function conflict(message: string): Error & { statusCode: number } {
  return Object.assign(new Error(message), { statusCode: 409 })
}

/** Handler lifetime, not socket lifetime: disconnect does not finish an active write. */
export class DatabaseRequestGate {
  private paused = false
  private active = new Map<symbol, (() => void) | undefined>()
  private idle: (() => void) | undefined

  async request<T>(run: () => Promise<T>, cancelStream?: () => void): Promise<T> {
    if (this.paused) throw conflict("Выполняется операция с базой приложения; повторите запрос позже")
    const lease = Symbol()
    this.active.set(lease, cancelStream)
    try { return await run() }
    finally {
      this.active.delete(lease)
      if (!this.active.size) this.idle?.()
    }
  }

  async exclusive<T>(run: () => Promise<T>, timeoutMs = 30_000): Promise<T> {
    if (this.paused) throw conflict("Операция с базой приложения уже выполняется")
    this.paused = true
    let timer: ReturnType<typeof setTimeout> | undefined
    try {
      for (const cancel of this.active.values()) cancel?.()
      if (this.active.size) await new Promise<void>((resolve, reject) => {
        this.idle = resolve
        timer = setTimeout(() => reject(conflict("Не удалось дождаться завершения API-запросов; операция с базой отменена")), timeoutMs)
      })
      return await run()
    } finally {
      if (timer) clearTimeout(timer)
      this.idle = undefined
      this.paused = false
    }
  }
}