embed_links: Take a lock on the message object while editing.

We leave the fetching of links outside of the lock, as they could take
seconds, which is an unreasonable amount of time to hold a lock on the
message row.  This may result in unnecessary work, in the case that
the message was since edited, but the unnecessary work is preferable
to blocking other work on the message row for the duration.

zerver/worker/queue_processors.py

@@ -869,12 +869,16 @@ class FetchLinksEmbedData(QueueProcessingWorker):
                 "Time spent on get_link_embed_data for %s: %s", url, time.time() - start_time
             )
 
-        message = Message.objects.get(id=event["message_id"])
+        with transaction.atomic():
+            message = Message.objects.select_for_update().get(id=event["message_id"])
             # If the message changed, we will run this task after updating the message
             # in zerver.actions.message_edit.check_update_message
             if message.content != event["message_content"]:
                 return
-        if message.content is not None:
+
+            if message.content is None:
+                return
+
             query = UserMessage.objects.filter(
                 message=message.id,
             )