sys/event: add API to start periodic event without initial delay

[fabian18]

Contribution description

Depending on what you want, a periodic event could happen the first time after the first interval, or without initial delay.
Only the first variant is implemented in RIOT so far. This adds an API to start it without delay.

Currently I am misusing the timed events for that. I initialize them with 0 delay and in their callback I re-initialize and add them again with actual delay. The reinit is an invitation for race conditions if the event could also be added from another thread as the event thread.

Testing procedure

I added a call to the new API in tests/sys/event_ztimer.

Help: Press s to start test, r to print it is ready
s
START
main(): This is RIOT! (Version: 2024.10-devel-292-gb6d800-pr/event_periodic_no_delay)
posting periodic timed callback with timeout 1sec
waiting for periodic callback to be triggered 4 times
trigger 1 of periodic timeout, elapsed time: 1000547 us
trigger 2 of periodic timeout, elapsed time: 1000076 us
stop periodic event
resume periodic event, 2 triggers remaining
trigger 3 of periodic timeout, elapsed time: 663 us
trigger 4 of periodic timeout, elapsed time: 999972 us
posting periodic timed callback with timeout 1sec and no delay
waiting for periodic callback to be triggered 4 times
trigger 1 of periodic timeout, elapsed time: 406 us
trigger 2 of periodic timeout, elapsed time: 1000223 us
stop periodic event
resume periodic event, 2 triggers remaining
trigger 3 of periodic timeout, elapsed time: 682 us
trigger 4 of periodic timeout, elapsed time: 1000017 us
posting timed callback with timeout 0.5sec, clear right after
posting timed callback with timeout 1sec
waiting for timed callback to trigger
triggered timed callback after 1000794us
[SUCCESS]
{ "threads": [{ "name": "idle", "stack_size": 8192, "stack_used": 600 }]}
{ "threads": [{ "name": "main", "stack_size": 12288, "stack_used": 2452 }]}

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

d4cd708 tests/sys/event_ztimer: add event_periodic_start_now() to test

Success	Failures	Total	Runtime
10215	0	10215	17m:31s

Artifacts

• Documentation preview

[fabian18]

Thank you for quick approval!