import {
  CanActivate,
  ExecutionContext,
  Injectable,
  ServiceUnavailableException,
  SetMetadata,
} from '@nestjs/common';
import { Reflector } from '@nestjs/core';

import { MaintenanceStateService } from './maintenance-state.service';

const MAINTENANCE_EXEMPT_KEY = 'maintenanceExempt';

/**
 * Marks routes that stay reachable while an in-app restore runs: the health
 * probes (monitors must keep seeing the instance) and the restore status
 * endpoint the maintenance screen polls.
 */
export const MaintenanceExempt = (): MethodDecorator & ClassDecorator =>
  SetMetadata(MAINTENANCE_EXEMPT_KEY, true);

/**
 * Global first-line guard (registered before SetupModule and AuthModule via
 * module order): while the backup sidecar restores the database, every
 * non-exempt route answers 503 `maintenance_mode` (issue #103) — nothing
 * may read or write mid-restore state.
 */
@Injectable()
export class MaintenanceGuard implements CanActivate {
  constructor(
    private readonly reflector: Reflector,
    private readonly state: MaintenanceStateService,
  ) {}

  canActivate(context: ExecutionContext): boolean {
    const exempt = this.reflector.getAllAndOverride<boolean>(MAINTENANCE_EXEMPT_KEY, [
      context.getHandler(),
      context.getClass(),
    ]);
    if (exempt) return true;
    if (this.state.isActive()) {
      throw new ServiceUnavailableException({ code: 'maintenance_mode' });
    }
    return true;
  }
}