Fix duplicate view template loading

Currently, when rendering an ActionView::Template, the template source
is loaded from the disk twice.

During one rendering process, Template#source is called at least twice
(once in encode! and once in strict_locals!), each of these calls runs
@source.to_s. As @source is usually a Template::Sources::File, #to_s
hits the disk to load the template.

It is unnecessary to load the file from disk more than once just to
check whether the "# locals:" comment is present.

actionview/lib/action_view/template.rb

@@ -198,7 +198,8 @@ def mime_types_implementation=(implementation)
     NONE = Object.new
 
     def initialize(source, identifier, handler, locals:, format: nil, variant: nil, virtual_path: nil)
-      @source            = source.dup
+      @_source           = source.dup
+      @source            = nil
       @identifier        = identifier
       @handler           = handler
       @compiled          = false
@@ -293,7 +294,7 @@ def inspect
     end
 
     def source
-      @source.to_s
+      @source ||= @_source.to_s.dup
     end
 
     LEADING_ENCODING_REGEXP = /\A#{ENCODING_FLAG}/
@@ -376,11 +377,11 @@ def strict_locals?
     # to ensure that references to the template object can be marshalled as well. This means forgoing
     # the marshalling of the compiler mutex and instantiating that again on unmarshalling.
     def marshal_dump # :nodoc:
-      [ @source, @identifier, @handler, @compiled, @locals, @virtual_path, @format, @variant ]
+      [ @_source, @identifier, @handler, @compiled, @locals, @virtual_path, @format, @variant ]
     end
 
     def marshal_load(array) # :nodoc:
-      @source, @identifier, @handler, @compiled, @locals, @virtual_path, @format, @variant = *array
+      @_source, @identifier, @handler, @compiled, @locals, @virtual_path, @format, @variant = *array
       @compile_mutex = Mutex.new
     end
 

actionview/test/template/template_test.rb

@@ -103,6 +103,16 @@ def test_template_does_not_lose_its_source_after_rendering_if_it_does_not_have_a
     assert_equal "Hello", @template.source
   end
 
+  def test_template_only_loads_template_source_once
+    source_loaded = 0
+    source = Class.new do |klass|
+      klass.define_method(:to_s) { source_loaded += 1; "hello" }
+    end.new
+    @template = new_template(source)
+    render
+    assert_equal 1, source_loaded
+  end
+
   def test_locals
     @template = new_template("<%= my_local %>", locals: [:my_local])
     assert_equal "I am a local", render(my_local: "I am a local")