Make attr_encrypted thread-safe

We ran into issues when encrypting in multiple threads because
attr_encrypted instances were sharing one encrypted_attributes
hash.

This was fixed in https://github.com/attr-encrypted/attr_encrypted/commit/d4ca0e2073ca6ba5035997ce25f7fc0b4bfbe39e
but a new version wasn't released. So we need to patch it
ourselves here.

config/initializers/attr_encrypted_thread_safe.rb

+# frozen_string_literal: true
+
+# As of v3.1.0, attr_encrypted is not thread-safe because all instances share the same `encrypted_attributes`
+# This was fixed in https://github.com/attr-encrypted/attr_encrypted/commit/d4ca0e2073ca6ba5035997ce25f7fc0b4bfbe39e
+# but no release was made after that so we have to patch it ourselves here
+
+module AttrEncrypted
+  module InstanceMethods
+    def encrypted_attributes
+      @encrypted_attributes ||= begin
+        duplicated = {}
+        self.class.encrypted_attributes.map { |key, value| duplicated[key] = value.dup }
+        duplicated
+      end
+    end
+  end
+end

spec/initializers/attr_encrypted_thread_safe_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe AttrEncrypted do
+  describe '#encrypted_attributes' do
+    subject do
+      Class.new(ActiveRecord::Base) do
+        self.table_name = 'projects'
+
+        attr_accessor :encrypted_foo
+        attr_accessor :encrypted_foo_iv
+
+        attr_encrypted :foo, key: 'This is a key that is 256 bits!!'
+      end
+    end
+
+    it 'does not share state with other instances' do
+      instance = subject.new
+      instance.foo = 'bar'
+
+      another_instance = subject.new
+
+      expect(instance.encrypted_attributes[:foo][:operation]).to eq(:encrypting)
+      expect(another_instance.encrypted_attributes[:foo][:operation]).to be_nil
+    end
+  end
+end