Re: infinity loop / recursion

[Sean Conner <sean@...>]

It was thus said that the Great Philipp Kraus once stated:
> Hello,
> 
> I'm using LUA for a simulation on a cluster system, so my user can define
> their own scripts. In some cases there exists a infinite loop or infinity
> recursion in the LUA code so the simulation does not stop anymore. My idea
> is to create a maximum execution time (like PHP) with a watchdog (I don't
> use a formal verification of the code e.g. wp-calculus). Each script has
> got a defined function which is called by my C++ class, so I would like to
> send the LUA script a "warning" that the max. execution time is reached,
> but if the script does not stop, the C++ side should stop the script.
> 
> Can I / How can I stop a running script from C side? Do you think it is a
> good idea to send a warning first, because the script does not use
> threads, so IMHO the warning can not be handled by the script? I hope for
> some ideas

  If you are using Unix, I can think of two ways that don't involve mucking
with Lua.

  1). SIGALRM---set a handler for SIGALRM (signal() or sigaction()), then
before running your script, trigger SIGALRM for the maximum amount of wall
time you think the script needs (using either alarm() or setitimer()).  When
the alarm goes off, your signal handler will get control.  

  2). Run the script in its own process, but set a CPU resource limit to the
maximum amount of CPU time [1] the process can run.  The process may be able
to catch SIGXCPU (CPU time exceeded) to know that it has exceeded its
allowable amount of time, but I'm not sure what, exactly, will happen if the
CPU time is exceeded (and thus, you have no more CPU to run).

  I'm not sure if it's a good idea to allow the script to check, because
what can the script do?  

  -spc

[1]	There is a difference between wall time and CPU time.  A program can
	sit there, waiting for input for 60 seconds and consume no CPU time. 
	A program can also sit there calculating a fractal for 60 seconds of
	CPU time.