----------------------

Since Python 3.3, ``signal.set_wakeup_fd()`` writes the signal number
into the file descriptor, whereas it only wrote a null byte before.
It becomes possible to distinguish between signals using the wakeup file
descriptor.

Linux has a ``signalfd()`` system call which provides more information on
each signal.  For example, it's possible to know the pid and uid who sent
the signal.  This function is not exposed in Python yet (see
`issue 12304 <http://bugs.python.org/issue12304>`_).

On Unix, the ``asyncio`` module uses the wakeup file descriptor to
wake up its event loop.


Multithreading
--------------

A C signal handler can be called from any thread, but Python
signal handlers will always be called in the main Python thread.

Python's C API provides the ``PyErr_SetInterrupt()`` function which calls
the ``SIGINT`` signal handler in order to interrupt the main Python thread.


Signals on Windows
------------------

Control events
^^^^^^^^^^^^^^

Windows uses "control events":

* ``CTRL_BREAK_EVENT``: Break (``SIGBREAK``)
* ``CTRL_CLOSE_EVENT``: Close event
* ``CTRL_C_EVENT``: CTRL+C (``SIGINT``)
* ``CTRL_LOGOFF_EVENT``: Logoff
* ``CTRL_SHUTDOWN_EVENT``: Shutdown

The `SetConsoleCtrlHandler() function
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms686016%28v=vs.85%29.aspx>`_
can be used to install a control handler.

The ``CTRL_C_EVENT`` and ``CTRL_BREAK_EVENT`` events can be sent to a
process using the `GenerateConsoleCtrlEvent() function
<http://msdn.microsoft.com/en-us/library/windows/desktop/ms683155%28v=vs.85%29.aspx>`_.
This function is exposed in Python as ``os.kill()``.


Signals
^^^^^^^

The following signals are supported on Windows:

* ``SIGABRT``
* ``SIGBREAK`` (``CTRL_BREAK_EVENT``): signal only available on Windows
* ``SIGFPE``
* ``SIGILL``
* ``SIGINT`` (``CTRL_C_EVENT``)
* ``SIGSEGV``
* ``SIGTERM``


SIGINT
^^^^^^

The default Python signal handler for ``SIGINT`` sets a Windows event
object: ``sigint_event``.

``time.sleep()`` is implemented with ``WaitForSingleObjectEx()``, it
waits for the ``sigint_event`` object using ``time.sleep()`` parameter
as the timeout.  So the sleep can be interrupted by ``SIGINT``.

``_winapi.WaitForMultipleObjects()`` automatically adds
``sigint_event`` to the list of watched handles, so it can also be
interrupted.

``PyOS_StdioReadline()`` also used ``sigint_event`` when ``fgets()``
failed to check if Ctrl-C or Ctrl-Z was pressed.


Links
-----

Misc
^^^^

* `glibc manual: Primitives Interrupted by Signals
  <http://www.gnu.org/software/libc/manual/html_node/Interrupted-Primitives.html>`_
* `Bug #119097 for perl5: print returning EINTR in 5.14
  <https://rt.perl.org/Public/Bug/Display.html?id=119097>`_.


Python issues related to EINTR
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The main issue is: `handle EINTR in the stdlib
<http://bugs.python.org/issue18885>`_.

Open issues:

* `Add a new signal.set_wakeup_socket() function
  <http://bugs.python.org/issue22018>`_
* `signal.set_wakeup_fd(fd): set the fd to non-blocking mode
  <http://bugs.python.org/issue22042>`_
* `Use a monotonic clock to compute timeouts
  <http://bugs.python.org/issue22043>`_
* `sys.stdout.write on OS X is not EINTR safe
  <http://bugs.python.org/issue22007>`_
* `platform.uname() not EINTR safe
  <http://bugs.python.org/issue21772>`_
* `asyncore does not handle EINTR in recv, send, connect, accept,
  <http://bugs.python.org/issue11266>`_
* `socket.create_connection() doesn't handle EINTR properly
  <http://bugs.python.org/issue20611>`_

Closed issues:

* `Interrupted system calls are not retried
  <http://bugs.python.org/issue9867>`_
* `Solaris: EINTR exception in select/socket calls in telnetlib
  <http://bugs.python.org/issue1049450>`_
* `subprocess: Popen.communicate() doesn't handle EINTR in some cases
  <http://bugs.python.org/issue12493>`_
* `multiprocessing.util._eintr_retry doen't recalculate timeouts
  <http://bugs.python.org/issue12338>`_
* `file readline, readlines & readall methods can lose data on EINTR
  <http://bugs.python.org/issue12268>`_
* `multiprocessing BaseManager serve_client() does not check EINTR on recv
  <http://bugs.python.org/issue17097>`_
* `selectors behaviour on EINTR undocumented
  <http://bugs.python.org/issue19849>`_
* `asyncio: limit EINTR occurrences with SA_RESTART
  <http://bugs.python.org/issue19850>`_
* `smtplib.py socket.create_connection() also doesn't handle EINTR properly
  <http://bugs.python.org/issue21602>`_
* `Faulty RESTART/EINTR handling in Parser/myreadline.c
  <http://bugs.python.org/issue11650>`_
* `test_httpservers intermittent failure, test_post and EINTR
  <http://bugs.python.org/issue3771>`_
* `os.spawnv(P_WAIT, ...) on Linux doesn't handle EINTR
  <http://bugs.python.org/issue686667>`_
* `asyncore fails when EINTR happens in pol
  <http://bugs.python.org/issue517554>`_
* `file.write and file.read don't handle EINTR
  <http://bugs.python.org/issue10956>`_
* `socket.readline() interface doesn't handle EINTR properly
  <http://bugs.python.org/issue1628205>`_
* `subprocess is not EINTR-safe
  <http://bugs.python.org/issue1068268>`_
* `SocketServer doesn't handle syscall interruption
  <http://bugs.python.org/issue7978>`_
* `subprocess deadlock when read() is interrupted
  <http://bugs.python.org/issue17367>`_
* `time.sleep(1): call PyErr_CheckSignals() if the sleep was interrupted
  <http://bugs.python.org/issue12462>`_
* `siginterrupt with flag=False is reset when signal received
  <http://bugs.python.org/issue8354>`_
* `need siginterrupt()  on Linux - impossible to do timeouts
  <http://bugs.python.org/issue1089358>`_
* `[Windows] Can not interrupt time.sleep()
  <http://bugs.python.org/issue581232>`_

Python issues related to signals
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Open issues:

* `signal.default_int_handler should set signal number on the raised
  exception <http://bugs.python.org/issue17182>`_
* `expose signalfd(2) in the signal module
  <http://bugs.python.org/issue12304>`_
* `missing return in win32_kill?
  <http://bugs.python.org/issue14484>`_
* `Interrupts are lost during readline PyOS_InputHook processing
  <http://bugs.python.org/issue3180>`_
* `cannot catch KeyboardInterrupt when using curses getkey()
  <http://bugs.python.org/issue1687125>`_
* `Deferred KeyboardInterrupt in interactive mode
  <http://bugs.python.org/issue16151>`_

Closed issues:

* `sys.interrupt_main()
  <http://bugs.python.org/issue753733>`_


Copyright
=========

This document has been placed in the public domain.



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