ashb opened a new pull request #20931:
URL: https://github.com/apache/airflow/pull/20931


   Set upstream dependencies when an XComArg is used in a MappedOperator, and 
(de)serialize them correctly.
   
   We can only re-create XComArg at the DAG level as we need to get hold of the 
Operator (not just a task_id) so we need a two phase approach here: when 
deserializing operators we create them as a place-holder class (`_XcomRef`) and 
then "up a level" when deserializing the DAG we turn these back in to XComArg 
objects.
   
   (And in so doing we needed to fix a bug or two in serializing MappedOperator 
that have a DAG -- it caused a recursion error.)
   
   This PR could possibly be split in to two (one to set deps, a second to 
serialize them.
   
   
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