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Apr 7, 2012, 2:06 PM

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peps: Publish pep-419

http://hg.python.org/peps/rev/fabdbf0c63d6
changeset: 4203:fabdbf0c63d6
user: Andrew Svetlov <andrew.svetlov [at] gmail>
date: Sun Apr 08 00:06:30 2012 +0300
summary:
Publish pep-419

files:
pep-0419.txt | 459 +++++++++++++++++++++++++++++++++++++++
1 files changed, 459 insertions(+), 0 deletions(-)


diff --git a/pep-0419.txt b/pep-0419.txt
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+PEP: 419
+Title: Protecting cleanup statements from interruptions
+Version: $Revision$
+Last-Modified: $Date$
+Author: Paul Colomiets <paul [at] colomiets>
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 06-Apr-2012
+Python-Version: 3.3
+
+
+Abstract
+========
+
+This PEP proposes a way to protect python code from being interrupted inside
+finally statement or context manager.
+
+
+Rationale
+=========
+
+Python has two nice ways to do cleanup. One is a ``finally`` statement
+and the other is context manager (or ``with`` statement). Although,
+neither of them is protected from ``KeyboardInterrupt`` or
+``generator.throw()``. For example::
+
+ lock.acquire()
+ try:
+ print('starting')
+ do_someting()
+ finally:
+ print('finished')
+ lock.release()
+
+If ``KeyboardInterrupt`` occurs just after ``print`` function is
+executed, lock will not be released. Similarly the following code
+using ``with`` statement is affected::
+
+ from threading import Lock
+
+ class MyLock:
+
+ def __init__(self):
+ self._lock_impl = lock
+
+ def __enter__(self):
+ self._lock_impl.acquire()
+ print("LOCKED")
+
+ def __exit__(self):
+ print("UNLOCKING")
+ self._lock_impl.release()
+
+ lock = MyLock()
+ with lock:
+ do_something
+
+If ``KeyboardInterrupt`` occurs near any of the ``print`` statements,
+lock will never be released.
+
+
+Coroutine Use Case
+------------------
+
+Similar case occurs with coroutines. Usually coroutine libraries want
+to interrupt coroutine with a timeout. There is a
+``generator.throw()`` method for this use case, but there are no
+method to know is it currently yielded from inside a ``finally``.
+
+Example that uses yield-based coroutines follows. Code looks
+similar using any of the popular coroutine libraries Monocle [1]_,
+Bluelet [2]_, or Twisted [3]_. ::
+
+ def run_locked()
+ yield connection.sendall('LOCK')
+ try:
+ yield do_something()
+ yield do_something_else()
+ finally:
+ yield connection.sendall('UNLOCK')
+
+ with timeout(5):
+ yield run_locked()
+
+In the example above ``yield something`` means pause executing current
+coroutine and execute coroutine ``something`` until it finished
+execution. So that library keeps stack of generators itself. The
+``connection.sendall`` waits until socket is writable and does thing
+similar to what ``socket.sendall`` does.
+
+The ``with`` statement ensures that all that code is executed within 5
+seconds timeout. It does so by registering a callback in main loop,
+which calls ``generator.throw()`` to the top-most frame in the
+coroutine stack when timeout happens.
+
+The ``greenlets`` extension works in similar way, except it doesn't
+need ``yield`` to enter new stack frame. Otherwise considerations are
+similar.
+
+
+Specification
+=============
+
+Frame Flag 'f_in_cleanup'
+-------------------------
+
+A new flag on frame object is proposed. It is set to ``True`` if this
+frame is currently in the ``finally`` suite. Internally it must be
+implemented as a counter of nested finally statements currently
+executed.
+
+The internal counter is also incremented when entering ``WITH_SETUP``
+bytecode and ``WITH_CLEANUP`` bytecode, and is decremented when
+leaving that bytecode. This allows to protect ``__enter__`` and
+``__exit__`` methods too.
+
+
+Function 'sys.setcleanuphook'
+-----------------------------
+
+A new function for the ``sys`` module is proposed. This function sets
+a callback which is executed every time ``f_in_cleanup`` becomes
+``False``. Callbacks gets ``frame`` as it's sole argument so it can
+get some evindence where it is called from.
+
+The setting is thread local and is stored inside ``PyThreadState``
+structure.
+
+
+Inspect Module Enhancements
+---------------------------
+
+Two new functions are proposed for ``inspect`` module:
+``isframeincleanup`` and ``getcleanupframe``.
+
+``isframeincleanup`` given ``frame`` object or ``generator`` object as
+sole argument returns the value of ``f_in_cleanup`` attribute of a
+frame itself or of the ``gi_frame`` attribute of a generator.
+
+``getcleanupframe`` given ``frame`` object as sole argument returns
+the innermost frame which has true value of ``f_in_cleanup`` or
+``None`` if no frames in the stack has the attribute set. It starts to
+inspect from specified frame and walks to outer frames using
+``f_back`` pointers, just like ``getouterframes`` does.
+
+
+Example
+=======
+
