cache all subnodes in the digest cache

this is for backwards compatibility.  We should remove this after
5-0-stable is created because digest calculation should be amortized.
Caching digests of subtrees will speed up digests for other trees that
share the same children.  However, this will also increase memory, and
after the app warms up, then those subtrees will never be reused which
means that memory is wasted.

actionview/lib/action_view/digestor.rb

@@ -28,7 +28,7 @@ module ActionView
         # (Concurrent::Map's lookups have volatile semantics)
         finder.digest_cache[cache_key] || @@digest_mutex.synchronize do
           finder.digest_cache.fetch(cache_key) do # re-check under lock
-            finder.digest_cache[cache_key] = tree(name, finder, dependencies).digest
+            finder.digest_cache[cache_key] = tree(name, finder, dependencies).digest(finder)
           end
         end
       end
@@ -83,17 +83,19 @@ module ActionView
         @children     = children
       end
 
-      def digest(stack = [])
-        Digest::MD5.hexdigest("#{template.source}-#{dependency_digest(stack)}")
+      def digest(finder, stack = [])
+        Digest::MD5.hexdigest("#{template.source}-#{dependency_digest(finder, stack)}")
       end
 
-      def dependency_digest(stack)
+      def dependency_digest(finder, stack)
         children.map do |node|
           if stack.include?(node)
             false
           else
-            stack.push node
-            node.digest(stack).tap { stack.pop }
+            finder.digest_cache[node.name] ||= begin
+                                                 stack.push node
+                                                 node.digest(finder, stack).tap { stack.pop }
+                                               end
           end
         end.join("-")
       end
@@ -106,11 +108,11 @@ module ActionView
     class Partial < Node; end
 
     class Missing < Node
-      def digest(_ = []) '' end
+      def digest(finder, _ = []) '' end
     end
 
     class Injected < Node
-      def digest(_ = []) name end
+      def digest(finder, _ = []) name end
     end
 
     class NullLogger

actionview/test/template/digestor_test.rb

@@ -319,7 +319,7 @@ class TemplateDigestorTest < ActionView::TestCase
       finder.variants = options.delete(:variants) || []
 
       node = ActionView::Digestor.tree(template_name, finder, options[:dependencies] || [])
-      node.digest
+      node.digest(finder)
     end
 
     def dependencies(template_name)