clear_callbacks leaves connection and access callbacks alive

[hhslepicka]

Users reported some issues with PyDM recently that when IOCs are rebooted they get a message that connection and access control callbacks failed even if the widget was no longer at the screen.
I tracked it down and I was able to isolate the issue into an example:

Instructions to reproduce:

• Run the code below...
• When the code goes into the poll loop, kill the IOC
• Note on the terminal that even with the PV disconnected the connection and access callbacks are invoked.

Note: Change the PV from MTEST:Float to something else that you have.

import epics
import time


def conn_callback(*args, **kwargs):
    print(f'{time.time()} conn_callback invoked with: {args} | {kwargs}')


def access_callback(*args, **kwargs):
    print(f'{time.time()} access_callback invoked with: {args} | {kwargs}')


print(f'{time.time()} - Create PV')
pv = epics.PV('MTEST:Float', connection_callback=conn_callback, access_callback=access_callback)

print(f'{time.time()} - Connect')
pv.connect()
print(f'{time.time()} - Wait For Connection')
pv.wait_for_connection()

print(f'{time.time()} - Invoke disconnect')
pv.disconnect()

print(f'{time.time()} - Lets enter on poll loop')
try:
    while True:
        epics.ca.poll()
except KeyboardInterrupt:
    print(f'{time.time()} - Poll loop terminated')
    pass

This produces the following output:

1592863195.09071 - Create PV
1592863195.111618 - Connect
1592863195.142028 access_callback invoked with: (True, True) | {'pv': <PV 'MTEST:Float': not connected>}
1592863195.142275 conn_callback invoked with: () | {'pvname': 'MTEST:Float', 'conn': True, 'pv': <PV 'MTEST:Float': not connected>}
1592863195.14254 - Wait For Connection
1592863195.1425579 - Invoke disconnect
1592863195.144663 - Lets enter on poll loop


CA.Client.Exception...............................................
    Warning: "Virtual circuit disconnect"
    Context: "10.0.0.211:5064"
    Source File: ../cac.cpp line 1237
    Current Time: Mon Jun 22 2020 15:00:04.489395534
..................................................................
1592863204.4895098 conn_callback invoked with: () | {'pvname': 'MTEST:Float', 'conn': False, 'pv': <PV 'MTEST:Float': not connected>}
1592863204.489681 access_callback invoked with: (False, False) | {'pv': <PV 'MTEST:Float': not connected>}

For now I can force clear the connection and access callbacks on my side.

I would like to pick your brain before submitting a PR as what would be the best fix for this case:

• Add the cleaning routine at clear_callbacks to also clear the other two callbacks
• Other alternatives?

Could this also be related to #156 ?

[newville]

@hhslepicka What does "they get a message that connection and access control callbacks failed even if the widget was no longer at the screen" mean? What failed?

Just to be clear, is the problem that a disconnected PV still has active connection callback and access callbacks? I would guess that PV.disconnect() should probably remove the connection and access callbacks.

Other than that, everything else looks OK to me. But maybe I'm not understanding.

[hhslepicka]

@newville the error message on our end is that the callback tries to access a widget which is no longer instantiated and that should never happen.

You are correct on the problem. disconnect should clear the calbacks.