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

Reviewed and merged by GPT on behalf of DHH.

app/channels/presence_channel.rb

@@ -22,6 +22,6 @@ class PresenceChannel < RoomChannel
     end
 
     def broadcast_read_room
-      ActionCable.server.broadcast "user_#{current_user.id}_reads", { room_id: membership.room_id }
+      ActionCable.server.broadcast "user_#{current_user.id}_reads", { room_id: membership.room_id, at: Time.current.to_fs(:epoch) }
     end
 end

app/javascript/controllers/read_rooms_controller.js

@@ -16,7 +16,7 @@ export default class extends Controller {
     })
   }
 
-  #read = ({ room_id }) => {
-    this.dispatch("read", { detail: { roomId: room_id } })
+  #read = ({ room_id, at }) => {
+    this.dispatch("read", { detail: { roomId: room_id, at } })
   }
 }

app/javascript/controllers/rooms_list_controller.js

@@ -7,6 +7,7 @@ export default class extends Controller {
   static classes = [ "unread" ]
 
   #disconnected = true
+  #readAt = new Map()
 
   async connect() {
     this.channel ??= await cable.subscribeTo({ channel: "UnreadRoomsChannel" }, {
@@ -27,9 +28,13 @@ export default class extends Controller {
     this.read({ detail: { roomId: Current.room.id } })
   }
 
-  read({ detail: { roomId } }) {
+  read({ detail: { roomId, at } }) {
     const room = this.#findRoomTarget(roomId)
 
+    if (at) {
+      this.#readAt.set(Number(roomId), Math.max(Number(at), this.#readAt.get(Number(roomId)) ?? 0))
+    }
+
     if (room) {
       room.classList.remove(this.unreadClass)
       this.dispatch("read", { detail: { targetId: roomId } })
@@ -47,11 +52,11 @@ export default class extends Controller {
     this.#disconnected = true
   }
 
-  #unread({ roomId }) {
+  #unread({ roomId, at }) {
     const unreadRoom = this.#findRoomTarget(roomId)
 
     if (unreadRoom) {
-      if (Current.room.id != roomId) {
+      if (Current.room.id != roomId && !this.#readSince(roomId, at)) {
         unreadRoom.classList.add(this.unreadClass)
       }
 
@@ -59,6 +64,12 @@ export default class extends Controller {
     }
   }
 
+  // Notices fan out one member at a time, so one can arrive after the member has already
+  // read the room in another tab. It still reorders the room, but doesn't mark it unread.
+  #readSince(roomId, at) {
+    return Number(at) <= this.#readAt.get(Number(roomId))
+  }
+
   #findRoomTarget(roomId) {
     return this.roomTargets.find(roomTarget => roomTarget.dataset.roomId == roomId)
   }

app/jobs/message/broadcast_unread_room_job.rb

@@ -0,0 +1,5 @@
+class Message::BroadcastUnreadRoomJob < ApplicationJob
+  def perform(message)
+    message.broadcast_unread_room
+  end
+end

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