Add Spring.after_environment_load hook for post-Rails preloading

lib/spring/application.rb

@@ -116,6 +116,17 @@ def preload
 
       require Spring.application_root_path.join("config", "environment")
 
+      # Run user-supplied post-environment-load callbacks (e.g. preloading the
+      # test runtime so forked children skip 2+s of require chains).
+      Spring.after_environment_load_callbacks.each do |callback|
+        begin
+          callback.call
+        rescue Exception => err
+          log "after_environment_load callback raised: #{err.class}: #{err.message}"
+          raise
+        end
+      end
+
       disconnect_database
 
       @preloaded = :success

lib/spring/configuration.rb

@@ -35,6 +35,18 @@ def after_fork(&block)
       after_fork_callbacks << block
     end
 
+    # Callbacks invoked once on the spring server, after config/environment
+    # has finished loading and Rails autoloads are wired up. Use this to
+    # preload heavy test/dev support files that depend on Rails being ready,
+    # so the forked child doesn't pay the cost on every command.
+    def after_environment_load_callbacks
+      @after_environment_load_callbacks ||= []
+    end
+
+    def after_environment_load(&block)
+      after_environment_load_callbacks << block
+    end
+
     def spawn_on_env
       @spawn_on_env ||= []
     end