ruby-grape / grape-entity / 664

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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 obj... (continued)

9 of 9 new or added lines in 4 files covered. (100.0%)

678 of 707 relevant lines covered (95.9%)

593.36 hits per line