FIX: Do not notify admins watching PM tags (#18103)

Admins received notifications if a PM was tagged with a tag they
watched even if they were not invited to the PM.

app/services/post_alerter.rb

@@ -170,6 +170,8 @@ class PostAlerter
 
       if topic.present?
         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)
       end
     end

spec/services/post_alerter_spec.rb

@@ -2004,4 +2004,21 @@ RSpec.describe PostAlerter do
       expect(liked_notification.data_hash[:custom_key]).to eq(custom_data)
     end
   end
+
+  it "does not create notifications for PMs if not invited" do
+    SiteSetting.pm_tags_allowed_for_groups = "#{Group::AUTO_GROUPS[:everyone]}"
+    watching_first_post_tag = Fabricate(:tag)
+    TagUser.change(admin.id, watching_first_post_tag.id, TagUser.notification_levels[:watching_first_post])
+    watching_tag = Fabricate(:tag)
+    TagUser.change(admin.id, watching_tag.id, TagUser.notification_levels[:watching])
+
+    post = create_post(tags: [watching_first_post_tag.name, watching_tag.name], archetype: Archetype.private_message, target_usernames: "#{evil_trout.username}")
+    expect { PostAlerter.new.after_save_post(post, true) }.to change { Notification.count }.by(1)
+
+    notification = Notification.last
+    expect(notification.user).to eq(evil_trout)
+    expect(notification.notification_type).to eq(Notification.types[:private_message])
+    expect(notification.topic).to eq(post.topic)
+    expect(notification.post_number).to eq(1)
+  end
 end