Fix errors when using deliver!

This makes the SMTP pool delivery method compatible with #deliver!

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60843

changelogs/unreleased/fix-smtp-pool-errors.yml

+---
+title: Fix SMTP errors when delivering service desk thank you emails with SMTP pool
+  enabled
+merge_request: 60843
+author:
+type: fixed

lib/gitlab/email/handler/service_desk_handler.rb

@@ -38,7 +38,7 @@ module Gitlab
 
           if from_address
             add_email_participant
-            send_thank_you_email!
+            send_thank_you_email
           end
         end
 
@@ -92,8 +92,8 @@ module Gitlab
           end
         end
 
-        def send_thank_you_email!
-          Notify.service_desk_thank_you_email(@issue.id).deliver_later!
+        def send_thank_you_email
+          Notify.service_desk_thank_you_email(@issue.id).deliver_later
         end
 
         def message_including_template

spec/lib/gitlab/email/handler/service_desk_handler_spec.rb

@@ -90,11 +90,6 @@ RSpec.describe Gitlab::Email::Handler::ServiceDeskHandler do
           context 'when quick actions are present' do
             let(:label) { create(:label, project: project, title: 'label1') }
             let(:milestone) { create(:milestone, project: project) }
-            let!(:user) { create(:user, username: 'user1') }
-
-            before do
-              project.add_developer(user)
-            end
 
             it 'applies quick action commands present on templates' do
               file_content = %(Text from template \n/label ~#{label.title} \n/milestone %"#{milestone.name}"")

vendor/gems/mail-smtp_pool/lib/mail/smtp_pool.rb

@@ -30,5 +30,11 @@ module Mail
     def deliver!(mail)
       @pool.with { |conn| conn.deliver!(mail) }
     end
+
+    # This makes it compatible with Mail's `#deliver!` method
+    # https://github.com/mikel/mail/blob/22a7afc23f253319965bf9228a0a430eec94e06d/lib/mail/message.rb#L271
+    def settings
+      {}
+    end
   end
 end

vendor/gems/mail-smtp_pool/spec/lib/mail/smtp_pool_spec.rb

@@ -64,5 +64,27 @@ describe Mail::SMTPPool do
 
       expect(MockSMTP.deliveries.size).to eq(1)
     end
+
+    context 'when called from Mail:Message' do
+      before do
+        mail.delivery_method(described_class, { pool: described_class.create_pool })
+      end
+
+      describe '#deliver' do
+        it 'delivers mail' do
+          mail.deliver
+
+          expect(MockSMTP.deliveries.size).to eq(1)
+        end
+      end
+
+      describe '#deliver!' do
+        it 'delivers mail' do
+          mail.deliver!
+
+          expect(MockSMTP.deliveries.size).to eq(1)
+        end
+      end
+    end
   end
 end