[Pythonmac-SIG] Hung like a snake...

[Steven D. Majewski]

On 10 Sep 1998, Jonathan "Hung like a snake" Wight wrote:

> If I were to create a mac app that could be extended with Python scripts
> how would I gracefully deal with the possibility of a script getting
> into an infinite loop and hanging? 
>
> Is threading the answer? Is threading implemented properly on the Mac?

I haven't tried threading & Python on the Mac, however, you may note
that when you run the Mac Python application, [1] you can background
the app by switching to another foreground app, and [2] you can cancel
a long command with Command-\period.

If you look at the main interpreter loop in ceval.c, just before the
#ifdef WITH_THREAD block of code, you'll see: 


#if !defined(HAVE_SIGNAL_H) || defined(macintosh)
			/* If we have true signals, the signal handler
			   will call Py_AddPendingCall() so we don't
			   have to call sigcheck().  On the Mac and
			   DOS, alas, we have to call it. */
			if (PyErr_CheckSignals()) {
				why = WHY_EXCEPTION;
				goto on_error;
			}
#endif


PyErr_CheckSignals() [ in sigcheck.c ] calls PyOS_InterruptOccurred() 
The "OS" indicates that this is an OS specific function, and it's 
defined in macglue.c -- and that's where the check for Command-period
is done. 


The exact flow of control when you add Tkinter and SOUIX and apple
events into the mix is something I haven't quite unravelled, but
SOMEWHERE MacPython makes occasional calls to PyMac_DoYield ( in 
macglue.c in the Mac/Python sources. ) to check for SOUIX and apple 
events. 


If you are embedding the python interpreter in another non-python
app, then I believe you'll probably be calling some other, lower
level entry point to the interpreter (in pythonrun.c), but you 
might do some similar setup in your app before calling the 
interpreter. 


> Is there some other way of detecting an infinite loop (timeouts) and
> then gracefully killing it and _hopefully_ rolling back the changes the
> script has made (if needed). 

However, *detecting* an infinite loop is another matter, and is not,
in general, solvable. But you may be able to make some assumptions
within your application to decide when that is likely ( For example,
the python interpreter itself, has a recursion limit to stop 
runaway stack growth. ) or allow the user a way to kill the 
interpreter. 


---|  Steven D. Majewski   (804-982-0831)  <sdm7g@Virginia.EDU>  |---
---|  Department of Molecular Physiology and Biological Physics  |---
---|  University of Virginia             Health Sciences Center  |---
---|  P.O. Box 10011            Charlottesville, VA  22906-0011  |---

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