fix(anpr): share one camera snapshot across bridge + advisory paths

On a vehicle entry, two paths captured the SAME Hikvision camera within ~1s —
the ANPR bridge (barrier-driving) and the advisory snapshotAsync (evidence/
telemetry) — each from a separate adapter instance. Hikvision serves snapshots
single-threaded, so the second concurrent GET returned HTTP 503; the bridge
then fail-softed and burned its 12s debounce, producing a ~74s "slow" subscriber
entry (observed 2026-06-25, Qazim Mulleti / AB816NN — plate read was instant at
conf 1.000; the delay was the 503/debounce churn, not recognition).

Add captureSnapshotShared() in snapshot.ts: a module-level, deviceId-keyed cache
that both paths call. It coalesces in-flight captures (the 2nd caller awaits the
1st's pull → no concurrent 503), serves a brief freshness window (1500ms) so the
bridge→advisory sequence for one vehicle reuses one frame, never caches a failure
(next caller retries), and keys by deviceId (no cross-camera/stale-vehicle reuse).
Wired into anpr-entry.ts (bridge) and snapshot.ts (advisory).

Tests: snapshot.test.ts (concurrent coalescing, TTL reuse, TTL-lapse re-pull,
failure-not-cached, per-camera keying); anpr-entry.test.ts mock updated. 168
server tests green.

NOTE: this removes the latency (the 503 collision). The separate double-entry
(two signed vehicle_entry for one car) — debounce-too-short / stamp-before-
success — is still open; less likely now but not eliminated.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/server/src/anpr-entry.test.ts

@@ -15,8 +15,13 @@ import type { SubscriptionFlow, SubscriptionMatch } from "./subscription-flow.js
 // Mock buildCamera so the bridge gets a fake camera whose captureSnapshot is a stub
 // (no registry, no network). The factory returns a fresh shot each call.
 const captureSnapshot = vi.fn(async () => ({ bytes: Buffer.from("jpg"), contentType: "image/jpeg" }));
+// The bridge now goes through captureSnapshotShared (the dedup wrapper, exercised in
+// snapshot.test.ts); here it just delegates to the fake camera's captureSnapshot so this
+// suite stays focused on the bridge's own match/debounce/emit logic.
 vi.mock("./snapshot.js", () => ({
   buildCamera: () => ({ captureSnapshot }),
+  captureSnapshotShared: (_id: string, camera: { captureSnapshot: typeof captureSnapshot }, ctx: unknown) =>
+    camera.captureSnapshot(ctx as never),
 }));
 
 // Import AFTER the mock is registered.

apps/server/src/anpr-entry.ts

@@ -3,7 +3,7 @@ import { devices, deviceEvents as deviceEventsTable, eq, siteConfig, type Db, ty
 import type { FastifyBaseLogger } from "fastify";
 import { deviceEvents, type DeviceReadEvent } from "./device-events.js";
 import { directionOf, type FlowDirection } from "./device-resolve.js";
-import { buildCamera } from "./snapshot.js";
+import { buildCamera, captureSnapshotShared } from "./snapshot.js";
 import type { SubscriptionFlow } from "./subscription-flow.js";
 import type { VisionClient } from "./vision-client.js";
 
@@ -100,7 +100,9 @@ export class AnprBridge {
       // "both" collapses to entry purely for the capture hint (it doesn't pick the lane —
       // the gated flow infers the verb from the camera's bound relay direction).
       const direction: FlowDirection = directionOf(this.#db, row) === "exit" ? "exit" : "entry";
-      const shot = await camera.captureSnapshot({ direction });
+      // Shared capture (deviceId-keyed): coalesces with the advisory snapshotAsync for
+      // the SAME vehicle so the single-threaded camera isn't hit twice (→ HTTP 503).
+      const shot = await captureSnapshotShared(deviceId, camera, { direction });
       const result = await this.#vision.analyze(shot.bytes, shot.contentType);
       if (!result || !result.plate) return; // nothing read
 

