Tweak the user account data remediation migration

- Send the unconfirm notification email first and 1 minute later, send
the confirmation instructions.
- Make sure `unconfirm_email` field is set to NULL.

db/post_migrate/20200615111857_unconfirm_wrongfully_verified_emails.rb

@@ -8,7 +8,7 @@ class UnconfirmWrongfullyVerifiedEmails < ActiveRecord::Migration[6.0]
   disable_ddl_transaction!
 
   INTERVAL = 5.minutes.to_i
-  BATCH_SIZE = 1000
+  BATCH_SIZE = 500
   MIGRATION = 'WrongfullyConfirmedEmailUnconfirmer'
   EMAIL_INDEX_NAME = 'tmp_index_for_email_unconfirmation_migration'
 

lib/gitlab/background_migration/wrongfully_confirmed_email_unconfirmer.rb

@@ -71,7 +71,7 @@ module Gitlab
       def update_user_records(user_ids)
         UserModel
           .where(id: user_ids)
-          .update_all("confirmed_at = NULL, confirmation_sent_at = NOW(), confirmation_token=md5(users.id::varchar || users.created_at || users.encrypted_password || '#{Integer(Time.now.to_i)}')")
+          .update_all("confirmed_at = NULL, confirmation_sent_at = NOW(), unconfirmed_email = NULL, confirmation_token=md5(users.id::varchar || users.created_at || users.encrypted_password || '#{Integer(Time.now.to_i)}')")
       end
 
       def email_query_for_update(start_id, stop_id)
@@ -81,15 +81,15 @@ module Gitlab
       end
 
       def send_emails(email_records)
-        email_records.each do |email|
-          DeviseMailer.confirmation_instructions(email, email.confirmation_token).deliver_later
-        end
-
         user_records = email_records.map(&:user).uniq
 
         user_records.each do |user|
-          DeviseMailer.confirmation_instructions(user, user.confirmation_token).deliver_later
           Gitlab::BackgroundMigration::Mailers::UnconfirmMailer.unconfirm_notification_email(user).deliver_later
+          DeviseMailer.confirmation_instructions(user, user.confirmation_token).deliver_later(wait: 1.minute)
+        end
+
+        email_records.each do |email|
+          DeviseMailer.confirmation_instructions(email, email.confirmation_token).deliver_later(wait: 1.minute)
         end
       end
     end

spec/lib/gitlab/background_migration/wrongfully_confirmed_email_unconfirmer_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe Gitlab::BackgroundMigration::WrongfullyConfirmedEmailUnconfirmer,
   let(:one_year_ago) { 1.year.ago }
 
   let!(:user_needs_migration_1) { users.create!(name: 'user1', email: 'test1@test.com', state: 'active', projects_limit: 1, confirmed_at: confirmed_at_2_days_ago, confirmation_sent_at: one_year_ago) }
-  let!(:user_needs_migration_2) { users.create!(name: 'user2', email: 'test2@test.com', state: 'active', projects_limit: 1, confirmed_at: confirmed_at_3_days_ago, confirmation_sent_at: one_year_ago) }
+  let!(:user_needs_migration_2) { users.create!(name: 'user2', email: 'test2@test.com', unconfirmed_email: 'unconfirmed@test.com', state: 'active', projects_limit: 1, confirmed_at: confirmed_at_3_days_ago, confirmation_sent_at: one_year_ago) }
   let!(:user_does_not_need_migration) { users.create!(name: 'user3', email: 'test3@test.com', state: 'active', projects_limit: 1) }
   let!(:inactive_user) { users.create!(name: 'user4', email: 'test4@test.com', state: 'blocked', projects_limit: 1, confirmed_at: confirmed_at_3_days_ago, confirmation_sent_at: one_year_ago) }
   let!(:alert_bot_user) { users.create!(name: 'user5', email: 'test5@test.com', state: 'active', user_type: 2, projects_limit: 1, confirmed_at: confirmed_at_3_days_ago, confirmation_sent_at: one_year_ago) }
@@ -48,6 +48,13 @@ RSpec.describe Gitlab::BackgroundMigration::WrongfullyConfirmedEmailUnconfirmer,
     expect(bad_email_4_bot_user.reload.confirmation_sent_at).to be_within(1.second).of(one_year_ago)
   end
 
+  it 'clears the `unconfirmed_email` field' do
+    subject
+
+    user_needs_migration_2.reload
+    expect(user_needs_migration_2.unconfirmed_email).to be_nil
+  end
+
   it 'does not change irrelevant user records' do
     subject