NullPool for the celery worker. (#1465)

caravel/sql_lab.py

@@ -10,6 +10,7 @@
     app, db, models, utils, dataframe, results_backend)
 from caravel.db_engine_specs import LimitMethod
 from caravel.jinja_context import process_template
+from sqlalchemy.pool import NullPool
 QueryStatus = models.QueryStatus
 
 celery_app = celery.Celery(config_source=app.config.get('CELERY_CONFIG'))
@@ -46,9 +47,15 @@ def create_table_as(sql, table_name, schema=None, override=False):
     return exec_sql.format(**locals())
 
 
-@celery_app.task
-def get_sql_results(query_id, return_results=True, store_results=False):
+@celery_app.task(bind=True)
+def get_sql_results(self, query_id, return_results=True, store_results=False):
     """Executes the sql query returns the results."""
+
+    # disable pooling for the celery task to prevent
+    # MySQL server gone issue.
+    if not self.request.called_directly:
+        db.engine.pool = NullPool(db.engine.create)
+
     session = db.session()
     session.commit()  # HACK
     query = session.query(models.Query).filter_by(id=query_id).one()