Calculate value to be compared instead of using updated_at

updated_at breaks when uploading attachments, as the same page will be
updated without us having a chance to update the header.

Instead, calculate an "etag" value from the meeting sections and agenda
items timestamps as well as the meeting's own lock_version.

frontend/src/stimulus/controllers/poll-for-changes.controller.ts

@@ -38,7 +38,7 @@ export default class PollForChangesController extends ApplicationController {
     updatedAt: String,
   };
 
-  declare updatedAtValue:string;
+  declare referenceValue:string;
   declare urlValue:string;
   declare intervalValue:number;
 
@@ -58,7 +58,7 @@ export default class PollForChangesController extends ApplicationController {
   }
 
   async triggerTurboStream():Promise<void> {
-    await fetch(`${this.urlValue}?updatedAt=${this.updatedAtValue}`, {
+    await fetch(`${this.urlValue}?reference=${this.referenceValue}`, {
       headers: {
         Accept: 'text/vnd.turbo-stream.html',
       },

modules/meeting/app/components/meetings/header_component.html.erb

@@ -3,7 +3,7 @@
     render(Primer::OpenProject::PageHeader.new(
       data: {
         controller: "poll-for-changes",
-        poll_for_changes_updated_at_value: @meeting.updated_at.iso8601,
+        poll_for_changes_reference_value: @meeting.changed_hash,
         poll_for_changes_url_value: check_for_updates_meeting_path(@meeting),
         poll_for_changes_interval_value: check_for_updates_interval,
       }

modules/meeting/app/controllers/meetings_controller.rb

@@ -76,10 +76,10 @@ def show
   end
 
   def check_for_updates
-    if Time.zone.parse(params[:updatedAt]) < @meeting.updated_at.change(usec: 0)
-      respond_with_flash(Meetings::UpdateFlashComponent.new(meeting: @meeting))
-    else
+    if params[:updatedAt] == @meeting.changed_hash
       head :no_content
+    else
+      respond_with_flash(Meetings::UpdateFlashComponent.new(meeting: @meeting))
     end
   end
 

modules/meeting/app/models/meeting.rb

@@ -110,6 +110,25 @@ class Meeting < ApplicationRecord
     closed: 5
   }
 
+  ##
+  # Cache key for detecting changes to be shown to the user
+  def changed_hash
+    sql = <<~SQL
+      SELECT MAX(meeting_agenda_items.updated_at), MAX(meeting_sections.updated_at), MAX(meetings.lock_version) FROM meetings
+      LEFT JOIN meeting_agenda_items ON meeting_agenda_items.meeting_id = meetings.id
+      LEFT JOIN meeting_sections ON meeting_sections.meeting_id = meetings.id
+      WHERE meetings.id = :id
+    SQL
+
+    parts = ActiveRecord::Base
+      .connection
+      .exec_query(OpenProject::SqlSanitization.sanitize(sql, id:))
+      .rows
+      .flatten
+
+    OpenProject::Cache::CacheKey.expand(parts)
+  end
+
   ##
   # Return the computed start_time when changed
   def start_time