RE: interrupting coroutines (was Re: Local Variables)

[Thijs Schreijer <thijs@...>]

> -----Original Message-----
> From: lua-l-bounces@lists.lua.org [mailto:lua-l-bounces@lists.lua.org] On
> Behalf Of Patrick Donnelly
> Sent: woensdag 20 augustus 2014 23:30
> To: Lua mailing list
> Subject: Re: interrupting coroutines (was Re: Local Variables)
> 
> On Tue, Aug 12, 2014 at 2:40 PM, Roberto Ierusalimschy
> <roberto@inf.puc-rio.br> wrote:
> >> > What is "gL" here?
> >>
> >>   lua_State *gL;
> >>
> >>   It's a C variable that is either global, or static to the file the
> >> routines are defined in.
> >
> > And where its value come from? (I thought the whole point of the
> > discussion was how to access the *running* coroutine when a signal
> > happens.)
> 
> I think it's fruitful to summarize the problems with interrupting Lua
> (coroutines):
> 
> * You cannot safely, asynchronously interact with Lua. Even
> lua_sethook is undesirable if you're using multiple Lua states.
> * You cannot find the main thread to set a hook on.
> * Hooks do not/cannot propagate errors to the root pcall (whoever is
> working on the main thread).
> 
> It seems to me the answer for solving this, if it should be solved, is
> to take the same approach most C programs do: set a static variable
> indicating a signal has been received and check it synchronously in
> the main program.
> 
> For Lua, this would mean providing an API to add a pointer to this
> static variable to the global state which is checked on each byte code
> instruction and other major lua_ entry points. There would also be a
> global state lua_Hook just for interrupts which is run in the "current
> thread" (which would just be whatever thread is currently being
> interacted with since C code doesn't *have* to use Lua's lua_resume
> interface to run code in a thread).

Can't you use the LOCK/UNLOCK macros for this purpose?

> 
> As for the third problem, propagating the error to the root pcall,
> perhaps the interrupt handler could leave the flag set and allow Lua
> to recall the interrupt hook which throws errors until the root pcall
> is reached.

That's how corowatch [1] implements it (Lua implementation)

Thijs

[1] see https://github.com/Tieske/corowatch#limitations