native getchar is blocking RIOT

[kfessel]

Description

On native:
getchar and therefor shell are blocking RIOT (lower than getchar priority tasks) (it does not change thread since it does no use the internal mutex)

This is an stdio on native problem ztimer is just used for demonstration purposes.

Steps to reproduce the issue

I just attached en example (inside the zip since zip is much saver than demo.c and a Makefile - maybe there is another reasoning for the choice of attachables)

bug_stdio.zip

$ make
$ make term

> hello HULK

second arg to hello will be interpreted as int offset for the priority of the hello (-1 will put it in front and block shell, this is expecteted as it is spinning on a ztimer)

The same problem exists if ztimer sleep is used but it does not show as nice

Expected results

HULK shall be greeted every 2 seconds while the shell keeps running

running that same thing on non native e.g. nucleo... works as expected

Actual results

no greetings to HULK shell keeps running

Versions

current RIOT

was discovered while processing #16808

#16822 does not yet help

bug_stdio.zip

[maribu]

I think @benpicco had/has a PR open for a similar issue?

[benpicco]

Yes #19002 should fix this.
The problem is that the blocking stdio masks #495 because it prevents a context switch.

With this fixed we have preemption working properly and therefore trigger floating point issues on native for tests that use floating point.