parking_solution/wiki/concepts/entry-double-press.md

type, tags, sources, updated, status

type	tags	sources	updated	status
concept	parking entry anti-fraud safety devices		2026-06-19	open

One car = one ticket (entry anti-double-press)

A transient parking-session is a button press → print a ticket → sign a vehicle_entry → open the barrier. Nothing stopped a driver pressing the button repeatedly and minting a fresh ticket each time — a real flaw found 2026-06-19. The damage is threefold:

• Ticket spam. One car walks away with a fistful of tickets.
• Occupancy corruption. Each press signs a vehicle_entry, so capacity-occupancy (a fold over open sessions) counts one car as many — the lot reads "full" with empty spaces.
• Ticket-shopping at exit. With several open sessions for the same car, the driver pays the cheapest and exits on it; the rest linger. A direct threat-model abuse.

The old #inFlight guard only blocked overlapping presses (it released in finally), so press → print → press again issued a second ticket immediately. That is not enough.

The fix is PER-RELAY CONFIG, chosen by available barrier feedback

The guard lives on the entry relay's spec (config.relays[] — see entry-exit-points), because whether real one-car-one-ticket is possible depends on the hardware at that lane. Two modes:

PRESENCE mode (preferred — when a vehicle-presence sensor is wired)

relays[].presenceInput = the 1-based input terminal of a vehicle-presence sensor on the same dingtian-relay (the Dingtian's inputs are decoupled from its relays). The sensor may be an induction loop OR a hikvision-radar (relays[].presenceKind: "loop"|"radar" — a label; the gate behaviour is identical). A radar wired to idle opposite the button needs presenceActiveLow: true so its edge reads correctly. The rule makes one-car-one-ticket physical:

• A press prints only while a car is present on the loop.
• After a ticket prints, the relay is disarmed — no second ticket — until the loop CLEARS (the car drove through = it entered) and a new car re-occupies it.

So mashing the button while sitting on the loop does nothing; a new car must physically arrive before another ticket can issue. The flow observes the loop's input edges (both directions) to track present + armed per relay. This is how real lanes behave.

COOLDOWN mode (fallback — no barrier feedback)

When no loop is wired, relays[].entryCooldownSec suppresses repeat presses on that relay for N seconds after a ticket (default unset = no guard; a sensible value is ~10–12 s — long enough for the car to pull through, short enough not to block the next legitimate car). It is a timer, a mitigation, not a guarantee — a determined abuser can wait it out. Use it only where presence feedback isn't available; prefer wiring a loop.

The two can coexist (presence first, cooldown as a backstop), but presence is authoritative when set.

A suppressed press is a NO-OP, not an anomaly

A blocked/repeat press is recorded as unsigned device-events (a device_events kind:"input" row with entrySuppressed:true + the reason), not a signed ledger anomaly. It isn't fraud — it's the system correctly refusing to double-issue — so it stays out of the immutable chain and off the red activity-log feed. (Operator's call, 2026-06-19.) The press is still auditable in telemetry if ever needed.

Invariants preserved

• Fail-closed entry is untouched. A suppressed press simply does nothing; the printer-down HOLD path (append-only-event-chain) and the fail-state-safety rules are unchanged.
• The barrier is still intent-only. No timed close; presence is only a gate on ticketing, not a barrier-close trigger (barrier-not-a-door).
• State is in-memory + rebuildable. The per-relay armed/present map is runtime state on the host (single-writer); it is derived from live input edges, never the source of truth. A restart starts armed (the first press after a restart works), which is the safe default.

As-built (2026-06-19)

• RelaySpec gains presenceInput? + entryCooldownSec? (device-resolve.ts); relayForButton carries them onto the ResolvedRelay, and a new relayForPresence() resolves a loop-input edge to the entry relay it gates.
• EntryFlow (entry-flow.ts) keeps a #guard map keyed controllerId:relay: #onPresenceEdge tracks the loop, #suppressReason decides presence/cooldown, #recordSuppressedPress writes the telemetry. The guard disarms + stamps the cooldown on print success (not on open).
• first-run-setup relay editor: entry/both relays expose a Presence loop (terminal) field and, when no loop is set, a Cooldown after ticket (s) field (localized sq+en — see i18n).

Open

• No automated test yet (the standing harness gap) — verify on hardware: with a loop, a held button issues one ticket; after the car clears the loop a new car gets a fresh one. Without a loop, a cooldown blocks the repeat and the suppressed press lands in telemetry.
• Exit side: the symmetric concern (re-reading a ticket at exit) is already handled differently — exit validates against an open session, so a second read finds the session closed (no double-exit). No presence gate needed there today.
• Loop as a safety/anti-tailgate signal is a larger future use of the same input (free-exit detection is noted in bom); this change uses it only to gate ticketing.