[Stackless] switching improvements

Christian Tismer tismer at stackless.com
Thu Nov 29 02:24:49 CET 2012

Hi Kristjan,

how about checking it into a branch firt, let us try and check that
intensively, and after enough confidence merge it in?
That's the way that works best for me with Mercurial/Git.

If you don't like to put a branch into python.org, you also can do a clone
on bitbucket easily and let us use that for review.

cheers - chris

On 28.11.12 16:11, Kristján Valur Jónsson wrote:
> Hello All.
> I was recently prompted to add a flag to stackless, a way to block all 
> tasklet switching.
> This springs from the way that we are embedding stackless python in an 
> game engine (UnReal) which sometimes makes callbacks into python. 
> Sometimes, this code will do nasty stuff that results in tasks 
> switching, causing havoc with the control flow of the game engine.
> To simplify this, I added a per-thread flag, switch_trap, which can be 
> controlled in a similar way to block_trap.  If the logic causes a 
> switch to be attempted, this should be trappable and the code should 
> be easily fixable, or we can otherwise deal with it.
> Anyway, doing this, adding it to slp_schedule_task(), and so on, 
> uncovered a subtle flaw in stackless:
> It turns out that slp_schedule_task() had no way of differentiating 
> whether an exception result from this call came as a result of a 
> failure to switch, or an exception being sent to the tasklet when it 
> wakes up again.
> So, I have changed the interface to be able to do this properly.  
> There are other reasons why switching can fail, including memory 
> allocation failures and so on, so this seems like a necessary change. 
> I also fixed code both in stacklesseval.c and taskletmodule and 
> channelobject to be able to cope with switch failure like this.
> Now:
> How does this sound to you?  The change is somewhat large and I would 
> hesitate to simply check it in without some sort of review or 
> otherwise approval.  Any suggestions?
> K
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