mirror of https://github.com/zulip/zulip.git
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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| .. | ||
| actions | ||
| data_import | ||
| integration_fixtures/nagios | ||
| lib | ||
| management | ||
| migrations | ||
| models | ||
| openapi | ||
| tests | ||
| tornado | ||
| transaction_tests | ||
| views | ||
| webhooks | ||
| worker | ||
| __init__.py | ||
| apps.py | ||
| context_processors.py | ||
| decorator.py | ||
| filters.py | ||
| forms.py | ||
| logging_handlers.py | ||
| middleware.py | ||
| signals.py | ||