According to the documentation, PyLong_FromString() sets *pend (if pend is not NULL) to the first character that could not be processed on error. But it does not do this in the following cases:

1. base is invalid (not 0 or in range from 2 to 36).
2. Binary base number exceeds sys.maxsize bits.
3. Decimal number exceeds the limit of digits sys.get_int_max_str_digits().
4. Other base numbers also have limits.
5. Memory allocation error.
6. User presses Ctrl-C when _pylong.int_from_string() is used.

The workaround is to initialize the pointer to some known value, for example the start of the string or NULL before using PyLong_FromString(). If initialize it to NULL, we can distinguish ValueError raised for invalid base or exceeding the sys.get_int_max_str_digits() limit from ValueError raised for invalid string content.

We should either document this and require initialization of the end pointer, or always set it on error in PyLong_FromString(). The latter is a potentially breaking change, because it can override the sentinel value used in the user code. But the former looks like bad design.