ha-handler/lib/ha_handler/drbd/watcher.ex

defmodule HAHandler.DRBD.Watcher do
  # TODO: add support for SSH public keys authentication.

  use GenServer

  require Logger

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts)
  end

  defp connect(hostname, password) do
    case :inet.gethostbyname(to_charlist(hostname), :inet6) do
      {:ok, {:hostent, _name, _aliases, _addrtype, _length, [addr]}} ->
        SSHEx.connect(
          ip: addr,
          user: 'root',
          password: password,
          silently_accept_hosts: true
        )
      err ->
        err
    end
  end

  @impl true
  def init(opts) do
    hostname = Keyword.get(opts, :hostname)
    password = Keyword.get(opts, :password)

    case connect(hostname, password) do
      {:ok, pid} ->
        state = %{
          backend: pid,
          hostname: hostname,
          password: password
        }

        {:ok, state}
      {:error, err} ->

        # Wait for 10 seconds so that the supervisor does not loop on dead node
        # (and reach the max_restart threshold / stop trying).
        Process.sleep(10_000)

        {:error, err}
    end
  end

  @impl true
  def handle_call({:execute, cmd}, _from, %{backend: backend} = state) do
    case SSHEx.run(backend, cmd) do
      {:ok, _output, _status} = reply->
        {:reply, reply, state}
      {:error, _err} = reply ->
        {:error, reply, state}
    end
  end
end