segfault in polygon_set_data<int>::get about polygon

[tvf]

I'm using GCC 7.4 on Ubuntu 18.04.4 LTS. Building with -fsanitize=address reports a stack buffer overflow.

[dmsteck]

I investigated this and the root cause of the problem lies in polygon_set_data<int>::clean(). There's a specific issue of two of the input line segments where compute_exact_intersection fails to, well, compute the exact intersection. This results in inconsistent geometry/violated preconditions of other functions and becomes a segfault way down the line.

It turns out that this kind of behaviour is expected (see https://lists.boost.org/boost-users/2010/11/63944.php) because the default implementation of compute_exact_intersection uses long double to compute the intersection points, which can produce incorrectly rounded results in certain cases.

A simple solution is to link the library against GMP by using the gmp_override.hpp header. This fixes the example presented here.

[dmsteck]

I think there are two approaches to prevent this kind of problem for users of the library:

• Make it more obvious that the library should be used with GMP, and make it easier to use the library in that way. Including gmp_override.hpp whenever you include polygon.hpp is error-prone. A simple solution would be to provide a feature macro that automatically configures the library for this kind of usage.
• A more ambitious alternative would be to attempt to fix compute_exact_intersection in its default state, i.e., using long double. It is relatively easy to come up with a quick win here (and my prototype fixes the segfault in this example), but I'm not sure/haven't analysed whether the issue can be fixed for all possible input values, and in a portable way.

@asydorchuk Could you let me know which of these solutions (if any) you would consider for inclusion in the library? I would be happy to raise a PR. Thank you.