Refactor listing files to be independent of exiting runners

Previously, we were always fetching files, even if not required (e.g., for score runs). Now, we reduce the number of file listings and use a dedicated callback.

lib/runner/connection.rb

@@ -5,7 +5,7 @@ require 'json_schemer'
 
 class Runner::Connection
   # These are events for which callbacks can be registered.
-  EVENTS = %i[start exit stdout stderr].freeze
+  EVENTS = %i[start exit stdout stderr files].freeze
   WEBSOCKET_MESSAGE_TYPES = %i[start stdout stderr error timeout exit].freeze
   BACKEND_OUTPUT_SCHEMA = JSONSchemer.schema(JSON.parse(File.read('lib/runner/backend-output.schema.json')))
   SENTRY_OP_NAME = 'websocket.client'
@@ -15,6 +15,7 @@ class Runner::Connection
   # @!attribute exit_callback
   # @!attribute stdout_callback
   # @!attribute stderr_callback
+  # @!attribute files_callback
   attr_writer :status
   attr_reader :error
 
@@ -158,12 +159,8 @@ class Runner::Connection
         @strategy.destroy_at_management
         @error = Runner::Error::ExecutionTimeout.new('Execution exceeded its time limit')
       when :terminated_by_codeocean, :terminated_by_management
-        files = begin
-          @strategy.retrieve_files
-        rescue Runner::Error::RunnerNotFound, Runner::Error::WorkspaceError
-          {'files' => []}
-        end
-        @exit_callback.call @exit_code, files
+        @exit_callback.call @exit_code
+        list_filesystem
       when :terminated_by_client, :error
         @strategy.destroy_at_management
       else # :established
@@ -181,6 +178,20 @@ class Runner::Connection
     @event_loop.stop
   end
 
+  def list_filesystem
+    files = {'files' => []}
+    begin
+      # Retrieve files from runner management ONLY IF the callback was defined outside of this class.
+      # Otherwise, we would call our default callback and retrieve the files without any further processing.
+      files = @strategy.retrieve_files if @files_callback.source_location.first != __FILE__
+    rescue Runner::Error::RunnerNotFound, Runner::Error::WorkspaceError
+      # Ignore errors when retrieving files. This is not critical and a suitable default is already provided.
+    ensure
+      @files_callback.call files
+    end
+  end
+  private :list_filesystem
+
   # === Message Handlers
   # Each message type indicated by the +type+ attribute in the JSON
   # sent be the runner management has a dedicated method.