Introduce a file type template, deprecate Template#refresh

Every template that specifies a "virtual path" loses the template source
when the template gets compiled:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template.rb#L275

The "refresh" method seems to think that the source code for a template
can be recovered if there is a virtual path:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template.rb#L171-L188

Every call site that allocates a template object *and* provides a
"virtual path" reads the template contents from the filesystem:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/template/resolver.rb#L229-L231

Templates that are inline or literals don't provide a "virtual path":

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/renderer/template_renderer.rb#L34

This commit introduces a `FileTemplate` type that subclasses `Template`.
The `FileTemplate` keeps a reference to the filename, and reads the
source from the filesystem.  This effectively makes the template source
immutable.

Other classes depended on the source to be mutated while being compiled,
so this commit also introduces a temporary way to pass the mutated
source to the ERB (or whatever) compiler.  See `LegacyTemplate`.

I think we should consider it an error to provide a virtual path on a
non file type template an non-file templates can't recover their source.
Here is an example:

  https://github.com/rails/rails/blob/eda0f574f129fcd5ad1fc58b55cb6d1db71ea95c/actionview/lib/action_view/testing/resolvers.rb#L53

This provides a "virtual path" so the source code (a string literal) is
thrown away after compilation.  Clearly we can't recover that string, so
I think this should be an error.

actionview/lib/action_view.rb

@@ -45,6 +45,7 @@ module ActionView
     autoload :Rendering
     autoload :RoutingUrlFor
     autoload :Template
+    autoload :FileTemplate
     autoload :ViewPaths
 
     autoload_under "renderer" do

actionview/lib/action_view/file_template.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require "action_view/template"
+
+module ActionView
+  class FileTemplate < Template
+    def initialize(filename, handler, details)
+      @filename = filename
+
+      super(nil, filename, handler, details)
+    end
+
+    def source
+      File.binread @filename
+    end
+
+    def refresh(_)
+      self
+    end
+
+    # Exceptions are marshalled when using the parallel test runner with DRb, so we need
+    # 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:
+      [ @identifier, @handler, @compiled, @original_encoding, @locals, @virtual_path, @updated_at, @formats, @variants ]
+    end
+
+    def marshal_load(array) # :nodoc:
+      @identifier, @handler, @compiled, @original_encoding, @locals, @virtual_path, @updated_at, @formats, @variants = *array
+      @compile_mutex = Mutex.new
+    end
+  end
+end

actionview/lib/action_view/template.rb

@@ -2,7 +2,9 @@
 
 require "active_support/core_ext/object/try"
 require "active_support/core_ext/kernel/singleton_class"
+require "active_support/deprecation"
 require "thread"
+require "delegate"
 
 module ActionView
   # = Action View Template
@@ -279,6 +281,15 @@ def compile!(view)
         end
       end
 
+      class LegacyTemplate < DelegateClass(Template) # :nodoc:
+        attr_reader :source
+
+        def initialize(template, source)
+          super(template)
+          @source = source
+        end
+      end
+
       # Among other things, this method is responsible for properly setting
       # the encoding of the compiled template.
       #
@@ -293,7 +304,7 @@ def compile!(view)
       # regardless of the original source encoding.
       def compile(mod)
         source = encode!
-        code = @handler.call(self)
+        code = @handler.call(LegacyTemplate.new(self, source))
 
         # Make sure that the resulting String to be eval'd is in the
         # encoding of the code

actionview/lib/action_view/template/resolver.rb

@@ -226,9 +226,8 @@ def query(path, details, formats, outside_app_allowed)
 
         template_paths.map do |template|
           handler, format, variant = extract_handler_and_format_and_variant(template)
-          contents = File.binread(template)
 
-          Template.new(contents, File.expand_path(template), handler,
+          FileTemplate.new(File.expand_path(template), handler,
             virtual_path: path.virtual,
             format: format,
             variant: variant,