mob

defmodule Mob.Dist do
  @moduledoc """
  Platform-aware Erlang distribution startup.

  On iOS, distribution is started at BEAM launch via flags in mob_beam.m
  (`-name mob_demo@127.0.0.1`), so nothing extra is needed here.

  On Android, starting distribution at BEAM launch races with Android's hwui
  thread pool initialization (~125ms window), corrupting an internal mutex and
  causing a SIGABRT. The fix is to defer `Node.start/2` until after the UI has
  fully settled.

  Additionally, `mix mob.connect` runs `adb reverse tcp:4369 tcp:4369` to tunnel
  Mac EPMD into the device. OTP's `Node.start/2` would ordinarily spawn a local
  `epmd` daemon that also tries to bind port 4369 — causing a port conflict and
  crash. The fix: set `start_epmd: false` and wait for the ADB-tunnelled EPMD to
  be reachable before calling `Node.start/2`. If the tunnel is not up within 10s
  (standalone launch, no `mix mob.connect`), distribution is skipped gracefully.

  ## Usage (in your app's start/0)

      Mob.Dist.ensure_started(node: :"mob_demo@127.0.0.1", cookie: :mob_secret)

  Options:
  - `:node`   — node name atom, e.g. `:"mob_demo@127.0.0.1"` (required on Android)
  - `:cookie` — cookie atom, e.g. `:mob_secret` (required on Android)
  - `:delay`  — ms to wait before starting dist on Android (default: 3_000)
  """

  @default_delay 3_000

  @doc """
  Ensure Erlang distribution is running for the current platform.

  - iOS: no-op (dist already started via BEAM args in mob_beam.m).
  - Android: spawns a process that sleeps for `:delay` ms then calls
    `Node.start/2` + `Node.set_cookie/1`. Pins the dist port to `:dist_port`
    (default 9100) so `dev_connect.sh` knows which port to forward.

  Options:
  - `:node`      — node name atom (required on Android)
  - `:cookie`    — cookie atom (required on Android)
  - `:delay`     — ms to wait before starting dist (default: 3_000)
  - `:dist_port` — Erlang dist listen port (default: 9100)
  """
  @spec ensure_started(keyword()) :: :ok
  def ensure_started(opts \\ []) do

    case :mob_nif.platform() do
      :ios ->
        :ok

      :android ->
        node      = Keyword.fetch!(opts, :node)
        cookie    = Keyword.fetch!(opts, :cookie)
        delay     = Keyword.get(opts, :delay, @default_delay)
        dist_port = Keyword.get(opts, :dist_port, 9100)
        spawn(fn -> start_after(node, cookie, delay, dist_port) end)
        :ok
    end
  end

  @doc """
  Stop Erlang distribution and shut down EPMD.

  Disconnects all connected nodes, stops the distribution listener, and
  terminates the local EPMD daemon if one was started by this node.

  Intended for use after an OTA update session or when forming a `Mob.Cluster`
  connection that should not persist. The app continues running normally after
  calling `stop/0` — only remote connectivity is removed.

  Returns `:ok` whether or not distribution was running.

      # OTA update session
      Mob.Dist.ensure_started(node: :"my_app@127.0.0.1", cookie: session_cookie)
      Node.connect(update_server_node)
      # ... receive BEAMs ...
      Mob.Dist.stop()

      # Mob.Cluster — rotate cookie between sessions
      Node.set_cookie(new_session_cookie)   # no restart needed
  """
  @spec stop() :: :ok
  def stop do
    if Node.alive?() do
      Enum.each(Node.list(), &Node.disconnect/1)
      :net_kernel.stop()
    end
    :ok
  end

  defp start_after(node, cookie, delay, dist_port) do
    Process.sleep(delay)
    # Wait for EPMD on port 4369 before starting distribution.
    #
    # When `mix mob.connect` is running, it sets up `adb reverse tcp:4369 tcp:4369`
    # which forwards device:4369 → Mac EPMD. We must not spawn a local epmd (which
    # would also try to bind 4369), so we set start_epmd: false and wait for the
    # Mac's EPMD to be reachable via the ADB tunnel.
    #
    # Without the tunnel (standalone launch), EPMD will never appear on 4369 and
    # we skip distribution entirely — the app runs fine, just not debuggable remotely.
    :mob_nif.log(~c"Mob.Dist: waiting for EPMD (adb reverse tcp:4369 tcp:4369)...")
    case wait_for_epmd(10_000) do
      :ready ->
        :mob_nif.log(~c"Mob.Dist: EPMD reachable, starting dist")
        # OTP auth tries to write HOME/.config/erlang/.erlang.cookie — ensure the dir exists.
        home = System.get_env("HOME") || "/data/data/com.mob.demo/files"
        File.mkdir_p("#{home}/.config/erlang")
        # Prevent OTP from spawning a local epmd daemon — the Mac's EPMD is available
        # via the ADB reverse tunnel and we must not fight it for port 4369.
        :application.set_env(:kernel, :start_epmd, false)
        # Pin the dist port so dev_connect.sh knows which port to adb-forward.
        :application.set_env(:kernel, :inet_dist_listen_min, dist_port)
        :application.set_env(:kernel, :inet_dist_listen_max, dist_port)
        result = Node.start(node, :longnames)
        :mob_nif.log(:lists.flatten(:io_lib.format(~c"Mob.Dist: result=~w", [result])))
        case result do
          {:ok, _} ->
            Node.set_cookie(cookie)
            :mob_nif.log(~c"Mob.Dist: distribution started")
          _ -> :ok
        end

      :timeout ->
        :mob_nif.log(~c"Mob.Dist: no EPMD on port 4369 after 10s — skipping dist (run mix mob.connect to enable)")
    end
  end

  # Poll port 4369 until the ADB-tunnelled Mac EPMD responds or we time out.
  defp wait_for_epmd(remaining_ms) when remaining_ms <= 0, do: :timeout
  defp wait_for_epmd(remaining_ms) do
    case :gen_tcp.connect(~c"127.0.0.1", 4369, [], 200) do
      {:ok, sock} ->
        :gen_tcp.close(sock)
        :ready
      {:error, _} ->
        Process.sleep(500)
        wait_for_epmd(remaining_ms - 700)
    end
  end
end