[YUNIKORN-2982] Send event when preemption occurs

[rhh777]

What is this PR for?

When a preemption occurs, send an event to be displayed in the victim's event list.

What type of PR is it?

• - Improvement

What is the Jira issue?

YUNIKORN-2982

How should this be tested?

Trigger preemption

Screenshots

Questions:

• - At present, the default interval for event sending is 2 seconds. When sending a preemptive event, the task may have already been removed, triggering an exception that the corresponding task cannot be found, resulting in the failure of event sending.
  Narrowing the time interval can be a temporary solution, but I haven't thought of a more suitable solution at the moment.

[pbacsko]

See comments.

[pbacsko]

1. Message creation should be in the Send* method
2. We usually don't refer to pod names in the "core", since it's a generic scheduler and we try to stay away from K8s-specific concepts. We use allocationKey.
3. We need a test to validate that the event is generated in case of a preemption. Use a MockEventSystem to do that. Just replace the Application.appEvents with:

mockEvents := mock.NewEventSystem()
app.appEvents = schedEvt.NewApplicationEvents(mockEvents)

You can re-use an already existing test which triggers preemption and check the contents of mockEvents.Events.

[pbacsko]

I looked at preemption_test.go, the simplest test case is TestTryPreemption. You can add the extra code there to verify the event.

[pbacsko]

This must be a type of REQUEST. That will result in a real K8s event being sent to the target pod.

The removal of the allocation will be a natural consequence of that, which will flow through the scheduler, so an app-specific event is not needed here.