Replicate current json gem behaviour in wrapper

This is a transparent change that will affect nothing, but it adds
support for emulating the old behaviour of the `json` gem, which will
be upgraded in the next iteration of this work.

lib/gitlab/json.rb

@@ -3,8 +3,13 @@
 module Gitlab
   module Json
     class << self
-      def parse(*args)
-        adapter.parse(*args)
+      def parse(string, *args, **named_args)
+        legacy_mode = legacy_mode_enabled?(named_args.delete(:legacy_mode))
+        data = adapter.parse(string, *args, **named_args)
+
+        raise parser_error if legacy_mode && [String, TrueClass, FalseClass].any? { |type| data.is_a?(type) }
+
+        data
       end
 
       def parse!(*args)
@@ -28,6 +33,17 @@ module Gitlab
       def adapter
         ::JSON
       end
+
+      def parser_error
+        ::JSON::ParserError
+      end
+
+      def legacy_mode_enabled?(arg_value)
+        # This will change to the following once the `json` gem is upgraded:
+        # arg_value.nil? ? true : arg_value
+
+        true
+      end
     end
   end
 end

spec/lib/gitlab/json_spec.rb

@@ -4,24 +4,52 @@ require "spec_helper"
 
 RSpec.describe Gitlab::Json do
   describe ".parse" do
-    it "parses an object" do
-      expect(subject.parse('{ "foo": "bar" }')).to eq({ "foo" => "bar" })
-    end
+    context "legacy_mode is on by default" do
+      it "parses an object" do
+        expect(subject.parse('{ "foo": "bar" }')).to eq({ "foo" => "bar" })
+      end
 
-    it "parses an array" do
-      expect(subject.parse('[{ "foo": "bar" }]')).to eq([{ "foo" => "bar" }])
-    end
+      it "parses an array" do
+        expect(subject.parse('[{ "foo": "bar" }]')).to eq([{ "foo" => "bar" }])
+      end
 
-    it "raises an error on a string" do
-      expect { subject.parse('"foo"') }.to raise_error(JSON::ParserError)
-    end
+      it "raises an error on a string" do
+        expect { subject.parse('"foo"') }.to raise_error(JSON::ParserError)
+      end
 
-    it "raises an error on a true bool" do
-      expect { subject.parse("true") }.to raise_error(JSON::ParserError)
+      it "raises an error on a true bool" do
+        expect { subject.parse("true") }.to raise_error(JSON::ParserError)
+      end
+
+      it "raises an error on a false bool" do
+        expect { subject.parse("false") }.to raise_error(JSON::ParserError)
+      end
     end
 
-    it "raises an error on a false bool" do
-      expect { subject.parse("false") }.to raise_error(JSON::ParserError)
+    context "legacy_mode is disabled" do
+      it "parses an object" do
+        expect(subject.parse('{ "foo": "bar" }', legacy_mode: false)).to eq({ "foo" => "bar" })
+      end
+
+      it "parses an array" do
+        expect(subject.parse('[{ "foo": "bar" }]', legacy_mode: false)).to eq([{ "foo" => "bar" }])
+      end
+
+      # These are expected errors now until we upgrade the `json` gem,
+      # and then it will be expected to not raise errors and these tests
+      # will be updated accordingly.
+
+      it "raises an error on a string" do
+        expect { subject.parse('"foo"') }.to raise_error(JSON::ParserError)
+      end
+
+      it "raises an error on a true bool" do
+        expect { subject.parse("true") }.to raise_error(JSON::ParserError)
+      end
+
+      it "raises an error on a false bool" do
+        expect { subject.parse("false") }.to raise_error(JSON::ParserError)
+      end
     end
   end