bpo-34423: Fix check for overflow when casting from a double to integral types.

[enedil]

The added comment explains the issue. This fix disallows overflow: if a double d passes the test, it can mean that d == 2**63. When casted to int64_t, the variable overflows to -2**63.

https://bugs.python.org/issue34423

[asottile]

this appears linked to the wrong issue -- is this the right one? https://bugs.python.org/issue34423

[enedil]

@asottile Yes, you're right. My branch is wrongly named too. Should I create new pull request?

[asottile]

It might be enough to edit the commit message and title? I don't actually know

[vstinner]

A test should be added to test_time.

[asottile]

unintentional change?

[jdemeyer]

I don't think that the condition sizeof(*v*) == sizeof(*type*) is really the right condition. Besides, you are applying the fix (changing <= to <) unconditionally, also when the maximal value can be represented exactly.

The problem is that the maximal value 2^N - 1 is hard to work with. So I suggest to replace

v < _Py_IntegralTypeMax(type)

by something like

v < ldexp(_Py_IntegralTypeSigned(type) ? 0.5 : 1.0, CHAR_BIT * sizeof(type))

[vstinner]

I'm not sure that we will be able to find a solution using a "simple" macro. Maybe we need new functions which takes a double as input and return an integer as output and report overflow. I'm not sure that it's possible to write a generic function.

Right now, _Py_InIntegralTypeRange() is only used to convert a double to _PyTime_t (int64_t) and time_t in pytime.c. Maybe 2 functions would be enough?

About generic code, we can maybe use the preprocessor as a template engine to generate these functions.

[jdemeyer]

Right now, _Py_InIntegralTypeRange() is only used to convert a double to _PyTime_t (int64_t) and time_t in pytime.c. Maybe 2 functions would be enough?

It would certainly be easier to support specific types only instead of being generic. We could for example assume that the number of bits in a double (53 assuming IEEE-754) is less than the number of bits in a int64_t (63).

[iritkatriel]

This has merge conflicts, as well as several unresolved issues raised by reviewers.

[bedevere-bot]

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[enedil]

I'm fine with leaving this as unresolved.

[iritkatriel]

I'm fine with leaving this as unresolved.

Sure, thanks.