parking_solution/apps/server/src/printer-monitor.ts

import type { FastifyBaseLogger } from "fastify";
import { eq, devices, type Db } from "@parking/db";
import {
  isMonitorable,
  registry,
  type PrinterStatus,
} from "@parking/devices";
import { deviceEvents, type PrinterStatusEvent } from "./device-events.js";

// Live printer-status monitor. Polls every enabled printer that supports
// readStatus() on an interval, caches the latest status in memory, and emits a
// "printer-status" event on the device bus whenever a printer's status CHANGES
// (so the UI/SSE stream and any future entry-flow logic react without polling
// the device themselves). See wiki/concepts/printer-status-monitoring.md.
//
// The poll is the booth's early warning: it surfaces "paper out" / "cover open"
// BEFORE a driver presses the entry button and no ticket prints. Reachability
// failures degrade to status "offline" — the same signal as a dead printer.

const POLL_MS = Number(process.env.PRINTER_POLL_MS ?? 5000);

/** A cached entry: the last status plus the device's identity for the UI. */
interface CachedStatus extends PrinterStatusEvent {}

export class PrinterMonitor {
  readonly #db: Db;
  readonly #log: FastifyBaseLogger;
  readonly #pollMs: number;
  /** Latest status per device id. */
  readonly #latest = new Map<string, CachedStatus>();
  /** Live adapter per device id (rebuilt when the set of printers changes). */
  readonly #devices = new Map<string, { build: () => ReturnType<typeof registry.create>; meta: Omit<PrinterStatusEvent, "status"> }>();
  #timer: ReturnType<typeof setInterval> | null = null;
  #ticking = false;

  constructor(db: Db, log: FastifyBaseLogger, pollMs = POLL_MS) {
    this.#db = db;
    this.#log = log;
    this.#pollMs = pollMs;
  }

  /** Begin polling. Idempotent. */
  start(): void {
    if (this.#timer) return;
    // Kick an immediate pass so status is populated without waiting a full cycle.
    void this.#tick();
    this.#timer = setInterval(() => void this.#tick(), this.#pollMs);
    // Don't keep the event loop alive solely for the monitor.
    this.#timer.unref?.();
    this.#log.info(`printer-monitor: polling every ${this.#pollMs}ms`);
  }

  stop(): void {
    if (this.#timer) {
      clearInterval(this.#timer);
      this.#timer = null;
    }
  }

  /** Current snapshot for the API. */
  snapshot(): CachedStatus[] {
    return [...this.#latest.values()];
  }

  /** Reload the set of monitored printers from lane_devices (call after assign). */
  async refreshDevices(): Promise<void> {
    const rows = await this.#db
      .select()
      .from(devices)
      .where(eq(devices.category, "printer"))
      .all();

    const seen = new Set<string>();
    for (const row of rows) {
      if (!row.enabled) continue;
      const driver = registry.get(row.driverId);
      if (!driver) continue;
      const cfg = row.config as Record<string, unknown>;
      // Probe-build once to check the driver yields a monitorable device.
      let monitorable: boolean;
      try {
        monitorable = isMonitorable(driver.create(cfg as never));
      } catch {
        monitorable = false;
      }
      if (!monitorable) continue;
      seen.add(row.id);
      this.#devices.set(row.id, {
        build: () => driver.create(cfg as never),
        meta: {
          deviceId: row.id,
          driverId: row.driverId,
          role: typeof cfg.role === "string" ? cfg.role : undefined,
        },
      });
    }
    // Drop devices that are no longer present/enabled.
    for (const id of [...this.#devices.keys()]) {
      if (!seen.has(id)) {
        this.#devices.delete(id);
        this.#latest.delete(id);
      }
    }
  }

  async #tick(): Promise<void> {
    if (this.#ticking) return; // never overlap polls
    this.#ticking = true;
    try {
      await this.refreshDevices();
      await Promise.all(
        [...this.#devices.entries()].map(([id, entry]) => this.#poll(id, entry)),
      );
    } catch (err) {
      this.#log.warn(`printer-monitor tick failed: ${(err as Error).message}`);
    } finally {
      this.#ticking = false;
    }
  }

  async #poll(id: string, entry: { build: () => ReturnType<typeof registry.create>; meta: Omit<PrinterStatusEvent, "status"> }): Promise<void> {
    let status: PrinterStatus;
    try {
      const device = entry.build();
      if (!isMonitorable(device)) return;
      status = await device.readStatus();
    } catch (err) {
      status = {
        status: "offline",
        detail: (err as Error).message,
        checkedAt: new Date().toISOString(),
      };
    }

    const event: PrinterStatusEvent = { ...entry.meta, status };
    const prev = this.#latest.get(id);
    this.#latest.set(id, event);

    if (!prev || statusChanged(prev.status, status)) {
      this.#log.info(
        `printer-monitor: ${entry.meta.role ?? "printer"} ${id} -> ${status.status}${status.detail ? ` (${status.detail})` : ""}`,
      );
      deviceEvents.emitPrinterStatus(event);
    }
  }
}

/** Did the operator-meaningful status change between two reads? */
function statusChanged(a: PrinterStatus, b: PrinterStatus): boolean {
  return (
    a.status !== b.status ||
    a.paperEnd !== b.paperEnd ||
    a.paperNearEnd !== b.paperNearEnd ||
    a.coverOpen !== b.coverOpen ||
    a.cutterError !== b.cutterError ||
    a.offline !== b.offline
  );
}