C++ error handling

[PonasKovas]

Hello, I'm trying to work with the C++ interface of Luau and I'm wondering about the error handling.

As I understand, the correct procedure is to use lua_pcall, but for that (forgive me if I get something wrong, I'm still only getting familiarised with lua and luau) you need to push your function that you need to call to the stack, along with it's arguments, right?

But isn't pushing to the stack fallible in itself (out of memory errors)? From the Lua spec I understand that whenever lua calls C, it guarantees at least LUA_MINSTACK = 20 free stack spaces, but obviously (though not documented) pushing something like a string will need an extra allocation which might fail.

So to safely (handling the errors) call a your own API function like lua_create_table, which (though undocumented, but I assume) is fallible due to allocations and might throw, I need to make a wrapper C++ function that calls it, push it to the stack (which is fallible in itself) along with the arguments, and then call lua_pcall?

It seems for me that this whole system is very convoluted (and I understand this is original lua legacy), but since luau is rewritten in C++ and you're already using C++ exceptions internally, why not expose the luau_exception class and let C++ users just catch the errors instead of going through all these hoops?

[vegorov-rbx]

You can catch lua_exception as an std::exception.

[PonasKovas]

Yeah but then I can't access the status code, only the message using what() method, and matching against that just feels like a hack

[vegorov-rbx]

We plan to make lua_checkstack return 0 instead of throwing on allocation failure.

This will make it match the documentation of Lua 5.1 better ("It returns false if it cannot grow the stack to that size") even though Lua 5.1 did not implement this guarantee for allocation failure.

Lua 5.2 extended the documentation to explicitly call this out ("or because it cannot allocate memory for the new stack size") and implement handling for it.

We'll post an update when it's implemented.

[vegorov-rbx]

You can now use lua_cpcall to enter a C function in a protected environment when you are not in one or cannot guarantee it.
Additionally, lua_checkstack will now return a failure value instead of throwing when OOM happens.
Hope that this allows you to be as robust as possible even in presence of OOM conditions.