DEV: Email notification filter plugin API (#24271)

app/services/notification_emailer.rb

@@ -138,6 +138,11 @@ class NotificationEmailer
 
     email_user = EmailUser.new(notification, no_delay: no_delay)
     email_method = Notification.types[notification.notification_type]
+
+    DiscoursePluginRegistry.email_notification_filters.each do |filter|
+      return unless filter.call(notification)
+    end
+
     email_user.public_send(email_method) if email_user.respond_to? email_method
   end
 end

lib/discourse_plugin_registry.rb

@@ -99,6 +99,7 @@ class DiscoursePluginRegistry
 
   define_filtered_register :presence_channel_prefixes
 
+  define_filtered_register :email_notification_filters
   define_filtered_register :push_notification_filters
 
   define_filtered_register :notification_consolidation_plans

lib/plugin/instance.rb

@@ -999,6 +999,12 @@ class Plugin::Instance
     DiscoursePluginRegistry.register_presence_channel_prefix([prefix, block], self)
   end
 
+  # Registers a new email notification filter. Notification is passed into block, and if all
+  # filters return `true`, the email notification will be sent.
+  def register_email_notification_filter(&block)
+    DiscoursePluginRegistry.register_email_notification_filter(block, self)
+  end
+
   # Registers a new push notification filter. User and notification payload are passed into block, and if all
   # filters return `true`, the push notification will be sent.
   def register_push_notification_filter(&block)

spec/services/notification_emailer_spec.rb

@@ -274,4 +274,31 @@ RSpec.describe NotificationEmailer do
       end
     end
   end
+
+  describe "with plugin-added email_notification_filters" do
+    let!(:plugin) { Plugin::Instance.new }
+    let!(:notification) { create_notification(:quoted) }
+    let(:no_delay) { true }
+    let(:type) { :user_quoted }
+
+    after { DiscoursePluginRegistry.reset! }
+
+    it "sends email when all filters return true" do
+      plugin.register_email_notification_filter { |_| true }
+      plugin.register_email_notification_filter { |_| true }
+
+      expect_enqueued_with(job: :user_email, args: { type: type }) do
+        NotificationEmailer.process_notification(notification, no_delay: no_delay)
+      end
+    end
+
+    it "doesn't send email when all one filter returns false" do
+      plugin.register_email_notification_filter { |_| true }
+      plugin.register_email_notification_filter { |_| false }
+
+      expect_not_enqueued_with(job: :user_email, args: { type: type }) do
+        NotificationEmailer.process_notification(notification, no_delay: no_delay)
+      end
+    end
+  end
 end