I was having a discussion with my friend the other day. I was saying how that, in pure Lua, you couldn't build a preemptive multitasking system. He claims you can, because of the following reason:
Both C and Lua have no inbuilt threading libraries [OP's note: well, Lua technically does, but AFAIK it's not useful for our purposes]. Windows, which is written in mostly C(++) has pre-emptive multitasking, which they built from scratch. Therefore, you should be able to do the same in Lua. The big problem i see with that is that the main way preemptive multitasking works (to my knowledge) is that it generates regular interrupts which the manager uses to get control and determine what code it should be working on next. I also don't think Lua has any facility that can do that.
My question is: is it possible to write a pure-Lua library that allows people to have pre-emptive multitasking?
I can't see how to do it, although without a formal semantics of Lua (like the semantics of yield for example), it's really hard to come up with an ironclad argument why it can't be done. (I've been wanting a formal semantics for ages, but evidently Roberto and lhf have better things to do.)
If I wanted pre-emptive multitasking for Lua, I wouldn't even try to do it in pure Lua. Instead I'd use an old trick I first saw 20 years ago in Standard ML of New Jersey:
Interrupt sets a flag in the lua_State saying "current coroutine has been preempted".
Alter the VM so that on every loop and every function call, it checks the flag and yields if necessary.
This patch would be easy to write and easy to maintain. It doesn't solve the problem of the long-running C function that can't be pre-empted, but if you have to solve that problem, you are wandering into much harder territory, and you may as well do all your threading at the C level, not the Lua level.