Async client: connect()->wait vs subscribe()->wait

[InventoryTech]

When I perform subscribe(...)->wait(), my understanding is that it's guaranteed that the callbacks for the iaction_listener will have completed before the wait() completes, is that correct?

When I perform connect(...)->wait(), it seems that neither the iaction_listener::on_success nor the callback::connected will necessarily have been called before the wait has completed, is that correct?

Can someone explain why? I can't seem to find any docs on the expected behaviour here.

[InventoryTech]

For example (pseudo code):

async_client client;
token_ptr subToken;
Callback callback;

void Callback::connected(const std::string& cause) {
    subToken = client->subscribe('a_topic');
}

initialise() {
    client->set_callback(callback);    
    token_ptr connTok = client->connect(callback);
    connTok->wait();
    
    // callback::connected may not have been called yet, and therefore subToken is still null
    subToken->wait();
}

[InventoryTech]

Reproduced the issue based on fairly minimal changes to the async_subscribe.cpp sample.

repro.txt

[InventoryTech]

Ok, digging in to the C library I think I understand what's going on now. on_success for the connToken is called before the callback::connected, see MqttAsync.c:

line 2236 (*(m->connect.onSuccess))(m->connect.context, &data);
...
line 2258 (*(m->connected))(m->connected_context, reason);

Token::on_success calls cond_.notify_all(); which unblocks/completes the token->wait. This is why my connected callback has not yet been called and subToken remains null.

I wonder whether the async_subscribe.cpp sample should make this clearer? Also the documentation for the Callback::connected should make clear that it will occur after Token::on_success. Documentation in this area is quite sparse in general.