core/thread: add thread_join()

[derMihai]

Contribution description

Motivation

Currently there is no clean way of ensuring a thread ends. One can e.g.

void *joinee_fn(void *arg)
{
    ...
    mutex_unlock(arg);
    return NULL;
}

int main(void)
{
    mutex_t exit_signal = MUTEX_INIT_LOCKED;
    thread_create(stack, sizeof(stack),
                           THREAD_PRIORITY_MAIN - 1,
                           0,
                           joinee_fn,
                           &exit_signal,
                           "joinee");

    ...

    mutex_lock(&exit_signal);
    // we can only guarantee joinee finished at this point it its prio was higher!
}

But this only works if the priority of the joinee (the thread we're joining) was higher,
otherwise there's no guarantee of progress past mutex_unlock(arg).

This PR adds thread_join(), which guarantees a thread is finished and its resources
can be reused.

Implementation

⚠️ This implementation is rather experimental. Some features are not implemented. It also aims to remain 100% backwards compatible, but it needs not be so. See below.

Backwards compatibility

The implementation adds a new thread creation flag. If not explicitly set, the threads life cycle remains unchanged, and it cannot be thread_join()ed. But if set, the thread goes into zombie state instead of stopping on exit, and thread_join() will eventually stop it. I added this distinction for compatibility with any application relying on threads to automatically stop on exit. However, one can also argue that any app doing so is buggy (see above), and as such there is a point in lifting this distinction and making all threads go into zombie mode on exit.

Return values

Return values are not implemented yet: the exit_value pointer passed to thread_join() will always be set to NULL. This is so because the thread function "returns" into sched_task_exit(), so without any CPU-specific preamble there is no way to catch the return value.

Testing procedure

Ran the test app on native and a cortex-m board.

[riot-ci]

Murdock results

✔️ PASSED

12c52cd core/thread: add thread_join()

Success	Failures	Total	Runtime
10525	0	10526	26m:05s

Artifacts

• Documentation preview