[issue10181] Problems with Py_buffer management in memoryobject.c (and elsewhere?)

[Pauli Virtanen]

Pauli Virtanen <pav at iki.fi> added the comment:

Nick's plan of action above seems mostly OK to me.

Though, it might be simpler to collapse the PyManagedBuffer object to PyMemoryView, even if this requires memoryviews to serve two purposes.

[Nick:]
> I'm still not comfortable with a convention that relies on *clients*
> of the PEP 3118 API not mucking with the internals of the Py_buffer
> struct.

Some points against: (i) Having to look up keys by memory address from an additional PyDict is more work for the exporter than just passing around some PyMem_Malloc'd information in `internal`. (ii) There is already an "obj" field in the structure that the consumers are supposed to not mess with. (iii) The exporter cannot use the `internal` field for anything if bf_releasebuffer cannot rely on it being intact.

If the recommended consumer API is changed so that Py_buffer mainly sits inside a PyObject, it becomes more clear that Py_buffer is read-only for the consumer (-- which is what I believe the PEP intended, but did not write down).

[Nick:]
> Altering release() to simply decrement the reference count of the 
> managed buffer would defeat the whole point of having that method, so
> it may be necessary to allow early release with outstanding references
> and then include a "still alive" check in the code that allows access
> to the buffer details (similar to the way weak references work).

Early release does not seem possible if the buffer does not come from the original object:

    lst = []
    with memoryview(a) as m:
        b = numpy.array(m)
        lst.append(b)

Now, m.__exit__ cannot release the buffer, since `b` holds a buffer-interface lock to `m`. `b` is 3rd party code, and does not know anything about MemoryViews.

Some alternatives: (i) require that bf_getbuffer always gives a new lock on all exported buffers, if there are multiple, (ii) allow memoryview.__exit__ to fail silently, (iii) drop the context management.

I guess (i) would be a sane choice -- I don't see many use cases for the same object exporting multiple different buffers. It's not needed by Numpy, and I suspect there is no existing 3rd party code that relies on this (because it doesn't work with the current implementation of memoryview :)

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<http://bugs.python.org/issue10181>
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