FIX: Improve tags in email subjects and add filter headers (#19760)

This commit does a couple of things:

1. Changes the limit of tags to include a subject for a
   notification email to the `max_tags_per_topic` setting
   instead of the arbitrary 3 limit
2. Adds both an X-Discourse-Tags and X-Discourse-Category
   custom header to outbound emails containing the tags
   and category from the subject, so people on mail clients
   that allow advanced filtering (i.e. not Gmail) can filter
   mail by tags and category, which is useful for mailing
   list mode users

c.f. https://meta.discourse.org/t/headers-for-email-notifications-so-that-gmail-users-can-filter-on-tags/249982/17

app/mailers/user_notifications.rb

@@ -538,7 +538,7 @@ class UserNotifications < ActionMailer::Base
           .visible_tags(Guardian.new(user))
           .joins(:topic_tags)
           .where("topic_tags.topic_id = ?", post.topic_id)
-          .limit(3)
+          .limit(SiteSetting.max_tags_per_topic)
           .pluck(:name)
 
       show_tags_in_subject = tags.any? ? tags.join(" ") : nil

lib/email/message_builder.rb

@@ -161,6 +161,11 @@ module Email
       result['X-Discourse-Post-Id']  = @opts[:post_id].to_s  if @opts[:post_id]
       result['X-Discourse-Topic-Id'] = @opts[:topic_id].to_s if @opts[:topic_id]
 
+      # at this point these have been filtered by the recipient's guardian for visibility,
+      # see UserNotifications#send_notification_email
+      result['X-Discourse-Tags'] = @template_args[:show_tags_in_subject] if @opts[:show_tags_in_subject]
+      result['X-Discourse-Category'] = @template_args[:show_category_in_subject] if @opts[:show_category_in_subject]
+
       # please, don't send us automatic responses...
       result['X-Auto-Response-Suppress'] = 'All'
 

spec/lib/email/message_builder_spec.rb

@@ -155,6 +155,8 @@ RSpec.describe Email::MessageBuilder do
         body: 'hello world',
         topic_id: 1234,
         post_id: 4567,
+        show_tags_in_subject: "foo bar baz",
+        show_category_in_subject: "random"
         }.merge(additional_opts)
       )
     end
@@ -171,6 +173,14 @@ RSpec.describe Email::MessageBuilder do
       expect(message_with_header_args.header_args['Reply-To']).to eq("\"Discourse\" <#{SiteSetting.notification_email}>")
     end
 
+    it "passes through the topic tags" do
+      expect(message_with_header_args.header_args['X-Discourse-Tags']).to eq('foo bar baz')
+    end
+
+    it "passes through the topic category" do
+      expect(message_with_header_args.header_args['X-Discourse-Category']).to eq('random')
+    end
+
     context "when allow_reply_by_email is enabled " do
       let(:additional_opts) { { allow_reply_by_email: true } }
 

spec/mailers/user_notifications_spec.rb

@@ -400,6 +400,19 @@ RSpec.describe UserNotifications do
       expect(mail_html.scan(/>bobmarley/).count).to eq(1)
     end
 
+    it "the number of tags shown in subject should match max_tags_per_topic" do
+      SiteSetting.email_subject = "[%{site_name}] %{optional_pm}%{optional_cat}%{optional_tags}%{topic_title}"
+      SiteSetting.max_tags_per_topic = 1
+      mail = UserNotifications.user_replied(
+        user,
+        post: response,
+        notification_type: notification.notification_type,
+        notification_data_hash: notification.data_hash
+      )
+      expect(mail.subject).to match(/Taggo/)
+      expect(mail.subject).not_to match(/Taggie/)
+    end
+
     it "doesn't include details when private_email is enabled" do
       SiteSetting.private_email = true
       mail = UserNotifications.user_replied(