003fa7adda
Fundamentally, we should take a write lock on the message, check its validity for a change, and then make and commit that change. Previously, `check_update_message` did not operate in a transaction, but `do_update_message` did -- which led to the ordering: - `check_update_message` reads Message, not in a transaction - `check_update_message` verifies properties of the Message - `do_update_message` starts a transaction - `do_update_message` takes a read lock on UserMessage - `do_update_message` writes on UserMessage - `do_update_message` writes Message - `do_update_message` commits This leads to race conditions, where the `check_update_message` may have verified based on stale data, and `do_update_message` may improperly overwrite it; as well as deadlocks, where other (properly-written) codepaths take a write lock on Message _before_ updating UserMessage, and thus deadlock with `do_update_message`. Change `check_update_message` to open a transaction, and take the write lock when first accessing the Message row. We update the comment above `do_update_message` to clarify this expectation. The new ordering is thus: - `check_update_message` starts a transaction - `check_update_message` takes a write lock on Message - `check_update_message` verifies properties of the Message - `do_update_message` writes on UserMessage - `do_update_message` writes Message - `check_update_message` commits
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.
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