apps/server/src/snapshot.test.ts

@@ -0,0 +1,93 @@
+import { describe, expect, it, vi } from "vitest";
+import type { CameraDevice, Snapshot } from "@parking/devices";
+import { captureSnapshotShared } from "./snapshot.js";
+
+// captureSnapshotShared: one HTTP pull per camera per vehicle. A Hikvision unit serves
+// snapshots SINGLE-THREADED (a 2nd concurrent GET → HTTP 503). On an entry the ANPR
+// bridge AND the advisory snapshotAsync both capture the same camera within ~1s, each
+// from a SEPARATE adapter instance — so this deviceId-keyed cache coalesces in-flight
+// captures and serves a brief freshness window, collapsing the two into one real pull.
+// (Root cause of the slow 2026-06-25 subscriber entry.)
+
+/** A fake camera whose captureSnapshot is controllable (count calls, delay, fail). */
+function fakeCamera(opts: { delayMs?: number; fail?: boolean; tag?: string } = {}): {
+  camera: CameraDevice;
+  calls: () => number;
+} {
+  let calls = 0;
+  const tag = opts.tag ?? "x";
+  const camera = {
+    async captureSnapshot(): Promise<Snapshot> {
+      calls++;
+      if (opts.delayMs) await new Promise((r) => setTimeout(r, opts.delayMs));
+      if (opts.fail) throw new Error("HTTP 503");
+      // Tag distinguishes frames from different cameras (the per-camera keying test).
+      return { bytes: Buffer.from(`shot-${tag}-${calls}`), contentType: "image/jpeg", capturedAt: new Date().toISOString() };
+    },
+  } as unknown as CameraDevice;
+  return { camera, calls: () => calls };
+}
+
+/** A unique deviceId per test so the module-level cache never bleeds across cases. */
+function id(): string {
+  return `cam-${Math.random().toString(36).slice(2)}`;
+}
+
+describe("captureSnapshotShared", () => {
+  it("coalesces CONCURRENT captures into a single hardware pull (the 503 fix)", async () => {
+    const { camera, calls } = fakeCamera({ delayMs: 20 });
+    const dev = id();
+    // The bridge and the advisory path fire at nearly the same instant.
+    const [a, b] = await Promise.all([
+      captureSnapshotShared(dev, camera, { direction: "entry" }),
+      captureSnapshotShared(dev, camera, { direction: "entry" }),
+    ]);
+    expect(calls()).toBe(1); // ONE GET, not two — no concurrent 503
+    expect(a.bytes.equals(b.bytes)).toBe(true); // both got the same frame
+  });
+
+  it("reuses a fresh capture within the TTL (sequential, same vehicle)", async () => {
+    const { camera, calls } = fakeCamera();
+    const dev = id();
+    const a = await captureSnapshotShared(dev, camera, { direction: "entry" });
+    const b = await captureSnapshotShared(dev, camera, { direction: "entry" }); // ~0ms later
+    expect(calls()).toBe(1); // 2nd call served from the freshness cache
+    expect(a.bytes.equals(b.bytes)).toBe(true);
+  });
+
+  it("pulls AGAIN after the TTL lapses (a later, different vehicle)", async () => {
+    vi.useFakeTimers();
+    try {
+      const { camera, calls } = fakeCamera();
+      const dev = id();
+      await captureSnapshotShared(dev, camera, { direction: "entry" });
+      expect(calls()).toBe(1);
+      await vi.advanceTimersByTimeAsync(2000); // past SNAPSHOT_TTL_MS (1500)
+      await captureSnapshotShared(dev, camera, { direction: "entry" });
+      expect(calls()).toBe(2); // stale → a real new pull (never a stale frame for a new car)
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("does NOT cache a failure — the next caller retries", async () => {
+    const dev = id();
+    const failing = fakeCamera({ fail: true });
+    await expect(captureSnapshotShared(dev, failing.camera, { direction: "entry" })).rejects.toThrow("503");
+    // A subsequent capture (camera recovered) must actually pull, not inherit the error.
+    const ok = fakeCamera();
+    const shot = await captureSnapshotShared(dev, ok.camera, { direction: "entry" });
+    expect(shot.bytes.toString()).toBe("shot-x-1");
+    expect(ok.calls()).toBe(1);
+  });
+
+  it("keys by deviceId — different cameras never share a frame", async () => {
+    const c1 = fakeCamera({ tag: "A" });
+    const c2 = fakeCamera({ tag: "B" });
+    const s1 = await captureSnapshotShared("cam-A", c1.camera, { direction: "entry" });
+    const s2 = await captureSnapshotShared("cam-B", c2.camera, { direction: "entry" });
+    expect(c1.calls()).toBe(1);
+    expect(c2.calls()).toBe(1);
+    expect(s1.bytes.equals(s2.bytes)).toBe(false);
+  });
+});

