usage of updatePVs in pcaspy

[Jameson Graef Rollins <jrollins@ligo.caltech.edu>]

Hi, Xiaoqiang.  I'm trying to understand when exactly driver.updatePVs()
needs to be called.  The reference documentation [0] implies that it
needs to be called after any setParam() call in order for clients to see
the update.  However, if I overload driver.write() and include some
setParam() calls, as far as I can tell it doesn't matter if I call
updatePVs() at the end or not, the behavior seems to be the same.

The peculiar thing I'm worried about is that in my current application
there are, under some circumstances, multiple record updates during a
single driver.write().  For example, before pcaspy 0.7.0, I was doing
something like this:

class MyDriver(pcaspy.Driver):
    ...
    def write(self, channel, value):
        ...
        self.setParam(channel, value)
        self.setParam(other_channel, other_value)
        ...
        self.pvDB[channel].flag = False
        self.updatePVs()
        return True

The 'pvDB[channel].flag = False' was to prevent subscribed clients from
seeing multiple updates for the primary record 'channel'.  In 0.7.0, it
seems that neither that, nor the final updatePVs() call are necessary.
However, I am now occasionally (less than 50% of the time) seeing
multiple client updates for the primary channel.  The sporadic nature of
the double updates makes me think there's some kind of race condition.
Regardless, I can't figure out what's going on, and how to uniformly
prevent redundant update in subscribed clients.

Any advice would be much appreciated.

jamie.

[0] http://pcaspy.readthedocs.io/en/latest/api.html?highlight=updatePVs

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