Implement custom AR type JsonPgSafe

Extends Rails' ActiveRecord::Type::Json data type to remove JSON encoded
null bytes `\u0000` to prevent PostgreSQL errors like
`PG::UntranslatableCharacter: ERROR: unsupported Unicode escape
sequence`.

lib/gitlab/database/type/json_pg_safe.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Type
+      # Extends Rails' ActiveRecord::Type::Json data type to remove JSON
+      # encooded nullbytes `\u0000` to prevent PostgreSQL errors like
+      # `PG::UntranslatableCharacter: ERROR: unsupported Unicode escape
+      # sequence`.
+      #
+      # Example:
+      #
+      #   class SomeModel < ApplicationRecord
+      #     # some_model.a_field is of type `jsonb`
+      #     attribute :a_field, Gitlab::Database::Type::JsonPgSafe.new
+      #   end
+      class JsonPgSafe < ActiveRecord::Type::Json
+        def serialize(value)
+          super&.gsub('\u0000', '')
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/type/json_pg_safe_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::Type::JsonPgSafe do
+  let(:type) { described_class.new }
+
+  describe '#serialize' do
+    using RSpec::Parameterized::TableSyntax
+
+    subject { type.serialize(value) }
+
+    where(:value, :json) do
+      nil                            | nil
+      1                              | '1'
+      1.0                            | '1.0'
+      "str\0ing\u0000"               | '"string"'
+      ["\0arr", "a\u0000y"]          | '["arr","ay"]'
+      { "key\0" => "value\u0000\0" } | '{"key":"value"}'
+    end
+
+    with_them do
+      it { is_expected.to eq(json) }
+    end
+  end
+end