Infer short vs longname from actual node and use longname in example (#284)

lib/livebook/config.ex

@@ -4,21 +4,21 @@ defmodule Livebook.Config do
   @type auth_mode() :: :token | :password | :disabled
 
   @doc """
-  Checks if the distribution mode is configured to use short names.
+  Returns the longname if the distribution mode is configured to use long names.
   """
-  @spec shortnames?() :: boolean()
-  def shortnames?() do
-    case Application.get_env(:livebook, :node) do
-      nil -> true
-      {:shortnames, _name} -> true
-      {:longnames, _name} -> false
+  @spec longname() :: binary() | nil
+  def longname() do
+    host = Livebook.Utils.node_host()
+
+    if host =~ "." do
+      host
     end
   end
 
   @doc """
   Returns the runtime module to be used by default.
   """
-  @spec default_runtime() :: Livebook.Runtime
+  @spec default_runtime() :: Livebook.Runtime.t()
   def default_runtime() do
     Application.fetch_env!(:livebook, :default_runtime)
   end

lib/livebook/runtime/standalone_init.ex

@@ -33,7 +33,7 @@ defmodule Livebook.Runtime.StandaloneInit do
   @spec elixir_flags(node()) :: list()
   def elixir_flags(node_name) do
     [
-      if(Livebook.Config.shortnames?(), do: "--sname", else: "--name"),
+      if(Livebook.Config.longname(), do: "--name", else: "--sname"),
       to_string(node_name),
       "--erl",
       # Minimize schedulers busy wait threshold,

lib/livebook/utils.ex

@@ -36,11 +36,19 @@ defmodule Livebook.Utils do
       String.to_atom(name)
     else
       # Default to the same host as the current node
-      [_, host] = node() |> Atom.to_string() |> String.split("@")
-      :"#{name}@#{host}"
+      :"#{name}@#{node_host()}"
     end
   end
 
+  @doc """
+  Returns the host part of a node.
+  """
+  @spec node_host() :: binary()
+  def node_host do
+    [_, host] = node() |> Atom.to_string() |> :binary.split("@")
+    host
+  end
+
   @doc """
   Registers the given process under `name` for the time of `fun` evaluation.
   """

lib/livebook_web/live/session_live/attached_live.ex

@@ -30,10 +30,10 @@ defmodule LivebookWeb.SessionLive.AttachedLive do
         Make sure to give the node a name and a cookie, for example:
       </p>
       <div class="text-gray-700 markdown">
-      <%= if Livebook.Config.shortnames? do %>
-        <pre><code>iex --sname test --cookie mycookie</code></pre>
+      <%= if longname = Livebook.Config.longname() do %>
+        <pre><code>iex --name test@<%= longname %> --cookie mycookie</code></pre>
       <% else %>
-        <pre><code>iex --name test@127.0.0.1 --cookie mycookie</code></pre>
+        <pre><code>iex --sname test --cookie mycookie</code></pre>
       <% end %>
       </div>
       <p class="text-gray-700">
@@ -43,8 +43,7 @@ defmodule LivebookWeb.SessionLive.AttachedLive do
         <div class="flex flex-col space-y-4">
           <div>
             <div class="input-label">Name</div>
-            <%= text_input f, :name, value: @data["name"], class: "input",
-                  placeholder: if(Livebook.Config.shortnames?, do: "test", else: "test@127.0.0.1") %>
+            <%= text_input f, :name, value: @data["name"], class: "input", placeholder: name_placeholder() %>
           </div>
           <div>
             <div class="input-label">Cookie</div>
@@ -90,6 +89,10 @@ defmodule LivebookWeb.SessionLive.AttachedLive do
     data["name"] != "" and data["cookie"] != ""
   end
 
+  defp name_placeholder do
+    if longname = Livebook.Config.longname(), do: "test@#{longname}", else: "test"
+  end
+
   defp runtime_error_to_message(:unreachable), do: "Node unreachable"
 
   defp runtime_error_to_message(:already_in_use),