WIP: Usubscribe from events on the next subscription (PyTango#292)

reszelaz commented 4 years ago

Similarly to #1091 and #1093, this PR tries to workaround https://github.com/tango-controls/pytango/issues/292.

I tried this PR with sardana-org/sardana#1312 ( which reverts the Sardana workaround and use zombie concept in tests) and it works corrrectly for the sardanatestsuite and for the stress macro posted in https://github.com/taurus-org/taurus/pull/1093#issuecomment-602042643. So, IMO it is a satisfactory workaround.

Note that it has one side effect - it keeps garbage collected attributes subscribed until some new attribute is subscribed. To mitigate this effect attributes are marked as zombies - their push event return immediately. Also this should happen only for the configuration events - the change events should be unsubscribed with the last TaurusModel.removeListener call. Also if one wants to increase the frequency of unsubscribes the tangoattribute._empty_unsub_queue() could be called more frequently.

reszelaz commented 4 years ago

One could also try to play manually with GC. Something like:

call gc.collect() before we subscribe to events. This way we will ensure that there are no pending unsubscribes to be done.
disable GC before subscribe and enable it after

(I have not tried neither of them)

reszelaz commented 4 years ago

The root issue was solved in https://github.com/tango-controls/cppTango/pull/699. It is available starting from cppTango 9.3.4rc5. Hence I do not foresee to work anymore on this PR.

cpascual commented 4 years ago

Thanks @reszelaz for taking care of this, and sorry for not having been as responsive as I should (I have been busy in other fronts).

One thing that remains to do is using the cppTango >= 9.3.4rc5 in our CI. This could be one way to try to fix #1042

taurus-org / taurus

WIP: Usubscribe from events on the next subscription (PyTango#292) #1095