Add invitation reminder email service

This service is to be called from the
MemberInvitationReminderEmailsWorker
cron job

app/services/members/invitation_reminder_email_service.rb

+# frozen_string_literal: true
+
+module Members
+  class InvitationReminderEmailService
+    include Gitlab::Utils::StrongMemoize
+
+    attr_reader :invitation
+
+    MAX_INVITATION_LIFESPAN = 14.0
+    REMINDER_RATIO = [2, 5, 10].freeze
+
+    def initialize(invitation)
+      @invitation = invitation
+    end
+
+    def execute
+      return unless experiment_enabled?
+
+      reminder_index = days_on_which_to_send_reminders.index(days_after_invitation_sent)
+      return unless reminder_index
+
+      invitation.send_invitation_reminder(reminder_index)
+    end
+
+    private
+
+    def experiment_enabled?
+      Gitlab::Experimentation.enabled_for_attribute?(:invitation_reminders, invitation.invite_email)
+    end
+
+    def days_after_invitation_sent
+      (Date.today - invitation.created_at.to_date).to_i
+    end
+
+    def days_on_which_to_send_reminders
+      # Don't send any reminders if the invitation has expired or expires today
+      return [] if invitation.expires_at && invitation.expires_at <= Date.today
+
+      # Calculate the number of days on which to send reminders based on the MAX_INVITATION_LIFESPAN and the REMINDER_RATIO
+      REMINDER_RATIO.map { |number_of_days| ((number_of_days * invitation_lifespan_in_days) / MAX_INVITATION_LIFESPAN).ceil }.uniq
+    end
+
+    def invitation_lifespan_in_days
+      # When the invitation lifespan is more than 14 days or does not expire, send the reminders within 14 days
+      strong_memoize(:invitation_lifespan_in_days) do
+        if invitation.expires_at
+          [(invitation.expires_at - invitation.created_at.to_date).to_i, MAX_INVITATION_LIFESPAN].min
+        else
+          MAX_INVITATION_LIFESPAN
+        end
+      end
+    end
+  end
+end

spec/services/members/invitation_reminder_email_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Members::InvitationReminderEmailService do
+  describe 'sending invitation reminders' do
+    subject { described_class.new(invitation).execute }
+
+    let_it_be(:frozen_time) { Date.today.beginning_of_day }
+    let_it_be(:invitation) { build(:group_member, :invited, created_at: frozen_time) }
+
+    context 'when the experiment is disabled' do
+      before do
+        allow(Gitlab::Experimentation).to receive(:enabled_for_attribute?).and_return(false)
+        invitation.expires_at = frozen_time + 2.days
+      end
+
+      it 'does not send an invitation' do
+        travel_to(frozen_time + 1.day) do
+          expect(invitation).not_to receive(:send_invitation_reminder)
+
+          subject
+        end
+      end
+    end
+
+    context 'when the experiment is enabled' do
+      before do
+        allow(Gitlab::Experimentation).to receive(:enabled_for_attribute?).and_return(true)
+        invitation.expires_at = frozen_time + expires_at_days.days if expires_at_days
+      end
+
+      using RSpec::Parameterized::TableSyntax
+
+      where(:expires_at_days, :send_reminder_at_days) do
+        0   | []
+        1   | []
+        2   | [1]
+        3   | [1, 2]
+        4   | [1, 2, 3]
+        5   | [1, 2, 4]
+        6   | [1, 3, 5]
+        7   | [1, 3, 5]
+        8   | [2, 3, 6]
+        9   | [2, 4, 7]
+        10  | [2, 4, 8]
+        11  | [2, 4, 8]
+        12  | [2, 5, 9]
+        13  | [2, 5, 10]
+        14  | [2, 5, 10]
+        15  | [2, 5, 10]
+        nil | [2, 5, 10]
+      end
+
+      with_them do
+        # Create an invitation today with an expiration date from 0 to 10 days in the future or without an expiration date
+        # We chose 10 days here, because we fetch invitations that were created at most 10 days ago.
+        (0..10).each do |day|
+          it 'sends an invitation reminder only on the expected days' do
+            next if day > (expires_at_days || 10) # We don't need to test after the invitation has already expired
+
+            # We are traveling in a loop from today to 10 days from now
+            travel_to(frozen_time + day.days) do
+              # Given an expiration date and the number of days after the creation of the invitation based on the current day in the loop, a reminder may be sent
+              if (reminder_index = send_reminder_at_days.index(day))
+                expect(invitation).to receive(:send_invitation_reminder).with(reminder_index)
+              else
+                expect(invitation).not_to receive(:send_invitation_reminder)
+              end
+
+              subject
+            end
+          end
+        end
+      end
+    end
+  end
+end