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While investigating #21297 (moved), I found several issues. Unfortunately, I can't find the cause of #21297 (moved) yet but maybe someone will with that list of issues.

1. Integer overflow in rend_consider_services_intro_points(). We have a safe guard against that except if the expiring_nodes list is not empty which is totally a possibility. This is bad because it will ultimately create a LOT of IP circuits and fail in unknown ways...

    if (smartlist_len(service->expiring_nodes) == 0 &&
        intro_nodes_len >= service->n_intro_points_wanted) {
      continue;
    }

    /* Number of intro points we want to open which is the wanted amount
     * minus the current amount of valid nodes. */
    n_intro_points_to_open = service->n_intro_points_wanted - intro_nodes_len;

2. In rend_service_intro_has_opened(), this if is subject to a bad "cast overflow" if we have a case that brings the expiring_nodes size bigger than the number of circuits. The following result is cast as an unsigned int so for instance "5 - 6" is actually a BIG number.

  if ((count_intro_point_circuits(service) -
       smartlist_len(service->expiring_nodes)) >
      service->n_intro_points_wanted) {

The consequence of this is that if we have a bug (maybe #21297 (moved)!!) that brings the size of expiring_nodes to 6 and then we try to open 5 IP circuits for the service (3 + 2 extras), all 5 will always be closed and it will loop non stop.

3. In remove_invalid_intro_points(), if we put an IP object in the retry_nodes list and this IP also expires, it will be put in the expiring_nodes list of the service and removed from the service valid list. After this, we'll retry a circuit to that IP object but since it's not in the valid list anymore, the rend_consider_services_intro_points() will launch a bunch of other IP circuit(s) ignoring the one being retried. This is OK because the one we want to retry should actually expire BUT we could avoid a useless circuit being created since we know it will expire and potential bug of losing the reference to that circuit if we have issues with cleaning up our expiring_nodes list.

My theory is that the solution to #21297 (moved) lies in a combination of the above bugs. Either we overflow with (2) or we have IP circuits that we've lost the reference to (1) making (2) always closing the 5 circuits we just opened and thus not having any valid IP object for the service. I don't think (1) is involved but you never know.