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.

4 files changed, 47 insertions, 3 deletions

diff --git a/actionview/lib/action_view.rb b/actionview/lib/action_view.rb
index c4a614fa6d..6ff2d70e35 100644
--- a/actionview/lib/action_view.rb
+++ b/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
diff --git a/actionview/lib/action_view/file_template.rb b/actionview/lib/action_view/file_template.rb
new file mode 100644
index 0000000000..4921074383
--- /dev/null
+++ b/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
diff --git a/actionview/lib/action_view/template.rb b/actionview/lib/action_view/template.rb
index a283d2467c..2d1762e568 100644
--- a/actionview/lib/action_view/template.rb
+++ b/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 @@ module ActionView
         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 @@ module ActionView
       # 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
diff --git a/actionview/lib/action_view/template/resolver.rb b/actionview/lib/action_view/template/resolver.rb
index 12ae82f8c5..3b4594942b 100644
--- a/actionview/lib/action_view/template/resolver.rb
+++ b/actionview/lib/action_view/template/resolver.rb
@@ -226,9 +226,8 @@ module ActionView
 
         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,