De-dup Templates, introduce UnboundTemplate

Previously it's possible to have multiple copies of the "same" Template.
For example, if index.html.erb is found both the :en and :fr locale, it
will return a different Template object for each. The same can happen
with formats, variants, and handlers.

This commit de-duplicates templates, there will now only be one template
per file/virtual_path/locals tuple.

We need to consider virtual_path because both `render "index"`, and
`render "index.html"` can both find the same file but will have
different virtual_paths. IMO this is rare and should be
deprecated/removed, but it exists now so we need to consider it in order
to cache correctly.

This commit introduces a new UnboundTemplate class, which represents a
template with unknown locals. Template objects can be built from it by
using `#with_locals`. Currently, this is just a convenience around
caching templates, but I hope it's a helpful concept that could have
more utility in the future.

1 files changed, 32 insertions, 0 deletions

diff --git a/actionview/lib/action_view/unbound_template.rb b/actionview/lib/action_view/unbound_template.rb
new file mode 100644
index 0000000000..db69b6d016
--- /dev/null
+++ b/actionview/lib/action_view/unbound_template.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "concurrent/map"
+
+module ActionView
+  class UnboundTemplate
+    def initialize(source, identifer, handler, options)
+      @source = source
+      @identifer = identifer
+      @handler = handler
+      @options = options
+
+      @templates = Concurrent::Map.new(initial_capacity: 2)
+    end
+
+    def bind_locals(locals)
+      @templates[locals] ||= build_template(locals)
+    end
+
+    private
+
+      def build_template(locals)
+        options = @options.merge(locals: locals)
+        Template.new(
+          @source,
+          @identifer,
+          @handler,
+          options
+        )
+      end
+  end
+end