Re: Properly binding C libraries using callbacks

[Tim Caswell <tim@...>]

I have a similar problem in my luvit project.  I'm wrapping an
inherently async library that provides non-blocking I/O.  I ended up
creating a helper that calls my lua callbacks from the main thread.

https://github.com/creationix/luvit/blob/master/src/utils.c#L7-37

As far as getting a reference to the lua state, the API I'm using
always has a place for an opaque data pointer and I put in some struct
that contains L and some other data (like a lua_ref integer) to get at
the real data.

On Thu, Oct 20, 2011 at 3:26 PM, Sam Roberts <vieuxtech@gmail.com> wrote:
> On Thu, Oct 20, 2011 at 2:07 PM, Rob Hoelz <rob@hoelz.ro> wrote:
>
> In:
>
>> static int l_setcompletion(lua_State *L)
>> {
>>    luaL_checktype(L, 1, LUA_TFUNCTION);
>>
>>    lua_pushvalue(L, 1);
>>    completionFuncRef = luaL_ref(L, LUA_REGISTRYINDEX);
>>    linenoiseSetCompletionCallback(completionCallbackWrapper);
>
> Don't do this:
>
>>    completionState = L;
>
> As soon as  l_setcompletion() returns, L should be treated as
> undefined (because it could become so, as you demonstrated).
>
> In ln.linenoise() -- you don't show the implementation but I guess it
> looks like below --  you should do this:
>
> void lua_linenoise(lua_State* L) {
>   completionState = L; /* L is guaranteed to be defined and valid
> until lua_linenoise() returns, so use it */
>
>   char* s = linenoise_do_your_thing(); /* this is the whatever
> function will call completionCallbackWrapper() */
>   lua_pushstring(L, s);
>   return 1;
> }
>
> That way the completionCallbackWrapper will use the correct lua_State.
>
> However, it calls lua_pcall(), and that lua code can call
> ln.linenoise()... is linenoise reentrant? Can you call back into it
> from it's callback? Probably not, and you might want to protect
> against that happening:
>
> void lua_linenoise(lua_State* L) {
>   if(completionState) {
>      luaL_error(L, "linenoise is not reentrant, naughty!");
>      }
>   completionState = L; /* L is guaranteed to be defined and valid
> until lua_linenoise() returns, so use it */
>   char* s = linenoise_do_your_thing(); /* this is the whatever
> function will call completionCallbackWrapper() */
>   completionState = NULL;
>   lua_pushstring(L, s);
>   return 1;
> }
>
> If it is reentrant, and you want to support that, and the
> completionCallbackWrapper() can be called multiple times before
> ln.linenoise() returns (all things I don't know), then after calling
> lua_pcall() in the completionCallbackWrapper, you have to again set
> the completionState back to the L that it saved, because that's the
> valid one.
>
>> Can someone give me some pointers on the proper way to bind a library that uses callbacks, especially one that doesn't
>> allow you to thread your own userdata through the callbacks?  Coroutines make it tricky, and my understanding is that
>> storing things in global variables in the binding are a faux pas.
>
> linenoise itself uses global state, it looks like, since
> linenoiseSetCompletionCallback()  doesn't take
> any kind of context as an argument. In that case, have the
> completionState be global doesn't make
> the binding any worse than the library it binds to.
>
> Cheers,
> Sam
>
>