+Example implementation of ``SIGINT`` handler that interrupts safely
+might look like::
+
+ import inspect, sys, functools
+
+ def sigint_handler(sig, frame)
+ if inspect.getcleanupframe(frame) is None:
+ raise KeyboardInterrupt()
+ sys.setcleanuphook(functools.partial(sigint_handler, 0))
+
+Coroutine example is out of scope of this document, because it's
+implemention depends very much on a trampoline (or main loop) used by
+coroutine library.
+
+
+Unresolved Issues
+=================
+
+Interruption Inside With Statement Expression
+---------------------------------------------
+
+Given the statement::
+
+ with open(filename):
+ do_something()
+
+Python can be interrupted after ``open`` is called, but before
+``SETUP_WITH`` bytecode is executed. There are two possible decisions:
+
+* Protect expression inside ``with`` statement. This would need
+ another bytecode, since currently there is no delimiter at the start
+ of ``with`` expression
+
+* Let user write a wrapper if he considers it's important for his
+ use-case. Safe wrapper code might look like the following::
+
+ class FileWrapper(object):
+
+ def __init__(self, filename, mode):
+ self.filename = filename
+ self.mode = mode
+
+ def __enter__(self):
+ self.file = open(self.filename, self.mode)
+
+ def __exit__(self):
+ self.file.close()
+
+ Alternatively it can be written using context manager::
+
+ @contextmanager
+ def open_wrapper(filename, mode):
+ file = open(filename, mode)
+ try:
+ yield file
+ finally:
+ file.close()
+
+ This code is safe, as first part of generator (before yield) is
+ executed inside ``WITH_SETUP`` bytecode of caller
+
+
+Exception Propagation
+---------------------
+
+Sometimes ``finally`` block or ``__enter__/__exit__`` method can be
+exited with an exception. Usually it's not a problem, since more
+important exception like ``KeyboardInterrupt`` or ``SystemExit``
+should be thrown instead. But it may be nice to be able to keep
+original exception inside a ``__context__`` attibute. So cleanup hook
+signature may grow an exception argument::
+
+ def sigint_handler(sig, frame)
+ if inspect.getcleanupframe(frame) is None:
+ raise KeyboardInterrupt()
+ sys.setcleanuphook(retry_sigint)
+
+ def retry_sigint(frame, exception=None):
+ if inspect.getcleanupframe(frame) is None:
+ raise KeyboardInterrupt() from exception
+
+.. note::
+
+ No need to have three arguments like in ``__exit__`` method since
+ we have a ``__traceback__`` attribute in exception in Python 3.x
+
+Although, this will set ``__cause__`` for the exception, which is not
+exactly what's intended. So some hidden interpeter logic may be used
+to put ``__context__`` attribute on every exception raised in cleanup
+hook.
+
+
+Interruption Between Acquiring Resource and Try Block
+-----------------------------------------------------
+
+Example from the first section is not totally safe. Let's look closer::
+
+ lock.acquire()
+ try:
+ do_something()
+ finally:
+ lock.release()
+
+There is no way it can be fixed without modifying the code. The actual
+fix of this code depends very much on use case.
+
+Usually code can be fixed using a ``with`` statement::
+
+ with lock:
+ do_something()
+
+Although, for coroutines you usually can't use ``with`` statement
+because you need to ``yield`` for both aquire and release operations.
+So code might be rewritten as following::
+
+ try:
+ yield lock.acquire()
+ do_something()
+ finally:
+ yield lock.release()
+
+The actual lock code might need more code to support this use case,
+but implementation is usually trivial, like check if lock has been
+acquired and unlock if it is.
+
+
+Setting Interruption Context Inside Finally Itself
+--------------------------------------------------
+
+Some coroutine libraries may need to set a timeout for the finally
+clause itself. For example::
+
+ try:
+ do_something()
+ finally:
+ with timeout(0.5):
+ try:
+ yield do_slow_cleanup()
+ finally:
+ yield do_fast_cleanup()
+
+With current semantics timeout will either protect
+the whole ``with`` block or nothing at all, depending on the
+implementation of a library. What the author is intended is to treat
+``do_slow_cleanup`` as an ordinary code, and ``do_fast_cleanup`` as a
+cleanup (non-interruptible one).
+
+Similar case might occur when using greenlets or tasklets.
+
+This case can be fixed by exposing ``f_in_cleanup`` as a counter, and
+by calling cleanup hook on each decrement. Corouting library may then
+remember the value at timeout start, and compare it on each hook
+execution.
+
+But in practice example is considered to be too obscure to take in
+account.
+
+
+Alternative Python Implementations Support
+==========================================
+
+We consider ``f_in_cleanup`` and implementation detail. The actual
+implementation may have some fake frame-like object passed to signal
+handler, cleanup hook and returned from ``getcleanupframe``. The only
+requirement is that ``inspect`` module functions work as expected on
