FIX: Don't publish notifications to MessageBus for inactive users (#15035)

We are pushing /notification-alert/#{user_id} and /notification/#{user_id}
messages to MessageBus from both PostAlerter and User#publish_notification_state.
This can cause memory issues on large sites with many users. This commit
stems the bleeding by only sending these alert messages if the user
in question has been seen in the last 30 days, which eliminates a large
chunk of users on some sites.

app/models/user.rb

@@ -590,6 +590,8 @@ class User < ActiveRecord::Base
   end
 
   def publish_notifications_state
+    return if !self.allow_live_notifications?
+
     # publish last notification json with the message so we can apply an update
     notification = notifications.visible.order('notifications.created_at desc').first
     json = NotificationSerializer.new(notification).as_json if notification
@@ -693,6 +695,10 @@ class User < ActiveRecord::Base
     last_seen_at.present?
   end
 
+  def seen_since?(datetime)
+    seen_before? && last_seen_at >= datetime
+  end
+
   def create_visit_record!(date, opts = {})
     user_stat.update_column(:days_visited, user_stat.days_visited + 1)
     user_visits.create!(visited_at: date, posts_read: opts[:posts_read] || 0, mobile: opts[:mobile] || false)
@@ -1438,6 +1444,10 @@ class User < ActiveRecord::Base
     ShelvedNotification.joins(:notification).where("notifications.user_id = ?", self.id)
   end
 
+  def allow_live_notifications?
+    seen_since?(30.days.ago)
+  end
+
   protected
 
   def badge_grant

app/services/post_alerter.rb

@@ -28,7 +28,11 @@ class PostAlerter
       }
 
       DiscourseEvent.trigger(:pre_notification_alert, user, payload)
-      MessageBus.publish("/notification-alert/#{user.id}", payload, user_ids: [user.id])
+
+      if user.allow_live_notifications?
+        MessageBus.publish("/notification-alert/#{user.id}", payload, user_ids: [user.id])
+      end
+
       push_notification(user, payload)
       DiscourseEvent.trigger(:post_notification_alert, user, payload)
     end

spec/models/user_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 describe User do
-  let(:user) { Fabricate(:user) }
+  let(:user) { Fabricate(:user, last_seen_at: 1.day.ago) }
 
   def user_error_message(*keys)
     I18n.t(:"activerecord.errors.models.user.attributes.#{keys.join('.')}")
@@ -1921,6 +1921,18 @@ describe User do
       expect(message.data[:unread_private_messages]).to eq(2)
       expect(message.data[:unread_high_priority_notifications]).to eq(2)
     end
+
+    it "does not publish to the /notification channel for users who have not been seen in > 30 days" do
+      notification = Fabricate(:notification, user: user)
+      notification2 = Fabricate(:notification, user: user, read: true)
+      user.update(last_seen_at: 31.days.ago)
+
+      message = MessageBus.track_publish("/notification/#{user.id}") do
+        user.publish_notifications_state
+      end.first
+
+      expect(message).to eq(nil)
+    end
   end
 
   describe "silenced?" do

spec/services/post_alerter_spec.rb

@@ -848,7 +848,7 @@ describe PostAlerter do
   end
 
   describe "create_notification_alert" do
-    it "does not nothing for suspended users" do
+    it "does nothing for suspended users" do
       evil_trout.update_columns(suspended_till: 1.year.from_now)
       post = Fabricate(:post)
 
@@ -869,6 +869,35 @@ describe PostAlerter do
       expect(messages.size).to eq(0)
       expect(Jobs::PushNotification.jobs.size).to eq(0)
     end
+
+    it "does not publish to MessageBus /notification-alert if the user has not been seen for > 30 days, but still sends a push notification" do
+      evil_trout.update_columns(last_seen_at: 31.days.ago)
+      post = Fabricate(:post)
+
+      SiteSetting.allowed_user_api_push_urls = "https://site2.com/push"
+      UserApiKey.create!(user_id: evil_trout.id,
+                         client_id: "xxx#1",
+                         application_name: "iPhone1",
+                         scopes: ['notifications'].map { |name| UserApiKeyScope.new(name: name) },
+                         push_url: "https://site2.com/push")
+
+      events = nil
+      messages = MessageBus.track_publish do
+        events = DiscourseEvent.track_events do
+          PostAlerter.create_notification_alert(
+            user: evil_trout,
+            post: post,
+            notification_type: Notification.types[:custom],
+            excerpt: "excerpt",
+            username: "username"
+          )
+        end
+      end
+
+      expect(events.size).to eq(2)
+      expect(messages.size).to eq(0)
+      expect(Jobs::PushNotification.jobs.size).to eq(1)
+    end
   end
 
   describe "watching_first_post" do