FIX: Don't attempt to migrate concurrently with other migrations (#14231)

lib/tasks/db.rake

@@ -114,6 +114,7 @@ task 'multisite:migrate' => ['db:load_config', 'environment', 'set_locale'] do |
     raise "Multisite migrate is only supported in production"
   end
 
+  DistributedMutex.synchronize('db_migration', redis: Discourse.redis.without_namespace, validity: 300) do
     # TODO: Switch to processes for concurrent migrations because Rails migration
     # is not thread safe by default.
     concurrency = 1
@@ -202,9 +203,11 @@ task 'multisite:migrate' => ['db:load_config', 'environment', 'set_locale'] do |
 
     Rake::Task['db:_dump'].invoke
   end
+end
 
 # we need to run seed_fu every time we run rake db:migrate
 task 'db:migrate' => ['load_config', 'environment', 'set_locale'] do |_, args|
+  DistributedMutex.synchronize('db_migration', redis: Discourse.redis.without_namespace, validity: 300) do
     migrations = ActiveRecord::Base.connection.migration_context.migrations
     now_timestamp = Time.now.utc.strftime('%Y%m%d%H%M%S').to_i
     epoch_timestamp = Time.at(0).utc.strftime('%Y%m%d%H%M%S').to_i
@@ -233,6 +236,7 @@ task 'db:migrate' => ['load_config', 'environment', 'set_locale'] do |_, args|
       system("RAILS_DB=discourse_test_multisite rake db:migrate")
     end
   end
+end
 
 task 'test:prepare' => 'environment' do
   I18n.locale = SiteSetting.default_locale rescue :en