3cbbf2307b
This prevents a deadlock between the thumbnailing worker and message sending, as follows: 1. A user uploads an image, making Attachment and ImageAttachment rows, as well as enqueuing a job in the thumbnailing queue. 2. Message sending starts a transaction, creates the Message row, and calls `do_claim_attachments`, which edits the Attachment row of the upload (implicitly locking it). 3. The thumbnailing worker starts a transaction, locks the ImageAttachment row for its image, thumbnails it, and then attempts to `select_for_update()` the message objects (joined to the Attachments table) to find the ones which link to the attachment in question. This query blocks, since "a locking clause without a table list affects all tables used in the statement"[^1] and the message-send request already has a write lock on the Attachments row in question. 4. The message-send request attempts to re-fetch the ImageAttachment row inside the transaction, which tries to pull a lock on it. 5. Deadlock, because the message-send request has the Attachment lock, and waits for the ImageAttachment lock; the thumbnailing worker has the ImageAttachment lock, and waits for the Attachment lock. We break this deadlock by limiting the `update_message_rendered_content` `select_for_update` to only take the lock on the Message table, and not also the Attachments table -- no changes will be made to the Attachments, so no lock is necessary there. This allows the thumbnailing worker to successfully pull the empty list of messages (since the message-send request has not commits its transaction, and thus the Message row is not visible yet), and release its ImageAttachment lock so that the message-send request can proceed. [^1]: https://www.postgresql.org/docs/current/sql-select.html#SQL-FOR-UPDATE-SHARE |
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README.md
Zulip overview
Zulip is an open-source team collaboration tool with unique topic-based threading that combines the best of email and chat to make remote work productive and delightful. Fortune 500 companies, leading open source projects, and thousands of other organizations use Zulip every day. Zulip is the only modern team chat app that is designed for both live and asynchronous conversations.
Zulip is built by a distributed community of developers from all around the world, with 74+ people who have each contributed 100+ commits. With over 1000 contributors merging over 500 commits a month, Zulip is the largest and fastest growing open source team chat project.
Come find us on the development community chat!
Getting started
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Contributing code. Check out our guide for new contributors to get started. We have invested in making Zulip’s code highly readable, thoughtfully tested, and easy to modify. Beyond that, we have written an extraordinary 150K words of documentation for Zulip contributors.
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Contributing non-code. Report an issue, translate Zulip into your language, or give us feedback. We'd love to hear from you, whether you've been using Zulip for years, or are just trying it out for the first time.
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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 about Zulip's unique approach to organizing conversations.
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Running a Zulip server. Self-host Zulip directly on Ubuntu or Debian Linux, in Docker, or with prebuilt images for Digital Ocean and Render. Learn more about self-hosting Zulip.
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Using Zulip without setting up a server. Learn about Zulip Cloud hosting options. Zulip sponsors free Zulip Cloud Standard for hundreds of worthy organizations, including fellow open-source projects.
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Participating in outreach programs like Google Summer of Code and Outreachy.
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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 help others find Zulip.
You may also be interested in reading our blog, and following us on Twitter and LinkedIn.
Zulip is distributed under the Apache 2.0 license.