upload: Generate thumbnails when images are uploaded.

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.

zerver/models/__init__.py

@@ -25,6 +25,7 @@ from zerver.models.messages import ArchivedSubMessage as ArchivedSubMessage
 from zerver.models.messages import ArchivedUserMessage as ArchivedUserMessage
 from zerver.models.messages import ArchiveTransaction as ArchiveTransaction
 from zerver.models.messages import Attachment as Attachment
+from zerver.models.messages import ImageAttachment as ImageAttachment
 from zerver.models.messages import Message as Message
 from zerver.models.messages import OnboardingUserMessage as OnboardingUserMessage
 from zerver.models.messages import Reaction as Reaction

zerver/models/messages.py

@@ -665,6 +665,18 @@ class ArchivedUserMessage(AbstractUserMessage):
         return f"{recipient_string} / {self.user_profile.email} ({self.flags_list()})"
 
 
+class ImageAttachment(models.Model):
+    realm = models.ForeignKey(Realm, on_delete=CASCADE)
+    path_id = models.TextField(db_index=True, unique=True)
+
+    original_width_px = models.IntegerField()
+    original_height_px = models.IntegerField()
+    frames = models.IntegerField()
+
+    # Contains a list of zerver.lib.thumbnail.StoredThumbnailFormat objects, serialized
+    thumbnail_metadata = models.JSONField(default=list, null=False)
+
+
 class AbstractAttachment(models.Model):
     file_name = models.TextField(db_index=True)