ros2 param set, on_parameter_event called twice

Bug report

Required Info:

• Operating System:
  • Ubuntu 18.04
• Installation type:
  • source
• Version or commit hash:
  • 2049b2f
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

ros2 param set /parameter_events plop 42

Expected behavior

Call on_parameter_event once with the new/changed/deleted parameters

Actual behavior

Call on_parameter_event twice, the first time with no parameters then with the new/changed/deleted parameters

Additional information

When I set a parameter using the parameter client in a node, on_parameter_event is called once, as it should be, but when I set a parameter using command line, the on_parameter_event is called twice. And the first time it's triggered, there is no parameters inside.
The output using a node :

[INFO] [parameter_events]: 
Parameter event:
 new parameters:
  foo
 changed parameters:
 deleted parameters:

The output using command line ros2 param set /parameter_events plop 42 :

[INFO] [parameter_events]: 
Parameter event:
 new parameters:
 changed parameters:
 deleted parameters:

[INFO] [parameter_events]: 
Parameter event:
 new parameters:
  plop
 changed parameters:
 deleted parameters:

I created a simple/minimal file to test this : here, based on the demos pkg.

I'm not 100% sure whether the issue should be here or in rclcpp, but since it only happens when I use the command line, I'm posting it here.

Start by reproducing ros2 param set /parameter_events plop 42 and compare its behavior with the parameter-client path described in the issue. Inspect the ros2 param set entry point and the linked parameter_events_async.cpp example from the demos package. Done means the command-line operation produces one parameter event containing the changed parameter, rather than an empty event followed by a populated one.