Merge branch 'master' into glitch-soc/merge-upstream

Conflicts:
- app/services/post_status_service.rb
  Small conflict due to handling of instance-local toots.
  A subsequent change is required to ensure instance-local polls are not leaked
  through Update.

app/controllers/settings/exports_controller.rb

@@ -9,11 +9,25 @@ class Settings::ExportsController < Settings::BaseController
   end
 
   def create
-    authorize :backup, :create?
+    raise Mastodon::NotPermittedError unless user_signed_in?
+
+    backup = nil
+
+    RedisLock.acquire(lock_options) do |lock|
+      if lock.acquired?
+        authorize :backup, :create?
+        backup = current_user.backups.create!
+      else
+        raise Mastodon::RaceConditionError
+      end
+    end
 
-    backup = current_user.backups.create!
     BackupWorker.perform_async(backup.id)
 
     redirect_to settings_export_path
   end
+
+  def lock_options
+    { redis: Redis.current, key: "backup:#{current_user.id}" }
+  end
 end