WIP: Debugging mode for CPython universal

[arigo]

Could be merged as it is, but probably only compiles on Linux. Also, there is no way to disable the extra checks, but they don't cost a lot.

[antocuni]

Are we sure this is the design we want to use?
If I understand it correctly, these changes merge the debug checks into the normal universal mode; even if these precise changes have a low overhead, it is likely that we will need to introduce a we_are_in_debug_mode flag to implement more expensive checks in the future.

An alternative design is to implement the debug mode as a wrapper around an existing HPyContext:

• one minor advantage is that it will have 0 overhead when it's not used
• the biggest advantage, IMHO, is that we will be able to use the very same debug mode on all interpreters. In particular, we will not need to implement it again on PyPy

[arigo]

While I see the long-term point, I have some doubts about the idea now. In "your" version, we need to wrap at least all functions of the API to check all handles. It doesn't exist so far because it is quite some work and it will add maintenance cost. It risks bugs of its own, because it needs to be complete and consistent. And it needs to be explicitly enabled by the user. By comparison this is a few lines of code, and adding the same ones inside PyPy is trivial.

[antocuni]

I see your point. My idea was to autogen all the wrappers, i.e. something like this:

HPy ctx_Long_FromLong(HPyContext ctx, long x)
{
    HPy res = ctx->original_ctx->ctx_Long_FromLong(ctx, x);
    ctx->_record_new_handle(res);
    return res;
}

If we do it this way, the only maintenance cost it adds is the autogen code (which, btw, it much nicer and easier to do in my WIP hpyfunc_autogen branch).
But, apart from handles, my main concern is about all the other debug checks that we will want to introduce. It would be nice to avoid redoing all the work twice, and moreover it would be nice if the debug mode on CPython and PyPy generate the very same warnings.

[antocuni]

we can probably use __VA_ARGS__ to avoid the weird usage pattern in which you have to put parenthesis in the call?

[antocuni]

In my idea, the "debug mode" be something different, i.e. a full wrapper around the ctx which does many extra checks, including potentially expensive ones, and will . So, maybe we should call this file differently?

[arigo]

Redone in PR #69.