Calling lua functions from C when using luajit ffi.

[Tim Caswell <tim@...>]

I know this was asked earlier this year, but it doesn't seem to have
gone anywhere.  The last thing I can find from Mike is:

> Callbacks are pretty high on my TODO list for the FFI. But I don't
> know when I'll get around to it. The sponsored work has priority.

Since I can't find anything about this feature being announced or
documented, I'm going to assume the sponsored work is still keeping
things busy. (The arm port is very useful btw)

I'm wondering if anyone has any ideas for how I could work around
this.  My program is luvit <https://github.com/creationix/luvit> and
I'm wrapping a couple C libraries that are heavily callback based.

One is an http_parser which is a sax style parser that calls my
"events" as it finds them synchronously.  This is simple because the
callbacks are called from the same stack that originated in lua.  I
can assume that once I get a handle to L (my lua_State*) that my
userdata is as position 1.

My other case is libuv which is a non-blocking I/O library.  By
definition of it's task, the callbacks get called by C level events
and don't originate from any lua call.  Currently in my C bindings I
store access to L and a lua_ref of my lua callback in the pointer that
my C callback has access to.  I then unpack everything and call the
lua callback from the main thread (to make it co-routine safe).

I have both of these cases working as manual C bindings, but I'd
really like to try out FFI and see if it's any faster.  Perhaps it
won't be because my loops cross stacks and all context is lost.

-Tim Caswell