Fix callback registration for the RunState on_stop handler

[Kami]

After #495 has been merged, I started to notice errors like this in the log:

2020-04-21 14:36:53.500Z ERROR [core] [scalyr_logging.py:603] [error="failedAgentMain"] Main run method for agent failed due to exception :stack_trace:
  Traceback (most recent call last):
    File "scalyr_agent/agent_main.py", line 992, in __run
      config_change_check_interval
    File "/home/kami/w/scalyr/scalyr-agent-2/scalyr_agent/util.py", line 708, in sleep_but_awaken_if_stopped
      self._wait_on_condition(timeout)
    File "/home/kami/w/scalyr/scalyr-agent-2/scalyr_agent/util.py", line 792, in _wait_on_condition
      self.__condition.wait(timeout)
    File "/home/kami/.pythonz/pythons/CPython-3.6.9/lib/python3.6/threading.py", line 299, in wait
      gotit = waiter.acquire(True, timeout)
    File "/home/kami/w/scalyr/scalyr-agent-2/scalyr_agent/platform_posix.py", line 627, in handle_interrupt
      handle_terminate(signal_num, frame)
    File "/home/kami/w/scalyr/scalyr-agent-2/scalyr_agent/platform_posix.py", line 641, in handle_terminate
      self.__termination_handler()
    File "scalyr_agent/agent_main.py", line 597, in __handle_terminate
      self.__run_state.stop()
    File "/home/kami/w/scalyr/scalyr-agent-2/scalyr_agent/util.py", line 751, in stop
      callback()
  TypeError: 'NoneType' object is not callable

It turns out, we didn't correctly register the callback - instead of registering the callable (function) we actually immediately called the function during registration phase and passed return value of that function to the register handler (that function returned None).

[Kami]

Ideally, we would also have some unit tests for it, but it might be a bit more involved (@yanscalyr - can you please look into it).

On that note - it looks like CodeSpeed indeed caught this regression / bug:

Right now I manually check the dashboard every so often so we would have caught this issue sooner or later, but in the future we should probably also try to come up with a more automated approach.

[codecov]

Codecov Report

Merging #513 into master will increase coverage by 0.02%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #513      +/-   ##
==========================================
+ Coverage   72.95%   72.97%   +0.02%     
==========================================
  Files         118      118              
  Lines       28253    28270      +17     
  Branches     3381     3381              
==========================================
+ Hits        20611    20631      +20     
+ Misses       7030     7029       -1     
+ Partials      612      610       -2     

Impacted Files	Coverage Δ
scalyr_agent/agent_main.py	56.77% <ø> (+0.71%)	⬆️
scalyr_agent/tests/util/util_test.py	97.38% <94.11%> (-0.09%)	⬇️
scalyr_agent/builtin_monitors/syslog_monitor.py	80.41% <0.00%> (-0.48%)	⬇️
.../builtin_monitors/tests/kubernetes_monitor_test.py	96.71% <0.00%> (ø)
scalyr_agent/util.py	86.06% <0.00%> (+0.27%)	⬆️

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[yanscalyr]

I actually had this fixed locally but I guess I got distracted by something because I never checked it in... My bad.

[yanscalyr]

I actually had this fixed locally but I guess I got distracted by something because I never checked it in... My bad.