+that objects. For this reason we also allow to pass a ``generator``
+object to a ``isframeincleanup`` function, this disables need to use
+``gi_frame`` attribute.
+
+It may need to be specified that ``getcleanupframe`` must return the
+same object that will be passed to cleanup hook at next invocation.
+
+
+Alternative Names
+=================
+
+Original proposal had ``f_in_finally`` flag. The original intention
+was to protect ``finally`` clauses. But as it grew up to protecting
+``__enter__`` and ``__exit__`` methods too, the ``f_in_cleanup``
+method seems better. Although ``__enter__`` method is not a cleanup
+routine, it at least relates to cleanup done by context managers.
+
+``setcleanuphook``, ``isframeincleanup`` and ``getcleanupframe`` can
+be unobscured to ``set_cleanup_hook``, ``is_frame_in_cleanup`` and
+``get_cleanup_frame``, althought they follow convention of their
+respective modules.
+
+
+Alternative Proposals
+=====================
+
+Propagating 'f_in_cleanup' Flag Automatically
+-----------------------------------------------
+
+This can make ``getcleanupframe`` unnecessary. But for yield based
+coroutines you need to propagate it yourself. Making it writable leads
+to somewhat unpredictable behavior of ``setcleanuphook``
+
+
+Add Bytecodes 'INCR_CLEANUP', 'DECR_CLEANUP'
+--------------------------------------------
+
+These bytecodes can be used to protect expression inside ``with``
+statement, as well as making counter increments more explicit and easy
+to debug (visible inside a disassembly). Some middle ground might be
+chosen, like ``END_FINALLY`` and ``SETUP_WITH`` imlicitly decrements
+counter (``END_FINALLY`` is present at end of ``with`` suite).
+
+Although, adding new bytecodes must be considered very carefully.
+
+
+Expose 'f_in_cleanup' as a Counter
+----------------------------------
+
+The original intention was to expose minimum needed functionality.
+Although, as we consider frame flag ``f_in_cleanup`` as an
+implementation detail, we may expose it as a counter.
+
+Similarly, if we have a counter we may need to have cleanup hook
+called on every counter decrement. It's unlikely have much performance
+impact as nested finally clauses are unlikely common case.
+
+
+Add code object flag 'CO_CLEANUP'
+---------------------------------
+
+As an alternative to set flag inside ``WITH_SETUP``, and
+``WITH_CLEANUP`` bytecodes we can introduce a flag ``CO_CLEANUP``.
+When interpreter starts to execute code with ``CO_CLEANUP`` set, it
+sets ``f_in_cleanup`` for the whole function body. This flag is set
+for code object of ``__enter__`` and ``__exit__`` special methods.
+Technically it might be set on functions called ``__enter__`` and
+``__exit__``.
+
+This seems to be less clear solution. It also covers the case where
+``__enter__`` and ``__exit__`` are called manually. This may be
+accepted either as feature or as a unnecessary side-effect (unlikely
+as a bug).
+
+It may also impose a problem when ``__enter__`` or ``__exit__``
+function are implemented in C, as there usually no frame to check for
+``f_in_cleanup`` flag.
+
+
+Have Cleanup Callback on Frame Object Itself
+----------------------------------------------
+
+Frame may be extended to have ``f_cleanup_callback`` which is called
+when ``f_in_cleanup`` is reset to 0. It would help to register
+different callbacks to different coroutines.
+
+Despite apparent beauty. This solution doesn't add anything. As there
+are two primary use cases:
+
+* Set callback in signal handler. The callback is inherently single
+ one for this case
+
+* Use single callback per loop for coroutine use case. And in almost
+ all cases there is only one loop per thread
+
+
+No Cleanup Hook
+---------------
+
+Original proposal included no cleanup hook specification. As there are
+few ways to achieve the same using current tools:
+
+* Use ``sys.settrace`` and ``f_trace`` callback. It may impose some
+ problem to debugging, and has big performance impact (although,
+ interrupting doesn't happen very often)
+
+* Sleep a bit more and try again. For coroutine library it's easy. For
+ signals it may be achieved using ``alert``.
+
+Both methods are considered too impractical and a way to catch exit
+from ``finally`` statement is proposed.
+
+
+References
+==========
+
+.. [1] Monocle
+ https://github.com/saucelabs/monocle
+
+.. [2] Bluelet
+ https://github.com/sampsyo/bluelet
+
+.. [3] Twisted: inlineCallbacks
+ http://twistedmatrix.com/documents/8.1.0/api/twisted.internet.defer.html
+
+.. [4] Original discussion
+ http://mail.python.org/pipermail/python-ideas/2012-April/014705.html
+
+
+Copyright
+=========
+
+This document has been placed in the public domain.
+
+
+
+..
+ Local Variables:
+ mode: indented-text
+ indent-tabs-mode: nil
+ sentence-end-double-space: t
+ fill-column: 70
+ coding: utf-8
+ End:

--
Repository URL: http://hg.python.org/peps

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