[Stackless] switching improvements

Kristján Valur Jónsson kristjan at ccpgames.com
Wed Nov 28 16:11:21 CET 2012

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.

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?

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