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
Messages are rendered outside of a transaction, for performance
reasons, and then sent inside of one. This opens thumbnailing up to a
race where the thumbnails have not yet been written when the message
is rendered, but the message has not been sent when thumbnailing
completes, causing `rewrite_thumbnailed_images` to be a no-op and the
message being left with a spinner which never resolves.
Explicitly lock and use he ImageAttachment data inside the
message-sending transaction, to rewrite the message content with the
latest information about the existing thumbnails.
Despite the thumbnailing worker taking a lock on Message rows to
update them, this does not lead to deadlocks -- the INSERT of the
Message rows happens in a transaction, ensuring that either the
message rending blocks the thumbnailing until the Message row is
created, or that the `rewrite_thumbnailed_images` and Message INSERT
waits until thumbnailing is complete (and updated no Message rows).
This allows clients to potentially lay out the thumbnails more
intelligently, or to provide a better "progressive-load" experience
when enlarging the thumbnail.
A new table is created to track which path_id attachments are images,
and for those their metadata, and which thumbnails have been created.
Using path_id as the effective primary key lets us ignore if the
attachment is archived or not, saving some foreign key messes.
A new worker is added to observe events when rows are added to this
table, and to generate and store thumbnails for those images in
differing sizes and formats.
Anders Kaseorg
Alex Vandiver