Sidekiq crashes with sd_markdown_render error when under high load

Sidekiq crashes almost daily with this error: ``` 2017-08-17T16:26:58.515Z 4084 TID-ox1fzyws8 ActionMailer::DeliveryJob JID-f2593e2cebd93caba716c59c INFO: done: 5.978 sec itlab [23 of 25 busy]: markdown.c:2897: sd_markdown_render: Assertion `md->work_bufs[0].size == 0' failed. ``` It would appear this occurs when sidekiq is under a fairly high load, with Sidekiq hitting 20+. Restarting gitlab resolves the issue until the next time. Note that after restarting Gitlab Sidekiq caps out at 25 until the queue is empty again. <details> <summary>Expand for output related to GitLab environment info</summary> <pre> System information System: Ubuntu 14.04 Current User: git Using RVM: no Ruby Version: 2.3.3p222 Gem Version: 2.5.2 Bundler Version:1.15.3 Rake Version: 10.5.0 Redis Version: 2.8.4 Git Version: 2.7.3 Sidekiq Version:5.0.0 Go Version: go1.8.3 linux/amd64 GitLab information Version: 9.4.4 Revision: 2c79d92 Directory: /home/git/gitlab DB Adapter: postgresql URL: https://gitlab.domain.com HTTP Clone URL: https://gitlab.domain.com/some-group/some-project.git SSH Clone URL: git@gitlab.domain.com:some-group/some-project.git Using LDAP: no Using Omniauth: yes Omniauth Providers: khronos GitLab Shell Version: 5.3.1 Repository storage paths: - default: /home/git/repositories Hooks: /home/git/gitlab-shell/hooks Git: /usr/local/bin/git </pre> </details> ### Possible fixes This error is possibly related to redcarpet and these issues: https://github.com/vmg/redcarpet/issues/570 https://github.com/vmg/redcarpet/issues/176

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