Merge pull request #296: Fan the unread room notice out from a job

Reviewed and merged by GPT on behalf of DHH.

app/models/message/broadcasts.rb

@@ -1,21 +1,29 @@
 module Message::Broadcasts
   def broadcast_create
     broadcast_append_to room, :messages, target: [ room, :messages ]
-    broadcast_unread_room
+    broadcast_unread_room_later
   end
 
   def broadcast_remove
     broadcast_remove_to room, :messages
   end
 
-  private
-    # Fanned out to the room's members rather than published on one global stream, so
-    # that the timing of activity in a room only reaches people who are in it.
-    def broadcast_unread_room
-      payload = ActiveSupport::JSON.encode(roomId: room.id)
+  # Fanned out to the room's members rather than published on one global stream, so
+  # that the timing of activity in a room only reaches people who are in it. Of those,
+  # only the ones who still have it unread or are in it now: by the time this runs,
+  # someone may have opened the room and moved on, and would see it marked unread again.
+  def broadcast_unread_room
+    payload = ActiveSupport::JSON.encode(roomId: room.id, at: created_at.to_fs(:epoch))
 
-      room.memberships.pluck(:user_id).each do |user_id|
-        ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
-      end
+    room.memberships.unread.or(room.memberships.connected).pluck(:user_id).each do |user_id|
+      ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
+    end
+  end
+
+  private
+    # The fanout is one publish per member, which in a big room takes far longer than the
+    # rest of posting a message, so it runs in a job rather than while the poster waits.
+    def broadcast_unread_room_later
+      Message::BroadcastUnreadRoomJob.perform_later(self)
     end
 end