FIX: Discourse.keep_readonly_mode spins a new thread each time it is called.

lib/discourse.rb

@@ -294,7 +294,12 @@ module Discourse
   def self.keep_readonly_mode(key)
     # extend the expiry by 1 minute every 30 seconds
     unless Rails.env.test?
-      Thread.new do
+      @threads ||= {}
+
+      active_thread = @threads[key]
+
+      unless active_thread&.alive?
+        @threads[key] = Thread.new do
           while readonly_mode?(key)
             $redis.expire(key, READONLY_MODE_KEY_TTL)
             sleep 30.seconds
@@ -302,6 +307,7 @@ module Discourse
         end
       end
     end
+  end
 
   def self.disable_readonly_mode(key = READONLY_MODE_KEY)
     $redis.del(key)