[docs] [issue15821] PyMemoryView_FromBuffer() behavior change (possible regression)
report at bugs.python.org
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
PyManagedBuffer_FreeArrays // shape, strides, suboffsets
/* 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
This is much more flexible than the existing function and does not suffer
from any abuse of getbuffer/releasebuffer.
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