Three months ago we discussed about an issue with PySlice_GetIndicesEx().
(https://mail.python.org/pipermail/python-dev/2016-August/145901.html)
The problem was that PySlice_GetIndicesEx() takes the size of the
sequence, but the size of the sequence can be changed when call custom
__index__() methods inside PySlice_GetIndicesEx().
The solution is to split PySlice_GetIndicesEx() into two functions: one
function convert Python objects to C integers by calling __index__()
methods, other function takes the size of the sequence and adjusts
indices, it is atomic from Python view.
The code
if (PySlice_GetIndicesEx(item, length,
&start, &stop, &step, &slicelength) < 0)
return -1;
should be replaced with
if (foo(item, &start, &stop, &step) < 0)
return -1;
slicelength = bar(&start, &stop, step, length);
PySlice_GetIndicesEx() can be converted to a macro calling these two
functions. It would be enough just recompile an extension to make it
invulnerable to the original bug.
But there is a problem. New functions should be added to stable ABI.
This means that we should design names and signatures of these functions
even if don't make them a part of public API.
Let's start bikeshedding. What are your ideas about names and the order
of arguments of two following functions?
1. Takes a slice object, returns it's start, stop and step as Py_ssize_t
values. Can fail.
2. Takes slice's start, stop, step, and the length of the sequence (all
are Py_ssize_t), returns adjusted start, stop, and the length of sliced
subsequence (as Py_ssize_t). Always success.
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