python-peps/pep-0298.txt

PEP: 298
Title: The Locked Buffer Interface
Version: $Revision$
Last-Modified: $Date$
Author: Thomas Heller <theller@python.net>
Status: Withdrawn
Type: Standards Track
Created: 26-Jul-2002
Python-Version: 2.3
Post-History: 30-Jul-2002, 1-Aug-2002


Abstract

    This PEP proposes an extension to the buffer interface called the
    'locked buffer interface'.

    The locked buffer interface avoids the flaws of the 'old' buffer
    interface [1] as defined in Python versions up to and including
    2.2, and has the following semantics:

        The lifetime of the retrieved pointer is clearly defined and
        controlled by the client.

        The buffer size is returned as a 'size_t' data type, which
        allows access to large buffers on platforms where sizeof(int)
        != sizeof(void *).

    (Guido comments: This second sounds like a change we could also
    make to the "old" buffer interface, if we introduce another flag
    bit that's *not* part of the default flags.)


Specification

    The locked buffer interface exposes new functions which return the
    size and the pointer to the internal memory block of any python
    object which chooses to implement this interface.

    Retrieving a buffer from an object puts this object in a locked
    state during which the buffer may not be freed, resized, or
    reallocated.

    The object must be unlocked again by releasing the buffer if it's
    no longer used by calling another function in the locked buffer
    interface.  If the object never resizes or reallocates the buffer
    during its lifetime, this function may be NULL.  Failure to call
    this function (if it is != NULL) is a programming error and may
    have unexpected results.

    The locked buffer interface omits the memory segment model which
    is present in the old buffer interface - only a single memory
    block can be exposed.

    The memory blocks can be accessed without holding the global
    interpreter lock.


Implementation

    Define a new flag in Include/object.h:

        /* PyBufferProcs contains bf_acquirelockedreadbuffer,
           bf_acquirelockedwritebuffer, and bf_releaselockedbuffer */
        #define Py_TPFLAGS_HAVE_LOCKEDBUFFER (1L<<15)


    This flag would be included in Py_TPFLAGS_DEFAULT:

        #define Py_TPFLAGS_DEFAULT  ( \
                             ....
                             Py_TPFLAGS_HAVE_LOCKEDBUFFER | \
                             ....
                            0)


    Extend the PyBufferProcs structure by new fields in
    Include/object.h:

        typedef size_t (*acquirelockedreadbufferproc)(PyObject *,
                                                      const void **);
        typedef size_t (*acquirelockedwritebufferproc)(PyObject *,
                                                       void **);
        typedef void (*releaselockedbufferproc)(PyObject *);

        typedef struct {
            getreadbufferproc bf_getreadbuffer;
            getwritebufferproc bf_getwritebuffer;
            getsegcountproc bf_getsegcount;
            getcharbufferproc bf_getcharbuffer;
            /* locked buffer interface functions */
            acquirelockedreadbufferproc bf_acquirelockedreadbuffer;
            acquirelockedwritebufferproc bf_acquirelockedwritebuffer;
            releaselockedbufferproc bf_releaselockedbuffer;
        } PyBufferProcs;


    The new fields are present if the Py_TPFLAGS_HAVE_LOCKEDBUFFER
    flag is set in the object's type.

    The Py_TPFLAGS_HAVE_LOCKEDBUFFER flag implies the
    Py_TPFLAGS_HAVE_GETCHARBUFFER flag.

    The acquirelockedreadbufferproc and acquirelockedwritebufferproc
    functions return the size in bytes of the memory block on success,
    and fill in the passed void * pointer on success.  If these
    functions fail - either because an error occurs or no memory block
    is exposed - they must set the void * pointer to NULL and raise an
    exception.  The return value is undefined in these cases and
    should not be used.

    If calls to these functions succeed, eventually the buffer must be
    released by a call to the releaselockedbufferproc, supplying the
    original object as argument.  The releaselockedbufferproc cannot
    fail.  For objects that actually maintain an internal lock count
    it would be a fatal error if the releaselockedbufferproc function
    would be called too often, leading to a negative lock count.

    Similar to the 'old' buffer interface, any of these functions may
    be set to NULL, but it is strongly recommended to implement the
    releaselockedbufferproc function (even if it does nothing) if any
    of the acquireread/writelockedbufferproc functions are
    implemented, to discourage extension writers from checking for a
    NULL value and not calling it.

    These functions aren't supposed to be called directly, they are
    called through convenience functions declared in
    Include/abstract.h:

        int PyObject_AquireLockedReadBuffer(PyObject *obj,
                                            const void **buffer,
                                            size_t *buffer_len);

        int PyObject_AcquireLockedWriteBuffer(PyObject *obj,
                                              void **buffer,
                                              size_t *buffer_len);

        void PyObject_ReleaseLockedBuffer(PyObject *obj);

    The former two functions return 0 on success, set buffer to the
    memory location and buffer_len to the length of the memory block
    in bytes. On failure, or if the locked buffer interface is not
    implemented by obj, they return -1 and set an exception.

    The latter function doesn't return anything, and cannot fail.


Backward Compatibility

    The size of the PyBufferProcs structure changes if this proposal
    is implemented, but the type's tp_flags slot can be used to
    determine if the additional fields are present.


Reference Implementation

    An implementation has been uploaded to the SourceForge patch
    manager as http://www.python.org/sf/652857.


Additional Notes/Comments

    Python strings, unicode strings, mmap objects, and array objects
    would expose the locked buffer interface.

    mmap and array objects would actually enter a locked state while
    the buffer is active, this is not needed for strings and unicode
    objects.  Resizing locked array objects is not allowed and will
    raise an exception. Whether closing a locked mmap object is an
    error or will only be deferred until the lock count reaches zero
    is an implementation detail.

    Guido recommends:

        But I'm still very concerned that if most built-in types
        (e.g. strings, bytes) don't implement the release
        functionality, it's too easy for an extension to seem to work
        while forgetting to release the buffer.

        I recommend that at least some built-in types implement the
        acquire/release functionality with a counter, and assert that
        the counter is zero when the object is deleted -- if the
        assert fails, someone DECREF'ed their reference to the object
        without releasing it.  (The rule should be that you must own a
        reference to the object while you've acquired the object.)

        For strings that might be impractical because the string
        object would have to grow 4 bytes to hold the counter; but the
        new bytes object (PEP 296) could easily implement the counter,
        and the array object too -- that way there will be plenty of
        opportunity to test proper use of the protocol.


Community Feedback

    Greg Ewing doubts the locked buffer interface is needed at all, he
    thinks the normal buffer interface could be used if the pointer is
    (re)fetched each time it's used.  This seems to be dangerous,
    because even innocent looking calls to the Python API like
    Py_DECREF() may trigger execution of arbitrary Python code.

    The first version of this proposal didn't have the release
    function, but it turned out that this would have been too
    restrictive: mmap and array objects wouldn't have been able to
    implement it, because mmap objects can be closed anytime if not
    locked, and array objects could resize or reallocate the buffer.

    This PEP will probably be rejected because nobody except the
    author needs it.



References

    [1] The buffer interface
        http://mail.python.org/pipermail/python-dev/2000-October/009974.html

    [2] The Buffer Problem
        http://www.python.org/dev/peps/pep-0296/


Copyright

    This document has been placed in the public domain.



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