HTML version: http://legacy.python.org/dev/peps/pep-0475/
PEP: 475 Title: Retry system calls failing with EINTR Version: $Revision$ Last-Modified: $Date$ Author: Charles-François Natali firstname.lastname@example.org, Victor Stinner email@example.com Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 29-July-2014 Python-Version: 3.5
Retry system calls failing with the ``EINTR`` error and recompute timeout if needed.
Interrupted system calls ------------------------
On POSIX systems, signals are common. Your program must be prepared to handle them. Examples of signals:
* The most common signal is ``SIGINT``, 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 signal handler is difficult, only "async-signal safe" functions can be called. For example, ``printf()`` and ``malloc()`` are not async-signal safe. When a signal is sent to a process calling 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. Depending on the platform, on the system call and the ``SA_RESTART`` flag, the system call may or may not fail with ``EINTR``.
If the signal handler was set with the ``SA_RESTART`` flag set, the kernel retries some the system call instead of failing with ``EINTR``. For example, ``read()`` is retried, whereas ``select()`` is not retried. The Python function ``signal.signal()`` clears the ``SA_RESTART`` flag when setting the signal handler: all system calls should fail with ``EINTR`` in Python.
The problem is that handling ``EINTR`` should be done for all system calls. The problem is similar to handling errors in the C language which does not have exceptions: you must check all function returns to check for error, and usually duplicate the code checking for errors. Python does not have this issue, it uses exceptions to notify errors.
Current status --------------
Currently in Python, the code to handle the ``InterruptedError`` exception (``EINTR`` error) is duplicated on case by case. Only a few Python modules 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 of the standard library which handle ``InterruptedError``:
* ``asyncio`` * ``asyncore`` * ``io``, ``_pyio`` * ``multiprocessing`` * ``selectors`` * ``socket`` * ``socketserver`` * ``subprocess``
Other programming languages like Perl, Java and Go already retry system calls failing with ``EINTR``.
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 an "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()`` can raises ``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 is that Python hides the ``InterruptedError``: retry system calls failing with the ``EINTR`` error.
Use Case 2: Be notified of signals as soon as possible ------------------------------------------------------
Sometimes, you expect some signals and you want to handle them as soon as possible. For example, you may want to quit immediatly a program using the ``CTRL+c`` keyboard shortcut.
Some signals are not interesting and should not interrupt the the application. There are two options to only interrupt an application on some signals:
* Raise an exception in the signal handler, like ``KeyboardInterrupt`` for ``SIGINT`` * Use a I/O multiplexing function like ``select()`` with the Python signal "wakeup" file descriptor: see the function ``signal.set_wakeup_fd()``.
If a system call fails with ``EINTR``, Python must call signal handlers: call ``PyErr_CheckSignals()``. If a signal handler raises an exception, the Python function fails with the exception. Otherwise, the system call is retried. If the system call takes a timeout parameter, the timeout is recomputed.
Modified functions ------------------
Example of functions that need to be modified:
* ``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.
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 application exist.
If such applications exist, they are not portable and are subject to race conditions (deadlock if the signal comes before the system call). These applications must be fixed to handle signals differently, to have a reliable behaviour on all platforms and all Python versions. For example, use a signal handler which raises an exception, or use a wakeup file descriptor.
For applications using event loops, ``signal.set_wakeup_fd()`` is the recommanded option to handle signals. The signal handler writes signal numbers into the file descriptor and the event loop is awaken to read them. The event loop can handle these signals without the restriction of signal handlers.
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 handle different signals using the wakeup file descriptor.
Linux has a ``signalfd()`` 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 the `issue 12304 http://bugs.python.org/issue12304`_).
On Unix, the ``asyncio`` module uses the wakeup file descriptor to wake up its event loop.
A C signal handler can be called from any thread, but the Python signal handler should only be called in the main thread.
Python has a ``PyErr_SetInterrupt()`` function which calls the ``SIGINT`` signal handler to interrupt the Python main 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()``.
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``
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.
* `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`_.
* `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`_
* `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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* `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`_
* `sys.interrupt_main() http://bugs.python.org/issue753733`_
This document has been placed in the public domain.
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