defmodule ProxmoxAgent.Diagnostics do
  @moduledoc """
  Optional diagnostic dump of external commands and outgoing samples.

  Controlled by `:dump_dir` in the application env. When unset or nil,
  `log_command/4` and `log_sample/2` are no-ops and incur no I/O.
  When set, they cast to `ProxmoxAgent.Diagnostics.Writer` (if running)
  which serializes appends to `commands.log` and `samples.log`.
  """

  require Logger

  @type command_result ::
          {:ok, String.t()}
          | {:error, term()}

  @spec configure(String.t() | nil) :: :ok
  def configure(nil), do: disable()
  def configure(""), do: disable()

  def configure(dir) when is_binary(dir) do
    case File.mkdir_p(dir) do
      :ok ->
        Application.put_env(:agent, :dump_dir, dir)
        Logger.info("diagnostics: enabled, writing to #{dir}")
        :ok

      {:error, reason} ->
        Logger.warning("diagnostics: mkdir_p #{dir} failed (#{inspect(reason)}); dumping disabled")
        disable()
    end
  end

  @spec enabled?() :: boolean()
  def enabled?, do: not is_nil(Application.get_env(:agent, :dump_dir))

  @spec dump_dir() :: String.t() | nil
  def dump_dir, do: Application.get_env(:agent, :dump_dir)

  @spec log_command(String.t(), [String.t()], command_result(), non_neg_integer()) :: :ok
  def log_command(cmd, args, result, duration_us) do
    cast({:command, cmd, args, result, duration_us})
  end

  @spec log_sample(String.t(), map()) :: :ok
  def log_sample(kind, payload) when is_binary(kind) and is_map(payload) do
    cast({:sample, kind, payload})
  end

  @spec log_read(Path.t(), command_result(), non_neg_integer()) :: :ok
  def log_read(path, result, duration_us) do
    log_command("cat", [to_string(path)], result, duration_us)
  end

  defp disable do
    Application.delete_env(:agent, :dump_dir)
    :ok
  end

  defp cast(msg) do
    if enabled?() do
      case Process.whereis(ProxmoxAgent.Diagnostics.Writer) do
        nil -> :ok
        pid -> GenServer.cast(pid, msg)
      end
    end

    :ok
  end
end