worker: fix reparenting for out-of-order emails

The current reparenting approach sometimes doesn't work. This commit
fixes three issues in the implementation:

1. If an email is being reparented that was previously the root of a
   thread, then it's `thread_id` is `None`. In this case, add its `id`
   to the list of thread IDs to update.
2. The update statement's where clause uses an `in` expressions, which
   is evaluated by Python immediately. Instead, use the `.in_` function.
3. The update statement is not tied to the session and never gets
   executed. Explicitly `execute()` it.

listssrht/process.py

@@ -264,10 +264,13 @@ def _archive(dest, envelope, do_webhooks=True):
     for child in children:
         child.parent_id = mail.id
         if child.thread_id != thread.id:
-            ex_threads.update({ child.thread_id })
+            ex_thread_id = child.thread_id if child.thread_id else child.id
+            ex_threads.update({ ex_thread_id })
         child.thread_id = thread.id
-    (Email.__table__.update().where(Email.thread_id in ex_threads)
-            .values(thread_id=thread.id))
+    if len(ex_threads) > 0:
+        db.session.execute(Email.__table__.update()
+                .where(Email.thread_id.in_(ex_threads))
+                .values(thread_id=thread.id))
 
     db.session.flush()
     # Update thread nreplies & nparticipants