Swap Grape over to Gitlab::Json

Forces Grape to use `Gitlab::Json` for rendering JSON,
rather than going through ActiveSupport or its own wrapper.

changelogs/unreleased/gitlab-json-grape.yml

+---
+title: Swap Grape over to Gitlab::Json
+merge_request: 36472
+author:
+type: performance

lib/api/api.rb

@@ -110,6 +110,7 @@ module API
     end
 
     format :json
+    formatter :json, Gitlab::Json::GrapeFormatter
     content_type :txt, "text/plain"
 
     # Ensure the namespace is right, otherwise we might load Grape::API::Helpers

lib/gitlab/json.rb

@@ -67,6 +67,15 @@ module Gitlab
         ::JSON.pretty_generate(object, opts)
       end
 
+      # Feature detection for using Oj instead of the `json` gem.
+      #
+      # @return [Boolean]
+      def enable_oj?
+        return false unless feature_table_exists?
+
+        Feature.enabled?(:oj_json, default_enabled: true)
+      end
+
       private
 
       # Convert JSON string into Ruby through toggleable adapters.
@@ -176,13 +185,6 @@ module Gitlab
         raise parser_error if INVALID_LEGACY_TYPES.any? { |type| data.is_a?(type) }
       end
 
-      # @return [Boolean]
-      def enable_oj?
-        return false unless feature_table_exists?
-
-        Feature.enabled?(:oj_json, default_enabled: true)
-      end
-
       # There are a variety of database errors possible when checking the feature
       # flags at the wrong time during boot, e.g. during migrations. We don't care
       # about these errors, we just need to ensure that we skip feature detection
@@ -195,5 +197,28 @@ module Gitlab
         false
       end
     end
+
+    # GrapeFormatter is a JSON formatter for the Grape API.
+    # This is set in lib/api/api.rb
+
+    class GrapeFormatter
+      # Convert an object to JSON.
+      #
+      # This will default to the built-in Grape formatter if either :oj_json or :grape_gitlab_json
+      # flags are disabled.
+      #
+      # The `env` param is ignored because it's not needed in either our formatter or Grape's,
+      # but it is passed through for consistency.
+      #
+      # @param object [Object]
+      # @return [String]
+      def self.call(object, env = nil)
+        if Gitlab::Json.enable_oj? && Feature.enabled?(:grape_gitlab_json, default_enabled: true)
+          Gitlab::Json.dump(object)
+        else
+          Grape::Formatter::Json.call(object, env)
+        end
+      end
+    end
   end
 end

spec/lib/gitlab/json_spec.rb

@@ -324,6 +324,12 @@ RSpec.describe Gitlab::Json do
     end
 
     it_behaves_like "json"
+
+    describe "#enable_oj?" do
+      it "returns true" do
+        expect(subject.enable_oj?).to be(true)
+      end
+    end
   end
 
   context "json gem" do
@@ -332,5 +338,73 @@ RSpec.describe Gitlab::Json do
     end
 
     it_behaves_like "json"
+
+    describe "#enable_oj?" do
+      it "returns false" do
+        expect(subject.enable_oj?).to be(false)
+      end
+    end
+  end
+
+  describe Gitlab::Json::GrapeFormatter do
+    subject { described_class.call(obj, env) }
+
+    let(:obj) { { test: true } }
+    let(:env) { {} }
+    let(:result) { "{\"test\":true}" }
+
+    context "oj is enabled" do
+      before do
+        stub_feature_flags(oj_json: true)
+      end
+
+      context "grape_gitlab_json flag is enabled" do
+        before do
+          stub_feature_flags(grape_gitlab_json: true)
+        end
+
+        it "generates JSON" do
+          expect(subject).to eq(result)
+        end
+
+        it "uses Gitlab::Json" do
+          expect(Gitlab::Json).to receive(:dump).with(obj)
+
+          subject
+        end
+      end
+
+      context "grape_gitlab_json flag is disabled" do
+        before do
+          stub_feature_flags(grape_gitlab_json: false)
+        end
+
+        it "generates JSON" do
+          expect(subject).to eq(result)
+        end
+
+        it "uses Grape::Formatter::Json" do
+          expect(Grape::Formatter::Json).to receive(:call).with(obj, env)
+
+          subject
+        end
+      end
+    end
+
+    context "oj is disabled" do
+      before do
+        stub_feature_flags(oj_json: false)
+      end
+
+      it "generates JSON" do
+        expect(subject).to eq(result)
+      end
+
+      it "uses Grape::Formatter::Json" do
+        expect(Grape::Formatter::Json).to receive(:call).with(obj, env)
+
+        subject
+      end
+    end
   end
 end