Set proper header to notification emails due to subscription

Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

app/models/notification_reason.rb

@@ -6,12 +6,14 @@ class NotificationReason
   OWN_ACTIVITY = 'own_activity'
   ASSIGNED = 'assigned'
   MENTIONED = 'mentioned'
+  SUBSCRIBED = 'subscribed'
 
   # Priority list for selecting which reason to return in the notification
   REASON_PRIORITY = [
     OWN_ACTIVITY,
     ASSIGNED,
-    MENTIONED
+    MENTIONED,
+    SUBSCRIBED
   ].freeze
 
   # returns the priority of a reason as an integer

app/services/notification_recipient_service.rb

@@ -181,7 +181,7 @@ module NotificationRecipientService
       def add_subscribed_users
         return unless target.respond_to? :subscribers
 
-        add_recipients(target.subscribers(project), :subscription, nil)
+        add_recipients(target.subscribers(project), :subscription, NotificationReason::SUBSCRIBED)
       end
 
       # rubocop: disable CodeReuse/ActiveRecord
@@ -240,7 +240,7 @@ module NotificationRecipientService
         return unless target.respond_to? :labels
 
         (labels || target.labels).each do |label|
-          add_recipients(label.subscribers(project), :subscription, nil)
+          add_recipients(label.subscribers(project), :subscription, NotificationReason::SUBSCRIBED)
         end
       end
     end

changelogs/unreleased/34416-subscribed-notification-header.yml

+---
+title: Set X-GitLab-NotificationReason header if notification reason is explicit subscription
+merge_request: 18812
+author:
+type: added

spec/services/notification_service_spec.rb

@@ -869,6 +869,18 @@ describe NotificationService, :mailer do
         should_email(user_4)
       end
 
+      it 'adds "subscribed" reason to subscriber emails' do
+        user_1 = create(:user)
+        label = create(:label, project: project, issues: [issue])
+        issue.reload
+        label.subscribe(user_1)
+
+        notification.new_issue(issue, @u_disabled)
+
+        email = find_email_for(user_1)
+        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::SUBSCRIBED)
+      end
+
       it_behaves_like 'project emails are disabled' do
         let(:notification_target)  { issue }
         let(:notification_trigger) { notification.new_issue(issue, @u_disabled) }
@@ -1272,6 +1284,17 @@ describe NotificationService, :mailer do
         let(:notification_target)  { issue }
         let(:notification_trigger) { notification.close_issue(issue, @u_disabled) }
       end
+
+      it 'adds "subscribed" reason to subscriber emails' do
+        user_1 = create(:user)
+        issue.subscribe(user_1)
+        issue.reload
+
+        notification.close_issue(issue, @u_disabled)
+
+        email = find_email_for(user_1)
+        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::SUBSCRIBED)
+      end
     end
 
     describe '#reopen_issue' do