ashb opened a new pull request #8909:
URL: https://github.com/apache/airflow/pull/8909


   We noticed our Celery tests failing sometimes with
   
   > (psycopg2.errors.UniqueViolation) duplicate key value violates unique
   > constraint "pg_type_typname_nsp_index"
   > DETAIL:  Key (typname, typnamespace)=(celery_tasksetmeta, 2200) already 
exists
   
   It appears this is a race condition in SQLAlchemy's "create_all()"
   function, where it first checks which tables exist, builds up a list of
   `CREATE TABLE` statements, then issues them. Thus if two celery worker
   processes start at the same time, they will find the the table doesn't
   yet exist, and both try to create it.
   
   This is _probably_ a bug in SQLA, but this should be an easy enough fix
   here, to just ensure that the table exists before launching any Celery tasks.
   
   Fixes #8909
   
   ---
   Make sure to mark the boxes below before creating PR: [x]
   
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   - [x] Relevant documentation is updated including usage instructions.
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Example](https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#contribution-workflow-example).
   
   ---
   In case of fundamental code change, Airflow Improvement Proposal 
([AIP](https://cwiki.apache.org/confluence/display/AIRFLOW/Airflow+Improvements+Proposals))
 is needed.
   In case of a new dependency, check compliance with the [ASF 3rd Party 
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[UPDATING.md](https://github.com/apache/airflow/blob/master/UPDATING.md).
   Read the [Pull Request 
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