apps/server/src/snapshot.ts

@@ -1,6 +1,6 @@
 import { randomUUID } from "node:crypto";
 import { deviceEvents as deviceEventsTable, snapshots, type Db } from "@parking/db";
-import { registry, type CameraDevice } from "@parking/devices";
+import { registry, type CameraDevice, type Snapshot } from "@parking/devices";
 import type { FastifyBaseLogger } from "fastify";
 import { devicesByDirection, type FlowDirection } from "./device-resolve.js";
 import type { VisionClient } from "./vision-client.js";
@@ -62,7 +62,9 @@ export function snapshotAsync(job: SnapshotJob): Promise<string[]> {
         return null;
       }
       try {
-        const shot = await camera.captureSnapshot({ direction });
+        // Shared capture: if the ANPR bridge just pulled this camera's frame for the
+        // same vehicle, reuse it instead of a 2nd concurrent GET (which 503s).
+        const shot = await captureSnapshotShared(row.id, camera, { direction });
         const id: string = randomUUID();
         db.insert(snapshots)
           .values({
@@ -153,6 +155,70 @@ export function buildCamera(row: { driverId: string; config: unknown }): CameraD
   }
 }
 
+// --- shared snapshot capture (one HTTP pull per camera per vehicle) -----------
+// A Hikvision camera serves /ISAPI/.../picture SINGLE-THREADED: two concurrent
+// snapshot GETs to the same unit return HTTP 503 "service busy". On a vehicle entry
+// TWO paths capture the SAME camera within ~1s — the ANPR bridge (barrier-driving,
+// anpr-entry.ts) and the advisory snapshotAsync (evidence + telemetry, below). They
+// each `buildCamera()` a SEPARATE adapter instance, so a per-instance cache can't
+// dedupe them. This module-level, deviceId-keyed cache does: it coalesces in-flight
+// captures (the 2nd caller awaits the 1st's pull) AND serves a result captured within
+// SNAPSHOT_TTL_MS, so the bridge + advisory share ONE frame instead of colliding into
+// a 503 (which then burned the bridge's 12s debounce → the slow entry observed
+// 2026-06-25; see wiki/concepts/lane-presence-and-anpr-entry.md).
+
+/** How long a fresh capture is reused for the same camera. A car is one event for a
+ *  couple of seconds; 1.5s comfortably spans the bridge→advisory gap without ever
+ *  serving a stale frame for a *different* vehicle (entries are seconds apart). */
+const SNAPSHOT_TTL_MS = 1500;
+
+interface CacheEntry {
+  /** A capture in flight — concurrent callers await this instead of issuing a 2nd GET. */
+  inflight?: Promise<Snapshot>;
+  /** The last SUCCESSFUL capture + when it resolved, for the freshness window. */
+  last?: { shot: Snapshot; at: number };
+}
+
+const snapshotCache = new Map<string, CacheEntry>();
+
+/**
+ * Capture a snapshot for a camera, sharing ONE HTTP pull across concurrent/near-
+ * simultaneous callers (the ANPR bridge and the advisory snapshot). Same contract as
+ * `camera.captureSnapshot` (throws on failure) — a failed pull is NOT cached, so the
+ * next caller retries rather than inheriting the error. Key by the stable `deviceId`.
+ */
+export function captureSnapshotShared(
+  deviceId: string,
+  camera: CameraDevice,
+  ctx: { direction: FlowDirection },
+): Promise<Snapshot> {
+  const now = Date.now();
+  let entry = snapshotCache.get(deviceId);
+  if (!entry) {
+    entry = {};
+    snapshotCache.set(deviceId, entry);
+  }
+  // Fresh enough → reuse the last frame (same vehicle, no second hardware hit).
+  if (entry.last && now - entry.last.at < SNAPSHOT_TTL_MS) {
+    return Promise.resolve(entry.last.shot);
+  }
+  // A capture is already running → join it (this is what prevents the 503 collision).
+  if (entry.inflight) return entry.inflight;
+  // Otherwise issue the single real pull; record it as the in-flight promise.
+  const pull = camera
+    .captureSnapshot(ctx)
+    .then((shot) => {
+      entry.last = { shot, at: Date.now() };
+      return shot;
+    })
+    .finally(() => {
+      // Clear the in-flight slot whether it resolved or threw; a failure is never cached.
+      if (entry.inflight === pull) entry.inflight = undefined;
+    });
+  entry.inflight = pull;
+  return pull;
+}
+
 function recordFailure(
   db: Db,
   direction: FlowDirection,