pass options to delegators when they accept it (#336)

This change reduces hash allocations.

Here is a simplified example what was happening.

    def do_not_accept_keywords(**)
    end

    100.times { do_not_accept_keywords(hash: true) }

After measuring via memory profiler, we got this:

    Total allocated: 69600 bytes (300 objects)

    allocated memory by location
    -----------------------------------
         69600  profile/sample.rb:15

It points to a line where the method gets called. So, every call created a separate hash. It was
happening for `Grape::Entity::Delegator::PlainObject`.

If options get extracted, the picture is better.

    def do_not_accept_keywords(**)
    end

    opts = { hash: true }

    100.times { do_not_accept_keywords(opts) }

Allocation:

    Total allocated: 46632 bytes (201 objects)

    allocated memory by location
    -----------------------------------
         46400  profile/sample.rb:15
           232  profile/sample.rb:13

However, there is no object allocation if nothing is passed to the method.
options.

    Total allocated: 0 bytes (0 objects)

Btw, if a method "swallows" arguments, but nothing is passed, there is still allocation.

    def do_not_accept_keywords(**)
    end

    100.times { do_not_accept_keywords }

Result:

    Total allocated: 23200 bytes (100 objects)

    allocated memory by location
    -----------------------------------
         23200  profile/sample.rb:15

The splat operator brings its cost.

So, now Grape Entity checks whether the delegetor accepts options before passing them.

I measured this change against our production:

**Before:**

    Total allocated: 1512362 bytes (12328 objects)

    allocated memory by location
    -----------------------------------
         605520  /usr/local/bundle/gems/grape-entity-0.8.0/lib/grape_entity/entity.rb:537

**After:**

    Total allocated: 908402 bytes (9733 objects)

    allocated memory by location
    -----------------------------------
         18000  /app/grape-entity/lib/grape_entity/entity.rb:553

0.57 MB of RAM were saved while serving the identical request.

Author: dnesteryuk

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 * Your contribution here.
 * [#333](https://github.com/ruby-grape/grape-entity/pull/333): Fix typo in CHANGELOG.md - [@eitoball](https://github.com/eitoball).
+* [#336](https://github.com/ruby-grape/grape-entity/pull/336): Pass options to delegators when they accept it - [@dnesteryuk](https://github.com/dnesteryuk).
 
 ### 0.8.0 (2020-02-18)
 

lib/grape_entity/delegator/fetchable_object.rb

@@ -4,7 +4,7 @@ module Grape
   class Entity
     module Delegator
       class FetchableObject < Base
-        def delegate(attribute, **)
+        def delegate(attribute)
           object.fetch attribute
         end
       end

lib/grape_entity/delegator/openstruct_object.rb

@@ -4,7 +4,7 @@ module Grape
   class Entity
     module Delegator
       class OpenStructObject < Base
-        def delegate(attribute, **)
+        def delegate(attribute)
           object.send attribute
         end
       end

lib/grape_entity/delegator/plain_object.rb

@@ -4,7 +4,7 @@ module Grape
   class Entity
     module Delegator
       class PlainObject < Base
-        def delegate(attribute, **)
+        def delegate(attribute)
           object.send attribute
         end
 

lib/grape_entity/entity.rb

@@ -130,6 +130,10 @@ def hash_access=(value)
             :to_sym
           end
       end
+
+      def delegation_opts
+        @delegation_opts ||= { hash_access: hash_access }
+      end
     end
 
     @formatters = {}
@@ -479,6 +483,9 @@ def initialize(object, options = {})
       @object = object
       @options = options.is_a?(Options) ? options : Options.new(options)
       @delegator = Delegator.new(object)
+
+      # Why not `arity > 1`? It might be negative https://ruby-doc.org/core-2.6.6/Method.html#method-i-arity
+      @delegator_accepts_opts = @delegator.method(:delegate).arity != 1
     end
 
     def root_exposures
@@ -531,8 +538,10 @@ def value_for(key, options = Options.new)
     def delegate_attribute(attribute)
       if is_defined_in_entity?(attribute)
         send(attribute)
+      elsif @delegator_accepts_opts
+        delegator.delegate(attribute, self.class.delegation_opts)
       else
-        delegator.delegate(attribute, hash_access: self.class.hash_access)
+        delegator.delegate(attribute)
       end
     end