[Python-Dev] PEP 298

[Scott Gilbert]

--- "Martin_v._Löwis" <martin@v.loewis.de> wrote:
> 
> > If you think it's an issue, the act of locking the buffer could
> > increase the refcount too...
> 
> I know something is flawed. Whether this is fixed by changing the spec,
> or by changing the implementation, I don't know.
> 
> If the spec means "the pointer is not valid if the reference count of
> the underlying object goes to zero", it should say so. If the spec means
> "if the reference count goes to zero while the object is locked, Python
> aborts", it should say so. If the spec means "the buffer will not be
> released while it is acquired", it should say so. From reading the spec,
> I cannot tell which one it is. From reading the implementation, I can
> tell "it depends on the type". I thought that "depends on the type" is
> what this API was meant to eliminate.
> 

The object should definitely be kept alive while someone has the buffer
locked.

I think it should be fixed by saying that PyObject_AcquireLockedReadBuffer
and PyObject_AcquireLockedWriteBuffer will also do a Py_INCREF and that
PyObject_ReleaseLockedBuffer will do a Py_DECREF.

If there is no disagreement, and Thomas Heller doesn't mind, I'll submit
the changes to the PEP...  Would this be sufficient to keep the PEP alive
(possibly in the "abandoned" state if Thomas Heller doesn't want to work on
it for 2.4)?

>
> > How would you use this at the Python level?  I don't see how it is
> > useful outside of a C extension.
> 
> I want to pass buffers to fcntl, and want fcntl to change the buffer
> in-place. Currently, fcntl will need to keep the interpreter lock for
> that.
> 
> For that to work, I need to create a locked buffer, and I need to find
> out its address. I can create a buffer using the buffer builtin, but it
> won't be a locked one, and I need to find out its address, but I can't.
> 

It sounds like you want to get your modifiable pointer as a Python integer.
 Then you want to pass that integer as the last argument of fcntl.  I think
it would be better if fcntl allowed the last argument to be a modifiable
buffer instead of just an "int or a string" as the docs indicate.

I think the fcntl C implementation could use the the locked buffer
interface when it is in place.  Then you could use the array module (or any
other modifiable buffer type object) for your argument.

BTW, the array object currently has the buffer_info() method which gives
you that integer.  I don't think this is the right long term solution, but
it might get you past a short term hump.



Cheers,
    -Scott








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