[issue1621] Do not assume signed integer overflow behavior

[Martin Panter]

Martin Panter added the comment:

Looking over r60793, the overflow check at Modules/cjkcodecs/multibytecodec.c:836 looks vulnerable to being optimized away, because it can only detect the overflow if the line above has already overflowed. Perhaps change PY_SSIZE_T_MAX to MAXDECPENDING. I wonder if any of the GCC optimization and warning modes can detect this case?

Also, Python/ast.c:3988 checks using PY_SIZE_MAX, but then passes the value to PyBytes_FromStringAndSize(), which expects ssize_t and in the best case would raise SystemError.

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