[Python-Dev] Assertion in _PyManagedBuffer_FromObject() (original) (raw)

Stefan Behnel stefan_ml at behnel.de
Fri Mar 2 15:39:22 CET 2012

Nick Coghlan, 02.03.2012 14:22:

On Fri, Mar 2, 2012 at 10:55 PM, Stefan Krah wrote:

Nick Coghlan wrote:

However, given the lack of control, an assert() isn't the appropriate tool here - PyObjectGetBuffer itself should be checking the constraint and then reporting an error if the check fails. Otherwise a misbehaving extension module could trivially crash the Python interpreter by returning a bad Pybuffer.

I'm not so sure. Extension modules that use the C-API in wrong or undocumented ways can always crash the interpreter. This assert() should be triggered in the first unit test of the module. Now, if the module does not have unit tests or they don't test against a new Python version is that really our problem? Crashing out with a C assert when we can easily give them a nice Python traceback instead is unnecessarily unfriendly. As Stefan Behnel pointed out, by tightening up the API semantics, we're already running the risk of breaking applications that relied on looking at what the old code did, since it clearly deviated from both spec (the PEP) and the documentation (which didn't explain how ReleaseBuffer works at all). Modules do need to be recompiled anyway due to the removal of Pybuffer.smalltable, otherwise they will almost certainly crash. Perhaps an addition to whatsnew/3.3 would be sufficient. That, updating the 2.7 and 3.2 docs with a reference to the fleshed out 3.3 semantics and converting the assert() to a Python exception should cover it.

One problem here: if the code raises an exception, it should properly clean up after itself. Meaning that it must call PyBuffer_Release() on the already acquired buffer - thus proving that the code actually works, except that it decides to raise an exception.

I keep failing to see the interest in making this an error in the first place. Why would the object that bf_getbuffer() is being called on have to be identical with the one that exports the buffer?

Stefan

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