[docs] [issue15821] PyMemoryView_FromBuffer() behavior change (possible regression)

Sat Sep 1 09:33:42 CEST 2012

Stefan Krah added the comment:
In general, the number of calls to PyObject_GetBuffer() must be equal to
the number of calls to PyBuffer_Release(), otherwise bad things will happen
in the same way as with malloc()/free().
Now, in my test case I omitted the call to PyObject_GetBuffer() *with*
the internal knowledge that in the case of the bytes object it does
not matter: The releasebufferproc is NULL, so PyBuffer_Release()
is just a way of decrementing the reference count of the bytes object.
No matter how you turn it, if info.obj != NULL you need some internal
knowledge what the obj's releasebufferproc will do. For example, I
suspect that many existing releasebufferprocs do not account for
the fact that a consumer may change shape, strides and suboffsets.
So you need to know the exporter really well.
My strategy here is this: I want to keep backwards compatibility
for PyMemoryView_FromBuffer(), which I think should be deprecated
at some point.
A proper function that returns a memoryview with automatic cleanup
would not involve getbuffer/releasebuffer at all and could look like
this:
Flags:
------
 PyManagedBuffer_FreeBuf
 PyManagedBuffer_FreeFormat
 PyManagedBuffer_FreeArrays // shape, strides, suboffsets
PyMemoryView_FromBufferWithCleanup(info, flags)
{
 /* info should never have been obtained by a call to PyObject_GetBuffer().
 Hence it should never be released. */
 assert(info.obj == NULL);
 /* buf, format, shape, strides and suboffsets MUST stay valid
 for the lifetime of the returned memoryview. Usually they
 will be dynamically allocated using PyMem_Malloc(). This is
 no problem, since there is automatic cleanup in mbuf_dealloc(). */
 mbuf = mbuf_alloc();
 mbuf->master = *info;
 mbuf->flags |= flags;
 ...
 return new memoryview
}
mbuf_dealloc(self)
{
 if (self->flags&PyManagedBuffer_FreeBuf)
 PyMem_Free(self->master.buf);
 ...
}
This is much more flexible than the existing function and does not suffer
from any abuse of getbuffer/releasebuffer.
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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15821>
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