Error handling in Lua: the Kyua approach

About a week ago, I detailed the different approaches I encountered to deal with errors raised by the Lua C API. Later, I announced the new C++ interface for Lua implemented within Kyua. And today, I would like to talk about the specific mechanism I implemented in this library to deal with the Lua errors.

The first thing to keep in mind is that the whole purpose of Lua in the context of Kyua is to parse configuration files. This is an infrequent operation, so high performance does not matter: it is more valuable to me to be able to write robust algorithms fast than to have them run at optimal speed. The other key point to consider is that I want Kyua to be able to use prebuilt Lua libraries, which are built as C binaries.

The approach I took is to wrap every single unsafe Lua C API call in a "thin" (FSVO thin depending on the case) wrapper that gets called by lua_pcall. Anything that runs inside the wrapper is safe to Lua errors, as they are caught and safely reported to the caller.

Lets examine how this works by taking a look at an example: the wrapping of lua_getglobal. We have the following code (copy pasted from the utils/lua/wrap.cpp file but hand-edited for publishing here):

static int
protected_getglobal(lua_State* state)
{
    lua_getglobal(state, lua_tostring(state, -1));
    return 1;
}

void
lua::state::get_global(const std::string& name)
{
    lua_pushcfunction(_pimpl->lua_state, protected_getglobal);
    lua_pushstring(_pimpl->lua_state, name.c_str());
    if (lua_pcall(_pimpl->lua_state, 1, 1, 0) != 0)
        throw lua::api_error::from_stack(_pimpl->lua_state,
            "lua_getglobal");
}

The state::get_global method is my public wrapper for the lua_getglobal Lua C API call. This wrapper first prepares the Lua stack by pushing the address of the C function to call and its parameters and then issues a lua_pcall call that executes the C function in a Lua protected environment.

In this case, the argument preparation for protected_getglobal is trivial because the lua_getglobal call does not require access to any preexisting values on the Lua stack. Things get much trickier when that happens as in the case of the lua_getglobal wrapper. I'll leave understanding how to do this as an exercise to the reader (but you can cheat by looking at line 154).

Anyway. The above looks all very nice and safe and the tests for the state::get_global function, even the ones that intentionally cause a failure, all work fine. So we are good, right? Nope! Unfortunately, the code above is not fully safe to Lua errors.

In order to prepare the lua_pcall execution, the code must push values on the stack. As it turns out, both lua_pushcfunction and lua_pushstring can fail if they run out of memory (OOM). Such failure would of course be captured inside a protected environment... but we have a little chicken'n'egg problem here. That said, OOM failures are rare so I'm going to leverage this fact and not worry about it. (Note to self: install a lua_atpanic handler to complain loudly if that ever happens.)

Addendum: Bundling Lua within my program and building it as a C++ binary with exception reporting enabled in luaconf.h would magically solve all my issues. I know. But I don't fancy the idea of bundling the library into my source tree for a variety of reasons.

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Error handling in Lua: the Kyua approach

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