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.
Earlier, we were using 'send_event' & 'queue_json_publish' in
'do_send_messages' which can lead to a situation where we enqueue
events but the transaction fails at a later stage.
Events should not be sent until we know we're not rolling back.
Previously, this code:
```python3
old_archived_attachments = ArchivedAttachment.objects.annotate(
has_other_messages=Exists(
Attachment.objects.filter(id=OuterRef("id"))
.exclude(messages=None)
.exclude(scheduled_messages=None)
)
).filter(messages=None, create_time__lt=delta_weeks_ago, has_other_messages=False)
```
...protected from removal any ArchivedAttachment objects where there
was an Attachment which had _both_ a message _and_ a scheduled
message, instead of _either_ a message _or_ a scheduled message.
Since files are removed from disk when the ArchivedAttachment rows are
deleted, this meant that if an upload was referenced in two messages,
and one was deleted, the file was permanently deleted when the
ArchivedMessage and ArchivedAttachment were cleaned up, despite being
still referenced in live Messages and Attachments.
Switch from `.exclude(messages=None).exclude(scheduled_messages=None)`
to `.exclude(messages=None, scheduled_messages=None)` which "OR"s
those conditions appropriately.
Pull the relevant test into its own file, and expand it significantly
to cover this, and other, corner cases.
Fixes#25414.
We add Attachment.scheduled_messages relation to track ScheduledMessages
which reference the attachment.
The import bits can be done after merging this, by updating #25345.
This function is oblivious to the existence of ArchivedAttachment, which
is incorrect. A file can be removed if and only if it is not referenced
by any Messages or ArchivedMessages.
Alex Vandiver
Anders Kaseorg
Prakhar Pratyush
bjorn3
Mateusz Mandera
Zixuan James Li