Added manager node monitor

Occasionally, our script that's designed to clean up after the project
node port doesn't seem to work correctly. Because of this, it makes
sense to monitor the manager node for the project node and to
shut down when the manager node dies.

apps/remote_control/lib/lexical/remote_control/application.ex

@@ -11,6 +11,7 @@ defmodule Lexical.RemoteControl.Application do
     children =
       if RemoteControl.project_node?() do
         [
+          RemoteControl.ManagerNodeMonitor,
           RemoteControl.Api.Proxy,
           RemoteControl.Commands.Reindex,
           RemoteControl.Module.Loader,

apps/remote_control/lib/lexical/remote_control/manager_node_monitor.ex

@@ -0,0 +1,43 @@
+defmodule Lexical.RemoteControl.ManagerNodeMonitor do
+  @moduledoc """
+  A node monitor that monitors the manager node for this project, and shuts down
+  the system if that node dies.
+  """
+  use GenServer
+  require Logger
+
+  def start_link(_) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  @impl true
+  def init(_) do
+    case fetch_manager_node() do
+      {:ok, manager_node} ->
+        Node.monitor(manager_node, true)
+        {:ok, manager_node}
+
+      :error ->
+        Logger.warning("Could not determine manager node for monitoring.")
+        :ignore
+    end
+  end
+
+  @impl true
+  def handle_info({:nodedown, _}, state) do
+    System.stop()
+    {:noreply, state}
+  end
+
+  defp fetch_manager_node do
+    Enum.find_value(Node.list(), :error, fn node_name ->
+      string_name = Atom.to_string(node_name)
+
+      if String.starts_with?(string_name, "manager-") do
+        {:ok, node_name}
+      else
+        false
+      end
+    end)
+  end
+end