[issue10181] get_shape0 in memoryobject.c not checked for error return

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

Hmm, if we're ever creating a new copy of a Py_buffer without calling GetBuffer again on the original object, then *that* is a bug (exactly comparable to copying a PyObject pointer without incrementing the reference count - it's OK if you can guarantee the lifecycle of your borrowed pointer is shorter than the lifecycle of the original, but disallowed otherwise).

This still doesn't sound like a problem with the spec to me, it sounds like a problem with the implementation strategy that was originally used when integrating the spec into the interpeter core (which I readily agree received far less attention than the spec itself did).

It really sounds like we should be creating a _Py_ManagedBuffer internal object, with each instance containing a PyObject* and a Py_buffer instance. We don't need to alter PEP 3118 to do that - such a change is entirely consumer side, so the objects providing the buffers don't need to care how the lifecycle of the Py_buffer struct is managed.

When we need another reference to the buffer, we can then just increment the refcount of the _Py_ManagedBuffer instance rather than having to call GetBuffer again on the original object.

The contents of Py_buffer objects that have been passed to GetBuffer really shouldn't be getting modified at all - for those cases, we should maintain *two* Py_buffer structs, one (unmodified) with the original results of the GetBuffer call, and a second owned by the caller (for manipulation).

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