DEV: Prevent race condition when keeping site in readonly mode.

lib/discourse.rb

@@ -311,7 +311,7 @@ module Discourse
       $redis.set(key, 1)
     else
       $redis.setex(key, READONLY_MODE_KEY_TTL, 1)
-      keep_readonly_mode(key)
+      keep_readonly_mode(key) if !Rails.env.test?
     end
 
     MessageBus.publish(readonly_channel, true)
@@ -321,16 +321,19 @@ module Discourse
 
   def self.keep_readonly_mode(key)
     # extend the expiry by 1 minute every 30 seconds
-    unless Rails.env.test?
+    @mutex ||= Mutex.new
+
+    @mutex.synchronize do
       @dbs ||= Set.new
       @dbs << RailsMultisite::ConnectionManagement.current_db
       @threads ||= {}
 
       unless @threads[key]&.alive?
         @threads[key] = Thread.new do
-          while @dbs.size > 0
+          while @dbs.size > 0 do
             sleep 30
 
+            @mutex.synchronize do
               @dbs.each do |db|
                 RailsMultisite::ConnectionManagement.with_connection(db) do
                   if !$redis.expire(key, READONLY_MODE_KEY_TTL)
@@ -343,6 +346,7 @@ module Discourse
         end
       end
     end
+  end
 
   def self.disable_readonly_mode(key = READONLY_MODE_KEY)
     $redis.del(key)