FIX: Create single notification per post and user (#18091)

A user could receive more than a notification for the same post if they
watched both the categories and tags at different levels. This commit
makes sure that only the watching notification is created.

* Add DiscourseEvent before post notifications are created

app/services/post_alerter.rb

@@ -110,6 +110,8 @@ class PostAlerter
   def after_save_post(post, new_record = false)
     notified = [post.user, post.last_editor].uniq
 
+    DiscourseEvent.trigger(:post_alerter_before_mentions, post, new_record, notified)
+
     # mentions (users/groups)
     mentioned_groups, mentioned_users, mentioned_here = extract_mentions(post)
 
@@ -139,6 +141,8 @@ class PostAlerter
       end
     end
 
+    DiscourseEvent.trigger(:post_alerter_before_replies, post, new_record, notified)
+
     # replies
     reply_to_user = post.reply_notification_target
 
@@ -146,25 +150,34 @@ class PostAlerter
       notified += notify_non_pm_users(reply_to_user, :replied, post)
     end
 
+    DiscourseEvent.trigger(:post_alerter_before_quotes, post, new_record, notified)
+
     # quotes
     quoted_users = extract_quoted_users(post)
     notified += notify_non_pm_users(quoted_users - notified, :quoted, post)
 
+    DiscourseEvent.trigger(:post_alerter_before_linked, post, new_record, notified)
+
     # linked
     linked_users = extract_linked_users(post)
     notified += notify_non_pm_users(linked_users - notified, :linked, post)
 
-    # private messages
+    DiscourseEvent.trigger(:post_alerter_before_post, post, new_record, notified)
+
     if new_record
       if post.topic.private_message?
-        notify_pm_users(post, reply_to_user, quoted_users, notified)
+        # private messages
+        notified += notify_pm_users(post, reply_to_user, quoted_users, notified)
       elsif notify_about_reply?(post)
-        notify_post_users(post, notified, new_record: new_record)
+        # posts
+        notified += notify_post_users(post, notified, new_record: new_record)
       end
     end
 
     sync_group_mentions(post, mentioned_groups)
 
+    DiscourseEvent.trigger(:post_alerter_before_first_post, post, new_record, notified)
+
     if new_record && post.post_number == 1
       topic = post.topic
 
@@ -172,7 +185,7 @@ class PostAlerter
         watchers = category_watchers(topic) + tag_watchers(topic) + group_watchers(topic)
         # Notify only users who can see the topic
         watchers &= topic.all_allowed_users.pluck(:id) if post.topic.private_message?
-        notify_first_post_watchers(post, watchers)
+        notified += notify_first_post_watchers(post, watchers, notified)
       end
     end
 
@@ -199,8 +212,8 @@ class PostAlerter
       .pluck(:user_id)
   end
 
-  def notify_first_post_watchers(post, user_ids)
-    return if user_ids.blank?
+  def notify_first_post_watchers(post, user_ids, notified = nil)
+    return [] if user_ids.blank?
     user_ids.uniq!
 
     warn_if_not_sidekiq
@@ -208,11 +221,14 @@ class PostAlerter
     # Don't notify the OP and last editor
     user_ids -= [post.user_id, post.last_editor_id]
     users = User.where(id: user_ids).includes(:do_not_disturb_timings)
+    users = users.where.not(id: notified.map(&:id)) if notified.present?
 
     DiscourseEvent.trigger(:before_create_notifications_for_users, users, post)
     each_user_in_batches(users) do |user|
       create_notification(user, Notification.types[:watching_first_post], post)
     end
+
+    users
   end
 
   def sync_group_mentions(post, mentioned_groups)
@@ -620,7 +636,7 @@ class PostAlerter
   end
 
   def notify_pm_users(post, reply_to_user, quoted_users, notified)
-    return unless post.topic
+    return [] unless post.topic
 
     warn_if_not_sidekiq
 
@@ -749,7 +765,7 @@ class PostAlerter
   end
 
   def notify_post_users(post, notified, group_ids: nil, include_topic_watchers: true, include_category_watchers: true, include_tag_watchers: true, new_record: false)
-    return unless post.topic
+    return [] unless post.topic
 
     warn_if_not_sidekiq
 
@@ -845,6 +861,8 @@ class PostAlerter
       opts[:display_username] = post.last_editor.username if notification_type == Notification.types[:edited]
       create_notification(user, notification_type, post, opts)
     end
+
+    notify
   end
 
   def warn_if_not_sidekiq

spec/services/post_alerter_spec.rb

@@ -2021,4 +2021,53 @@ RSpec.describe PostAlerter do
     expect(notification.topic).to eq(post.topic)
     expect(notification.post_number).to eq(1)
   end
+
+  it "does not create multiple notifications for same post" do
+    category = Fabricate(:category)
+    CategoryUser.set_notification_level_for_category(
+      user,
+      NotificationLevels.all[:tracking],
+      category.id,
+    )
+    watching_first_post_tag = Fabricate(:tag)
+    TagUser.change(user.id, watching_first_post_tag.id, TagUser.notification_levels[:watching_first_post])
+    watching_tag = Fabricate(:tag)
+    TagUser.change(user.id, watching_tag.id, TagUser.notification_levels[:watching])
+
+    post = create_post(category: category, tags: [watching_first_post_tag.name, watching_tag.name])
+    expect { PostAlerter.new.after_save_post(post, true) }.to change { Notification.count }.by(1)
+
+    notification = Notification.last
+    expect(notification.user).to eq(user)
+    expect(notification.notification_type).to eq(Notification.types[:posted])
+    expect(notification.topic).to eq(post.topic)
+    expect(notification.post_number).to eq(1)
+  end
+
+  it 'triggers all discourse events' do
+    expected_events = [
+      :post_alerter_before_mentions,
+      :post_alerter_before_replies,
+      :post_alerter_before_quotes,
+      :post_alerter_before_linked,
+      :post_alerter_before_post,
+      :post_alerter_before_first_post,
+      :post_alerter_after_save_post,
+    ]
+
+    events = DiscourseEvent.track_events do
+      PostAlerter.new.after_save_post(post, true)
+    end
+
+    # Expect all the notification events are called
+    # There are some other events triggered from outside after_save_post
+    expect(events.map { |e| e[:event_name] }).to include(*expected_events)
+
+    # Expect each notification event is called with the right parameters
+    events.each do |event|
+      if expected_events.include?(event[:event_name])
+        expect(event[:params]).to eq([post, true, [post.user]])
+      end
+    end
+  end
 end