Handle namespaced models

We encountered issues with setting module headers for namespaced models.
These changes address this. We retain the namespacing, but transform the
classnames to make them into safe email headers.

app/mailers/notify.rb

@@ -125,9 +125,8 @@ class Notify < BaseMailer
   def mail_thread(model, headers = {})
     add_project_headers
     add_unsubscription_headers_and_links
+    add_model_headers(model)
 
-    headers["X-GitLab-#{model.class.name}-ID"] = model.id
-    headers["X-GitLab-#{model.class.name}-IID"] = model.iid if model.respond_to?(:iid)
     headers['X-GitLab-Reply-Key'] = reply_key
 
     @reason = headers['X-GitLab-NotificationReason']
@@ -196,6 +195,18 @@ class Notify < BaseMailer
     @reply_key ||= SentNotification.reply_key
   end
 
+  # This method applies threading headers to the email to identify
+  # the instance we are discussing.
+  #
+  # All model instances must have `#id`, and may implement `#iid`.
+  def add_model_headers(object)
+    # Use replacement so we don't strip the module.
+    prefix = "X-GitLab-#{object.class.name.gsub(/::/, '-')}"
+
+    headers["#{prefix}-ID"] = object.id
+    headers["#{prefix}-IID"] = object.iid if object.respond_to?(:iid)
+  end
+
   def add_project_headers
     return unless @project
 

spec/mailers/notify_spec.rb

@@ -543,6 +543,73 @@ describe Notify do
       end
     end
 
+    describe '#mail_thread' do
+      set(:mail_thread_note) { create(:note) }
+
+      let(:headers) do
+        {
+          from: 'someone@test.com',
+          to: 'someone-else@test.com',
+          subject: 'something',
+          template_name: '_note_email' # re-use this for testing
+        }
+      end
+
+      let(:mailer) do
+        mailer = described_class.new
+        mailer.instance_variable_set(:@note, mail_thread_note)
+        mailer
+      end
+
+      context 'the model has no namespace' do
+        class TopLevelThing
+          include Referable
+          include Noteable
+
+          def to_reference(*_args)
+            'tlt-ref'
+          end
+
+          def id
+            'tlt-id'
+          end
+        end
+
+        subject do
+          mailer.send(:mail_thread, TopLevelThing.new, headers)
+        end
+
+        it 'has X-GitLab-Namespaced-Thing-ID header' do
+          expect(subject.header['X-GitLab-TopLevelThing-ID'].value).to eq('tlt-id')
+        end
+      end
+
+      context 'the model has a namespace' do
+        module Namespaced
+          class Thing
+            include Referable
+            include Noteable
+
+            def to_reference(*_args)
+              'some-reference'
+            end
+
+            def id
+              'some-id'
+            end
+          end
+        end
+
+        subject do
+          mailer.send(:mail_thread, Namespaced::Thing.new, headers)
+        end
+
+        it 'has X-GitLab-Namespaced-Thing-ID header' do
+          expect(subject.header['X-GitLab-Namespaced-Thing-ID'].value).to eq('some-id')
+        end
+      end
+    end
+
     context 'for issue notes' do
       let(:host) { Gitlab.config.gitlab.host }