Two Waypoints, One Name — Java Bug Hunt

Briefing

Modelled on the NATS flight-plan failure of 28 August 2023, the UK bank-holiday Monday when air traffic control had to fall back to manual flight-plan processing. NATS's investigation found that a single flight plan referred to two different waypoints that share the same identifier. The flight-plan processing system could not resolve the route, treated it as a critical error, and — to protect safety — both the primary and the backup system stopped processing, triggering widespread cancellations. This reconstruction's RouteResolver refuses any identifier with more than one match, and FlightPlanProcessor.processAll lets that one exception stop the whole batch.

Fix the resolver to use the route's context, and the processor to send a single unresolvable plan to the manual queue.

Bug report

BUG-NATS0828 · Priority: Critical · Reported by: operations

RouteResolver.resolve(route, db):

• an id with exactly one entry resolves to it
• an unknown id throws RouteException
• an id with several entries resolves to the candidate NEAREST (squared distance, Waypoint.dist2) to the previous resolved waypoint of the route
• if the ambiguous id is the first waypoint (no context), or two candidates are equally near, throw RouteException

FlightPlanProcessor.processAll(plans, db):

• each plan is resolved independently; success -> its callsign is appended to processed
• a RouteException -> its callsign is appended to manual and processing CONTINUES with the next plan; processAll never throws for one bad plan

Observed: one plan with a duplicated waypoint name stopped processing for every plan behind it.

Logs

[fprsa] plan BAW123 route ... KOBA ... -> 2 candidates for KOBA
[fprsa] critical exception, entering maintenance mode
[fprsa-backup] same plan, same exception, entering maintenance mode

The code as shipped

src/atc/FlightPlanProcessor.java (editable)

class RouteResolver {
    static List<Waypoint> resolve(List<String> route, WaypointDb db) {
        List<Waypoint> out = new ArrayList<>();
        for (String id : route) {
            List<Waypoint> candidates = db.lookup(id);
            if (candidates.size() != 1) {
                throw new RouteException("cannot resolve " + id);
            }
            out.add(candidates.get(0));
        }
        return out;
    }
}

class FlightPlanProcessor {
    final List<String> processed = new ArrayList<>();
    final List<String> manual = new ArrayList<>();

    void processAll(List<FlightPlan> plans, WaypointDb db) {
        for (FlightPlan p : plans) {
            RouteResolver.resolve(p.route, db);
            processed.add(p.callsign);
        }
    }
}

Read-only context: src/atc/Charts.java, src/atc/Waypoints.java.