livebook/lib/live_book/session.ex

defmodule LiveBook.Session do
  @moduledoc false

  # Server corresponding to a single notebook session.
  #
  # The process keeps the current notebook state and serves
  # as a source of truth that multiple clients talk to.
  # Receives update requests from the clients and notifies
  # them of any changes applied to the notebook.
  #
  # The core concept is the `Data` structure
  # to which we can apply reproducible opreations.
  # See `Data` for more information.

  use GenServer, restart: :temporary

  alias LiveBook.Session.Data
  alias LiveBook.{Evaluator, Utils, Notebook, Delta, Runtime}
  alias LiveBook.Notebook.{Cell, Section}

  @type state :: %{
          session_id: id(),
          data: Data.t(),
          evaluators: %{Section.t() => Evaluator.t()},
          client_pids: list(pid()),
          runtime_monitor_ref: reference()
        }

  @typedoc """
  An id assigned to every running session process.
  """
  @type id :: Utils.id()

  ## API

  @doc """
  Starts the server process and registers it globally using the `:global` module,
  so that it's identifiable by the given id.
  """
  @spec start_link(id()) :: GenServer.on_start()
  def start_link(session_id) do
    GenServer.start_link(__MODULE__, [session_id: session_id], name: name(session_id))
  end

  defp name(session_id) do
    {:global, {:session, session_id}}
  end

  @doc """
  Returns session pid given its id.
  """
  @spec get_pid(id()) :: pid() | nil
  def get_pid(session_id) do
    GenServer.whereis(name(session_id))
  end

  @doc """
  Registers a session client, so that the session is aware of it.

  The client process is automatically unregistered when it terminates.

  Returns the current session data, which the client can than
  keep in sync with the server by subscribing to the `sessions:id` topic
  and reciving operations to apply.
  """
  @spec register_client(id(), pid()) :: Data.t()
  def register_client(session_id, pid) do
    GenServer.call(name(session_id), {:register_client, pid})
  end

  @doc """
  Returns the current session data.
  """
  @spec get_data(id()) :: Data.t()
  def get_data(session_id) do
    GenServer.call(name(session_id), :get_data)
  end

  @doc """
  Asynchronously sends section insertion request to the server.
  """
  @spec insert_section(id(), non_neg_integer()) :: :ok
  def insert_section(session_id, index) do
    GenServer.cast(name(session_id), {:insert_section, index})
  end

  @doc """
  Asynchronously sends cell insertion request to the server.
  """
  @spec insert_cell(id(), Section.id(), non_neg_integer(), Cell.type()) ::
          :ok
  def insert_cell(session_id, section_id, index, type) do
    GenServer.cast(name(session_id), {:insert_cell, section_id, index, type})
  end

  @doc """
  Asynchronously sends section deletion request to the server.
  """
  @spec delete_section(id(), Section.id()) :: :ok
  def delete_section(session_id, section_id) do
    GenServer.cast(name(session_id), {:delete_section, section_id})
  end

  @doc """
  Asynchronously sends cell deletion request to the server.
  """
  @spec delete_cell(id(), Cell.id()) :: :ok
  def delete_cell(session_id, cell_id) do
    GenServer.cast(name(session_id), {:delete_cell, cell_id})
  end

  @doc """
  Asynchronously sends cell evaluation request to the server.
  """
  @spec queue_cell_evaluation(id(), Cell.id()) :: :ok
  def queue_cell_evaluation(session_id, cell_id) do
    GenServer.cast(name(session_id), {:queue_cell_evaluation, cell_id})
  end

  @doc """
  Asynchronously sends cell evaluation cancellation request to the server.
  """
  @spec cancel_cell_evaluation(id(), Cell.id()) :: :ok
  def cancel_cell_evaluation(session_id, cell_id) do
    GenServer.cast(name(session_id), {:cancel_cell_evaluation, cell_id})
  end

  @doc """
  Asynchronously sends notebook name update request to the server.
  """
  @spec set_notebook_name(id(), String.t()) :: :ok
  def set_notebook_name(session_id, name) do
    GenServer.cast(name(session_id), {:set_notebook_name, name})
  end

  @doc """
  Asynchronously sends section name update request to the server.
  """
  @spec set_section_name(id(), Section.id(), String.t()) :: :ok
  def set_section_name(session_id, section_id, name) do
    GenServer.cast(name(session_id), {:set_section_name, section_id, name})
  end

  @doc """
  Asynchronously sends a cell delta to apply to the server.
  """
  @spec apply_cell_delta(id(), pid(), Cell.id(), Delta.t(), Data.cell_revision()) :: :ok
  def apply_cell_delta(session_id, from, cell_id, delta, revision) do
    GenServer.cast(name(session_id), {:apply_cell_delta, from, cell_id, delta, revision})
  end

  @doc """
  Asynchronously connects to the given runtime.

  Note that this results in initializing the corresponding remote node
  with modules and processes required for evaluation.
  """
  @spec connect_runtime(id(), Runtime.t()) :: :ok
  def connect_runtime(session_id, runtime) do
    GenServer.cast(name(session_id), {:connect_runtime, runtime})
  end

  @doc """
  Asynchronously disconnects from the current runtime.

