Sidekiq の rack gem アップグレード後のレンダリング失敗

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説明:
Discourse のコミット 9e399b42b96f115a8367c368862d62aeecfac680 へのアップグレード時に、Sidekiq が読み込めなくなります。これは 2 コンテナ構成（./discourse-setup 2container から取得した Web と DB）で実行されています。

不具合の動作:
<あなたの Discourse ドメイン名.tld>/sidekiq にアクセスすると、「このディスカッションフォーラムを動作させるソフトウェアで予期しない問題が発生しました。ご迷惑をおかけして申し訳ありません」というエラーが表示されます。

ブートストラップ、破壊、再起動を行ってもこの動作は続きます。DB コンテナと Web コンテナの両方を再構築しても、同様の動作が発生します。

期待される動作:
Sidekiq のダッシュボードが開くことです。

自己診断:
<あなたの Discourse ドメイン名.tld>/logs にアクセスすると、エラーが 1 つだけ表示されます（すべてのエラーをクリアし、Sidekiq ページを表示してから確認しました）。エラーのバックトレースと情報は、以下の関連する「詳細を隠す」ボックスにあります。

情報

NoMethodError (undefined method transform_keys' for #<ActionDispatch::Request::Session:0x7fb46388f648 not yet loaded>) /var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:212:in stringify_keys’

バックトレース 
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:212:in `stringify_keys'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:148:in `update'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:317:in `prepare_session'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:276:in `context'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:271:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:77:in `block in call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:61:in `each'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:61:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/builder.rb:176:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/sidekiq-6.0.4/lib/sidekiq/web.rb:104:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/sidekiq-6.0.4/lib/sidekiq/web.rb:109:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/routing/mapper.rb:19:in `block in <class:Constraints>'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/routing/mapper.rb:48:in `serve'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/journey/router.rb:49:in `block in serve'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/journey/router.rb:32:in `each'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/journey/router.rb:32:in `serve'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/routing/route_set.rb:837:in `call'
/var/www/discourse/lib/middleware/omniauth_bypass_middleware.rb:68:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/tempfile_reaper.rb:17:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/conditional_get.rb:27:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/head.rb:14:in `call'
/var/www/discourse/lib/content_security_policy/middleware.rb:12:in `call'
/var/www/discourse/lib/middleware/anonymous_cache.rb:318:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:277:in `context'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/session/abstract/id.rb:271:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/cookies.rb:648:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/callbacks.rb:27:in `block in call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/activesupport-6.0.1/lib/active_support/callbacks.rb:101:in `run_callbacks'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/callbacks.rb:26:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/actionable_exceptions.rb:17:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/debug_exceptions.rb:32:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/logster-2.5.1/lib/logster/middleware/reporter.rb:43:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.1/lib/rails/rack/logger.rb:38:in `call_app'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.1/lib/rails/rack/logger.rb:28:in `call'
/var/www/discourse/config/initializers/100-quiet_logger.rb:18:in `call'
/var/www/discourse/config/initializers/100-silence_logger.rb:31:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/request_id.rb:27:in `call'
/var/www/discourse/lib/middleware/enforce_hostname.rb:17:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/method_override.rb:24:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/executor.rb:14:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/sendfile.rb:113:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/actionpack-6.0.1/lib/action_dispatch/middleware/host_authorization.rb:77:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-mini-profiler-1.1.4/lib/mini_profiler/profiler.rb:184:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/message_bus-2.2.3/lib/message_bus/rack/middleware.rb:57:in `call'
/var/www/discourse/lib/middleware/request_tracker.rb:181:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.1/lib/rails/engine.rb:526:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.1/lib/rails/railtie.rb:190:in `public_send'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/railties-6.0.1/lib/rails/railtie.rb:190:in `method_missing'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:77:in `block in call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:61:in `each'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rack-2.1.1/lib/rack/urlmap.rb:61:in `call'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/unicorn-5.5.2/lib/unicorn/http_server.rb:605:in `process_client'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/unicorn-5.5.2/lib/unicorn/http_server.rb:700:in `worker_loop'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/unicorn-5.5.2/lib/unicorn/http_server.rb:548:in `spawn_missing_workers'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/unicorn-5.5.2/lib/unicorn/http_server.rb:144:in `start'
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/unicorn-5.5.2/bin/unicorn:128:in `<top (required)>'
/var/www/discourse/vendor/bundle/ruby/2.6.0/bin/unicorn:23:in `load'
/var/www/discourse/vendor/bundle/ruby/2.6.0/bin/unicorn:23:in `<main>'

バックトレースの解釈に基づくと、この問題は以下の PR による Rack gem のアップグレードが原因ではないかと考えています。

「いいね！」 5
3

これは以下の通り修正済みです：

Rack のアップグレードについては、別の TODO を追跡しています。

「いいね！」 7