sys/event: manage event thread size via build system

[maribu]

Contribution description

This adds a mechanism for modules to declare requirements on the thread stack size in their Makefile.dep and let the build system then override the default stack size, if any requirements are declared.

The motivation is to allow multiple modules to have special requirements without causing conflicts, as just adding the following to their Makefile.include would do:

CFLAGS += -DEVENT_THREAD_MEDIUM_STACKSIZE=<MAGIC_NUMBER>

Instead, the new mechanism would work by having them both declare in their Makefile.dep:

EVENT_THREAD_MEDIUM_STACKSIZE_MIN +=

The build system then picks the maximum number in EVENT_THREAD_MEDIUM_STACKSIZE_MIN and exposes this as stack size, if any module did declare a minimum requirement.

Testing procedure

The test in tests/sys/event_thread_shared has been extended to test correct passing of the stack size value at compile time. So a green CI would be sufficient.

Issues/PRs references

None

[riot-ci]

Murdock results

✔️ PASSED

2db5023 fixup! tests/sys/event_thread_shared: test stack size handling

Success	Failures	Total	Runtime
10950	0	10950	12m:04s

Artifacts

• Documentation preview

[maribu]

I defused the footgun with potential confusion of event thread names and event queue names.

E.g. if event_thread is in use, we only have the lowest prio event thread handling all three event queues (but handling the event queues in the order of their priorities).

I think a module posting to EVENT_PRIO_MEDIUM rightfully may expect that EVENT_THREAD_MEDIUM_STACKSIZE_MIN += 1024 would ensure that the thread handling EVENT_PRIO_MEDIUM has a stack size of at least 1024. With new change this is the case, even when the medium queue is handled by the same thread as EVENT_PRIO_LOWEST.