[Twisted-Python] Deterministic TimerService

[Norm Petterson]

Hi,

Thanks for such a useful framework.

While working on an industrial data logger using Twisted, I found that
application.internet.TimerService calls my own callLater callable function
before reissuing the next callLater. Consequently, instead of a time period
c, the callable function runs every c+x, where x is the duration of my own
function, and thus appears to slip over time.

Based on my past experiences with DEC RSX "mark time" and VMS "set timer" in
similar situations, I recommend changing:

def setupCall():
    self.callable(*self.args, **self.kwargs)
    self._call  =  reactor.callLater(self.step, setupCall)

to

def setupCall():
    self._call  =  reactor.callLater(self.step, setupCall)
    self.callable(*self.args, **self.kwargs)

Reissuing the timer request first appears to work more as I'd expect,
particularly as I am using timestamp information for each scan to compute an
appropriate bsddb record number in a persistent circular buffer. Any other
opinions?

Regards,

Norm Petterson