Make sure write-protected files can only be forked (#182)

* Format file errors into readable messages

* Make sure write-protected files can only be forked

* Show tooltip when the file is write protected

lib/livebook/session.ex

@@ -675,7 +675,8 @@ defmodule Livebook.Session do
         handle_operation(state, {:mark_as_not_dirty, self()})
       else
         {:error, reason} ->
-          broadcast_error(state.session_id, "failed to save notebook - #{reason}")
+          message = :file.format_error(reason)
+          broadcast_error(state.session_id, "failed to save notebook - #{message}")
           state
       end
     else

lib/livebook_web/live/home_live.ex

@@ -57,10 +57,15 @@ defmodule LivebookWeb.HomeLive do
                   <%= live_patch "Join session", to: Routes.session_path(@socket, :page, session_id_by_path(@path, @session_summaries)),
                     class: "button button-blue" %>
                 <% else %>
-                  <%= content_tag :button, "Open",
-                    class: "button button-blue",
-                    phx_click: "open",
-                    disabled: not path_openable?(@path, @session_summaries) %>
+                  <%= tag :span, if(File.regular?(@path) and not file_writable?(@path),
+                        do: [class: "tooltip top", aria_label: "This file is write-protected, please fork instead"],
+                        else: []
+                      ) %>
+                    <%= content_tag :button, "Open",
+                      class: "button button-blue",
+                      phx_click: "open",
+                      disabled: not path_openable?(@path, @session_summaries) %>
+                  </span>
                 <% end %>
               </div>
             <% end %>
@@ -170,7 +175,7 @@ defmodule LivebookWeb.HomeLive do
   end
 
   defp path_openable?(path, session_summaries) do
-    File.regular?(path) and not path_running?(path, session_summaries)
+    File.regular?(path) and not path_running?(path, session_summaries) and file_writable?(path)
   end
 
   defp path_running?(path, session_summaries) do
@@ -178,6 +183,13 @@ defmodule LivebookWeb.HomeLive do
     path in running_paths
   end
 
+  defp file_writable?(path) do
+    case File.stat(path) do
+      {:ok, stat} -> stat.access in [:read_write, :write]
+      {:error, _} -> false
+    end
+  end
+
   defp create_session(socket, opts \\ []) do
     case SessionSupervisor.create_session(opts) do
       {:ok, id} ->