Ensure portdb selects _all_ rows with negative rowids (#13226)

changelog.d/13226.bugfix

+Fix a long-standing bug where the `synapse_port_db` script could fail to copy rows with negative row ids.

synapse/_scripts/synapse_port_db.py

@@ -418,12 +418,15 @@ class Porter:
             self.progress.update(table, table_size)  # Mark table as done
             return
 
+        # We sweep over rowids in two directions: one forwards (rowids 1, 2, 3, ...)
+        # and another backwards (rowids 0, -1, -2, ...).
         forward_select = (
             "SELECT rowid, * FROM %s WHERE rowid >= ? ORDER BY rowid LIMIT ?" % (table,)
         )
 
         backward_select = (
-            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid LIMIT ?" % (table,)
+            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid DESC LIMIT ?"
+            % (table,)
         )
 
         do_forward = [True]