Performance: Memoize Encryptor in ActiveRecordEncryptedString::Type

[Bhacaz]

Hi, I would like to suggest a small change that has a big impact on performance.

Currently, instantiating the encryptor, especially due to ActiveSupport::KeyGenerator#generate_key, is a CPU-intensive operation. This can lead to significant overhead if an encryptor is initialized repeatedly for the same attribute type.

Rails attribute types, like ActiveRecordEncryptedString::Type, are (if I understand it correctly) typically instantiated once per attribute definition (e.g., attribute :secret_field, :encrypted_string). This PR leverages this by memoizing the encryptor instance variable (e.g., using @encryptor ||= ...).

With the help of the specs I benchmark that change

it 'benchmark' do
  Benchmark.bm do |x|
    x.report('encrypt') do
      100.times do |i|
        dummy_klass.create!(plain: plain, encrypted: "test#{i}")
      end
    end
    x.report('decrypt') do
      dummy_klass.pluck(:encrypted)
    end
  end
end

# Without
#              user     system      total        real
# encrypt  6.315960   0.063629   6.379589 (  6.431360)
# decrypt  3.325287   0.023737   3.349024 (  3.364617)
#
# With memorization
#              user     system      total        real
# encrypt  0.126073   0.020950   0.147023 (  0.147913)
# decrypt  0.005078   0.001927   0.007005 (  0.007034)

This how I found out

Details

it 'flamegraph' do
  100.times do |i|
    dummy_klass.create!(plain: plain, encrypted: "test#{i}")
  end
  flamegraph { dummy_klass.pluck(:encrypted) }
end