[knot-dns-users] [Knot-Resolver] "I yielded, not you" issue

Hi everyone, While working on my Knot-Resolver (kr) module, I came to think that the current YIELD mechanism for layers will not work properly, if multiple YIELD-enabled modules are loaded simultaneously. My understanding of how the current YIELD system works is as follows. A layer receives the current state through ((knot_layer_t*) ctx)->state. The current state can be modified by previous layers returned value. When a layer returns a YIELD state, the chain of responsability, maintained by the lib/resolve.c:ITERATE_LAYERS macro is broken (break;) and the resolution plan is evaluated once more to handle the tail element of the rplan. Once the rplan popped up all rplan_pushed queries, then the query that yielded is once more the last element of the rplan stack. This element is then evaluated, calling once more all layers, starting with the first layer. This time, the state that is provided to the layers is YIELD, except if a layer returns another value instead of returning the state as is. A layer could behave differently, based on whether it receives a YIELD state or not as "input state". This is, in fact, what is provided as example in the documentation: consume = function (state, req, answer) answer = kres.pkt_t(answer) if state == kres.YIELD then print('continuing yielded layer') return kres.DONE else if answer:qtype() == kres.type.NS then req = kres.request_t(req) local qry = req:push(answer:qname(), kres.type.SOA, kres.class.IN) qry.flags = kres.query.AWAIT_CUT print('planned SOA query, yielding') return kres.YIELD end return state end end <<< The issue, I think, is that a module has no way of knowing whether it is the one that yielded or not. Let's say I have three module, A, B, and C. B and C can yield, and A only returns YIELD, when it receives YIELD as input state. On the first evaluation, A and B returns DONE. C yields. Then on the second evaluation, A returns YIELD, because it does not handle this state and only passes it along. B receives the YIELD from A. B, as said above, may yield in some cases. B will therefore handle the YIELD, and do his "YIELD" actions, ignoring the fact that the module that actually yielded was C. I suppose B and C could define flags to be capable of remembering whether they yielded on this query or not. Unfortunately, there is currently no framework to ensure uniqueness of these flags. Sadly, this email does not come up with a proposed solution. I'm only sharing a thought about what I think could be an issue. What is your take on this? Have I missed something or misunderstood how the yield state works? Thank you. Cheers, Florian