  Note that this results in clearing the evaluation state.
  """
  @spec disconnect_runtime(id()) :: :ok
  def disconnect_runtime(session_id) do
    GenServer.cast(name(session_id), :disconnect_runtime)
  end

  @doc """
  Synchronously stops the server.
  """
  @spec stop(id()) :: :ok
  def stop(session_id) do
    GenServer.stop(name(session_id))
  end

  ## Callbacks

  @impl true
  def init(session_id: session_id) do
    {:ok,
     %{
       session_id: session_id,
       data: Data.new(),
       client_pids: [],
       evaluators: %{},
       runtime_monitor_ref: nil
     }}
  end

  @impl true
  def handle_call({:register_client, pid}, _from, state) do
    Process.monitor(pid)
    {:reply, state.data, %{state | client_pids: [pid | state.client_pids]}}
  end

  def handle_call(:get_data, _from, state) do
    {:reply, state.data, state}
  end

  @impl true
  def handle_cast({:insert_section, index}, state) do
    # Include new id in the operation, so it's reproducible
    operation = {:insert_section, index, Utils.random_id()}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:insert_cell, section_id, index, type}, state) do
    # Include new id in the operation, so it's reproducible
    operation = {:insert_cell, section_id, index, type, Utils.random_id()}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:delete_section, section_id}, state) do
    operation = {:delete_section, section_id}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:delete_cell, cell_id}, state) do
    operation = {:delete_cell, cell_id}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:queue_cell_evaluation, cell_id}, state) do
    case ensure_runtime(state) do
      {:ok, state} ->
        operation = {:queue_cell_evaluation, cell_id}
        {:noreply, handle_operation(state, operation)}

      {:error, error} ->
        broadcast_error(state.session_id, "failed to setup runtime - #{error}")
        {:noreply, state}
    end
  end

  def handle_cast({:cancel_cell_evaluation, cell_id}, state) do
    operation = {:cancel_cell_evaluation, cell_id}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:set_notebook_name, name}, state) do
    operation = {:set_notebook_name, name}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:set_section_name, section_id, name}, state) do
    operation = {:set_section_name, section_id, name}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:apply_cell_delta, from, cell_id, delta, revision}, state) do
    operation = {:apply_cell_delta, from, cell_id, delta, revision}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_cast({:connect_runtime, runtime}, state) do
    if state.data.runtime do
      Runtime.disconnect(state.data.runtime)
    end

    runtime_monitor_ref = Runtime.connect(runtime)

    {:noreply,
     %{state | runtime_monitor_ref: runtime_monitor_ref}
     |> handle_operation({:set_runtime, runtime})}
  end

  def handle_cast(:disconnect_runtime, state) do
    Runtime.disconnect(state.data.runtime)

    {:noreply,
     %{state | runtime_monitor_ref: nil}
     |> handle_operation({:set_runtime, nil})}
  end

  @impl true
  def handle_info({:DOWN, ref, :process, _, _}, %{runtime_monitor_ref: ref} = state) do
    broadcast_info(state.session_id, "runtime node terminated unexpectedly")

    {:noreply,
     %{state | runtime_monitor_ref: nil}
     |> handle_operation({:set_runtime, nil})}
  end

  def handle_info({:DOWN, _, :process, pid, _}, state) do
    {:noreply, %{state | client_pids: List.delete(state.client_pids, pid)}}
  end

  def handle_info({:evaluation_stdout, cell_id, string}, state) do
    operation = {:add_cell_evaluation_stdout, cell_id, string}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_info({:evaluation_response, cell_id, response}, state) do
    operation = {:add_cell_evaluation_response, cell_id, response}
    {:noreply, handle_operation(state, operation)}
  end

  def handle_info(_message, state), do: {:noreply, state}

  # ---

  # Given any opeation on `Data`, the process does the following:
  #
  #   * broadcasts the operation to all clients immediately,
  #     so that they can update their local `Data`
  #   * applies the operation to own local `Data`
  #   * if necessary, performs the relevant actions (e.g. starts cell evaluation),
  #     to reflect the new `Data`
  #
  defp handle_operation(state, operation) do
    broadcast_operation(state.session_id, operation)

    case Data.apply_operation(state.data, operation) do
      {:ok, new_data, actions} ->
        new_state = %{state | data: new_data}
        handle_actions(new_state, actions)

      :error ->
        state
    end
  end

  defp handle_actions(state, actions) do
    Enum.reduce(actions, state, &handle_action(&2, &1))
  end

  defp handle_action(state, {:start_evaluation, cell, section}) do
    start_evaluation(state, cell, section)
  end

  defp handle_action(state, {:stop_evaluation, section}) do
    if state.data.runtime do
      Runtime.drop_container(state.data.runtime, section.id)
    end

    state
  end

  defp handle_action(state, {:forget_evaluation, cell, section}) do
    if state.data.runtime do
      Runtime.forget_evaluation(state.data.runtime, section.id, cell.id)
    end

    state
  end

  defp handle_action(state, _action), do: state

  defp broadcast_operation(session_id, operation) do
    broadcast_message(session_id, {:operation, operation})
  end

  defp broadcast_error(session_id, error) do
    broadcast_message(session_id, {:error, error})
  end

  defp broadcast_info(session_id, info) do
    broadcast_message(session_id, {:info, info})
  end

  defp broadcast_message(session_id, message) do
    Phoenix.PubSub.broadcast(LiveBook.PubSub, "sessions:#{session_id}", message)
  end

  defp start_evaluation(state, cell, section) do
    prev_ref =
      case Notebook.parent_cells(state.data.notebook, cell.id) do
        [parent | _] -> parent.id
        [] -> :initial
      end

    Runtime.evaluate_code(state.data.runtime, cell.source, section.id, cell.id, prev_ref)

    state
  end

  # Checks if a runtime already set, and if that's not the case
  # starts a new standlone one.
  defp ensure_runtime(%{data: %{runtime: nil}} = state) do
    with {:ok, runtime} <- Runtime.Standalone.init(self()) do
      runtime_monitor_ref = Runtime.connect(runtime)

      {:ok,
       %{state | runtime_monitor_ref: runtime_monitor_ref}
       |> handle_operation({:set_runtime, runtime})}
    end
  end

  defp ensure_runtime(state), do: {:ok, state}
end