Method join

src/bthread/task_group.cpp:627–656

                                   void* __restrict arg);

int TaskGroup::join(bthread_t tid, void** return_value) {
    if (__builtin_expect(!tid, 0)) {  // tid of bthread is never 0.
        return EINVAL;
    }
    TaskMeta* m = address_meta(tid);
    if (BAIDU_UNLIKELY(NULL == m)) {
        // The bthread is not created yet, this join is definitely wrong.
        return EINVAL;
    }
    TaskGroup* g = tls_task_group;
    if (g != NULL && g->current_tid() == tid) {
        // joining self causes indefinite waiting.
        return EINVAL;
    }
    const uint32_t expected_version = get_version(tid);
    while (*m->version_butex == expected_version) {
        if (butex_wait(m->version_butex, expected_version, NULL) < 0 &&
            errno != EWOULDBLOCK && errno != EINTR) {
            return errno;
        }
    }
    // Ensure all memory writes made by the joined bthread are visible to
    // the joining thread after join returns. This matches the semantic
    // guarantee provided by pthread_join() across supported architectures.
    butil::atomic_thread_fence(butil::memory_order_acquire);
    if (return_value) {
        *return_value = NULL;
    }
    return 0;
}

bool TaskGroup::exists(bthread_t tid) {
    if (tid != 0) {  // tid of bthread is never 0.