[WIP] bpo-28708: Add setsize keywoard-only parameter to select.select()

[vstinner]

It becomes possible to configure the set size used internally by
select.select() at runtime.

https://bugs.python.org/issue28708

[vstinner]

I have no idea if this PR works :-) Is it really possible to change FD_SETSIZE at runtime?

[andzn]

I think these three sets will need to be allocated differently. We'll probably need a different fd_set structure something like:

typedef struct fd_set_2
{
    u_int fd_count
    SOCKET *fd_array;
} fd_set_2;

and then dynamically allocate the fd_array to be a SOCKET* array of size set_size.
Knowing that the select implementation only looks at the fd_count for the size of the array (and doesn't do any boundary checking), we can cast those sets to fd_set and then pass them to the select call.

[andzn]

Actually that struct type isn't going to work, because that adds a level of indirection that the select implementation doesn't expect.
We'll need to allocate a continuous area of memory of size (sizeof(u_int) + setsize*sizeof(SOCKET))

[vstinner]

It seems like @andzn went further than me in PR #13842, I close my WIP PR.