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Python's behaviour on `sys.exit` is to wait for all non-daemon threads to exit. In the context of the missedmessage_emails worker, if any work is pending, a non-daemon Timer thread exists, which is waiting for 5 seconds. As soon as that thread is serviced, it sets up another 5-second Timer, a process which repeats until all ScheduledMessageNotificationEmail records have been handled. This likely takes two minutes, but may theoretically take up to a week until the thread exits, and thus sys.exit can complete. Supervisor only gives the process 30 seconds to shut down, so something else must prevent this endless Timer. When `stop` is called, take the lock so we can mutate the timer. However, since `stop` may have been called from a signal handler, our thread may _already_ have the lock. As Python provides no way to know if our thread is the one which has the lock, make the lock a re-entrant one, allowing us to always try to take it. With the lock in hand, cancel any outstanding timers. A race exists where the timer may not be able to be canceled because it has finished, maybe_send_batched_emails has been called, and is itself blocked on the lock. Handle this case by timing out the thread join in `stop()`, and signal the running thread to exit by unsetting the timer event, which will be detected once it claims the lock.
Zulip overview
Zulip is a powerful, open source group chat application that combines the immediacy of real-time chat with the productivity benefits of threaded conversations. Zulip is used by open source projects, Fortune 500 companies, large standards bodies, and others who need a real-time chat system that allows users to easily process hundreds or thousands of messages a day. With over 700 contributors merging over 500 commits a month, Zulip is also the largest and fastest growing open source group chat project.
Getting started
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You might be interested in:
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Contributing code. Check out our guide for new contributors to get started. Zulip prides itself on maintaining a clean and well-tested codebase, and a stock of hundreds of beginner-friendly issues.
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Contributing non-code. Report an issue, translate Zulip into your language, write for the Zulip blog, or give us feedback. We would love to hear from you, even if you're just trying the product out.
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Supporting Zulip. Advocate for your organization to use Zulip, become a sponsor, write a review in the mobile app stores, or upvote Zulip on product comparison sites.
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Checking Zulip out. The best way to see Zulip in action is to drop by the Zulip community server. We also recommend reading Zulip for open source, Zulip for companies, or Zulip for communities.
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Running a Zulip server. Use a preconfigured DigitalOcean droplet, install Zulip directly, or use Zulip's experimental Docker image. Commercial support is available; see https://zulip.com/plans for details.
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Using Zulip without setting up a server. https://zulip.com offers free and commercial hosting, including providing our paid plan for free to fellow open source projects.
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Participating in outreach programs like Google Summer of Code.
You may also be interested in reading our blog or following us on Twitter. Zulip is distributed under the Apache 2.0 license.