FIX: Create new PM notifications for `watching_first_post` groups (#21997)

At the moment, PMs to groups with default notification level set to
`watching_first_post` do not generate "emailable" notifications. This happens
because, topic user notification level which is indirectly derived
from the group's default notification level is set to `tracking` if the
group's notification level happens to be `watching_first_post`.

This leads to a `group_message_summary` notification being created
instead of a `private_message` notification which results in no email
alerts being sent when a  topic is created.

As this `watching_first_post` --> `tracking` switcheroo appears to be
intentional instead being a bug, this change extends `PostAlerter`'s
`notify_pm_users` method to create a `private_message` notification for
first posts created in a `watching_first_post` group even if the topic
user notification level is set to `tracking`

app/services/post_alerter.rb

@@ -196,7 +196,7 @@ class PostAlerter
     if new_record
       if post.topic.private_message?
         # private messages
-        notified += notify_pm_users(post, reply_to_user, quoted_users, notified)
+        notified += notify_pm_users(post, reply_to_user, quoted_users, notified, new_record)
       elsif notify_about_reply?(post)
         # posts
         notified +=
@@ -724,7 +724,7 @@ class PostAlerter
     end
   end
 
-  def notify_pm_users(post, reply_to_user, quoted_users, notified)
+  def notify_pm_users(post, reply_to_user, quoted_users, notified, new_record = false)
     return [] unless post.topic
 
     warn_if_not_sidekiq
@@ -761,7 +761,12 @@ class PostAlerter
       when TopicUser.notification_levels[:watching]
         create_pm_notification(user, post, emails_to_skip_send)
       when TopicUser.notification_levels[:tracking]
-        if is_replying?(user, reply_to_user, quoted_users)
+        # TopicUser is the canonical source of topic notification levels, except for
+        # new topics created within a group with default notification level set to
+        # `watching_first_post`. TopicUser notification level is set to `tracking`
+        # for these.
+        if is_replying?(user, reply_to_user, quoted_users) ||
+             (new_record && group_watched_first_post?(user, post))
           create_pm_notification(user, post, emails_to_skip_send)
         else
           notify_group_summary(user, post.topic)
@@ -1009,4 +1014,8 @@ class PostAlerter
       notification_level: TopicUser.notification_levels[:watching],
     )
   end
+
+  def group_watched_first_post?(user, post)
+    post.is_first_post? && group_watchers(post.topic).include?(user.id)
+  end
 end

spec/services/post_alerter_spec.rb

@@ -124,6 +124,15 @@ RSpec.describe PostAlerter do
       fab!(:group) do
         Fabricate(:group, users: [user2], name: "TestGroup", default_notification_level: 2)
       end
+      fab!(:watching_first_post_group) do
+        Fabricate(
+          :group,
+          name: "some_group",
+          users: [evil_trout, coding_horror],
+          messageable_level: Group::ALIAS_LEVELS[:everyone],
+          default_notification_level: NotificationLevels.all[:watching_first_post],
+        )
+      end
       fab!(:pm) do
         Fabricate(:topic, archetype: "private_message", category_id: nil, allowed_groups: [group])
       end
@@ -334,6 +343,86 @@ RSpec.describe PostAlerter do
           )
         }.not_to change(user2.notifications, :count)
       end
+
+      context "with watching_first_post notification level" do
+        it "notifies group members of first post" do
+          post =
+            PostCreator.create!(
+              user,
+              title: "Hi there, welcome to my topic",
+              raw: "This is my awesome message",
+              archetype: Archetype.private_message,
+              target_group_names: watching_first_post_group.name,
+            )
+
+          PostAlerter.new.after_save_post(post, true)
+
+          expect(
+            evil_trout
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(1)
+          expect(
+            coding_horror
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(1)
+        end
+
+        it "doesn't notify group members of replies" do
+          post =
+            PostCreator.create!(
+              user,
+              title: "Hi there, welcome to my topic",
+              raw: "This is my awesome message",
+              archetype: Archetype.private_message,
+              target_group_names: watching_first_post_group.name,
+            )
+
+          expect(
+            evil_trout
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(0)
+          expect(
+            coding_horror
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(0)
+
+          PostAlerter.new.after_save_post(post, true)
+
+          expect(
+            evil_trout
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(1)
+          expect(
+            coding_horror
+              .notifications
+              .where(notification_type: Notification.types[:private_message])
+              .count,
+          ).to eq(1)
+
+          reply =
+            Fabricate(
+              :post,
+              raw: "Reply to PM",
+              user: user,
+              topic: post.topic,
+              reply_to_post_number: post.post_number,
+            )
+
+          expect do PostAlerter.new.after_save_post(reply, false) end.to_not change {
+            Notification.count
+          }
+        end
+      end
     end
   end