DEV add modifiers to message_builder so plugins can customize subject/body/html (#26867)

app/mailers/user_notifications.rb

@@ -760,6 +760,7 @@ class UserNotifications < ActionMailer::Base
       subject_pm: subject_pm,
       participants: participants,
       include_respond_instructions: !(user.suspended? || user.staged?),
+      notification_type: notification_type,
       template: template,
       use_topic_title_subject: use_topic_title_subject,
       site_description: SiteSetting.site_description,

lib/email/message_builder.rb

@@ -126,7 +126,7 @@ module Email
       else
         subject = @opts[:subject]
       end
-      subject
+      DiscoursePluginRegistry.apply_modifier(:message_builder_subject, subject, @opts)
     end
 
     def html_part
@@ -164,6 +164,7 @@ module Email
             html_body: html_override.html_safe,
           },
         )
+      html = DiscoursePluginRegistry.apply_modifier(:message_builder_html_part, html, @opts)
 
       Mail::Part.new do
         content_type "text/html; charset=UTF-8"
@@ -184,8 +185,7 @@ module Email
         body << "\n"
         body << @template_args[:unsubscribe_instructions]
       end
-
+      DiscoursePluginRegistry.apply_modifier(:message_builder_body, body, @opts)
-      body
     end
 
     def build_args

spec/lib/email/message_builder_spec.rb

@@ -10,6 +10,9 @@ RSpec.describe Email::MessageBuilder do
   let(:build_args) { builder.build_args }
   let(:header_args) { builder.header_args }
   let(:allow_reply_header) { described_class::ALLOW_REPLY_BY_EMAIL_HEADER }
+  let(:subject_modifier_block) { Proc.new { |subject, opts| "modified subject" } }
+
+  let(:body_modifier_block) { Proc.new { |subject, opts| "modified body" } }
 
   it "has the correct to address" do
     expect(build_args[:to]).to eq(to_address)
@@ -23,6 +26,30 @@ RSpec.describe Email::MessageBuilder do
     expect(builder.body).to eq(body)
   end
 
+  it "uses the message_builder subject modifier properly" do
+    plugin_instance = Plugin::Instance.new
+    plugin_instance.register_modifier(:message_builder_subject, &subject_modifier_block)
+    expect(builder.subject).to eq("modified subject")
+  ensure
+    DiscoursePluginRegistry.unregister_modifier(
+      plugin_instance,
+      :message_builder_subject,
+      &subject_modifier_block
+    )
+  end
+
+  it "uses the message_builder body modifier properly" do
+    plugin_instance = Plugin::Instance.new
+    plugin_instance.register_modifier(:message_builder_body, &body_modifier_block)
+    expect(builder.body).to eq("modified body")
+  ensure
+    DiscoursePluginRegistry.unregister_modifier(
+      plugin_instance,
+      :message_builder_body,
+      &body_modifier_block
+    )
+  end
+
   it "has a utf-8 charset" do
     expect(builder.build_args[:charset]).to eq("UTF-8")
   end