neiam /gitgud

defmodule GitGud.Ssh.Server do
  @moduledoc """
  Embedded SSH daemon for git push/pull.

  Started under the app supervisor when configured `enabled: true`.
  Host key is generated to `host_key_dir` on first boot if not present
  (ed25519 + rsa) — the key is gitignored; treat the directory like a
  TLS cert directory.

  Configure:

      config :git_gud, GitGud.Ssh.Server,
        enabled: true,
        port: 2222,
        host_key_dir: Path.expand("priv/ssh")

  Run on a privileged port via setcap or fronting with sshd
  ProxyCommand; for dev, port 2222 is fine.
  """

  use GenServer
  require Logger

  alias GitGud.Ssh.Channel
  alias GitGud.Ssh.KeyApi

  def start_link(opts), do: GenServer.start_link(__MODULE__, opts, name: __MODULE__)

  @impl true
  def init(_opts) do
    config = Application.get_env(:git_gud, __MODULE__, [])

    cond do
      not Keyword.get(config, :enabled, false) ->
        Logger.info("SSH daemon disabled (config :git_gud, GitGud.Ssh.Server, enabled: true)")
        :ignore

      true ->
        :ok = Application.ensure_started(:ssh)
        do_start(config)
    end
  end

  defp do_start(config) do
    port = Keyword.get(config, :port, 2222)
    host_key_dir = Keyword.get(config, :host_key_dir) |> resolve_dir!()
    ensure_host_keys!(host_key_dir)

    opts =
      [
        system_dir: String.to_charlist(host_key_dir),
        key_cb: KeyApi,
        auth_methods: ~c"publickey",
        subsystems: [],
        # Channel module receives exec / shell requests and bidirectional
        # data via handle_ssh_msg. We refuse shells inside the channel.
        ssh_cli: {Channel, []},
        max_sessions: 256,
        no_auth_needed: false
      ]

    # Bind the daemon. `:ssh.daemon` expects a charlist for IP literals.
    case :ssh.daemon(port, opts) do
      {:ok, ref} ->
        Logger.info("SSH daemon listening on port #{port}")
        {:ok, %{ref: ref, port: port}}

      {:error, reason} ->
        Logger.error("Failed to start SSH daemon: #{inspect(reason)}")
        {:stop, {:ssh_daemon_failed, reason}}
    end
  end

  defp resolve_dir!(nil), do: resolve_dir!("priv/ssh")
  defp resolve_dir!(dir), do: Path.expand(dir)

  # Generate host keys if they don't already exist. The daemon expects
  # OpenSSH-format files: `ssh_host_ed25519_key` / `ssh_host_rsa_key`.
  defp ensure_host_keys!(dir) do
    File.mkdir_p!(dir)

    ed = Path.join(dir, "ssh_host_ed25519_key")
    rsa = Path.join(dir, "ssh_host_rsa_key")

    if !File.exists?(ed) and !File.exists?(rsa) do
      Logger.info("Generating SSH host key in #{dir}…")

      case System.cmd("ssh-keygen", ["-t", "ed25519", "-N", "", "-f", ed, "-q"],
             stderr_to_stdout: true
           ) do
        {_, 0} ->
          File.chmod!(ed, 0o600)

        {out, code} ->
          raise """
          Could not generate host key. ssh-keygen exited #{code}.
          #{out}

          Provide one manually at #{ed} or pre-create the directory.
          """
      end
    end
  end

  @impl true
  def terminate(_reason, %{ref: ref}) do
    :ssh.stop_daemon(ref)
    :ok
  end

  def terminate(_, _), do: :ok
end