[druid] Fixing issue 3894 multi-processing w/ Gunicorn (apache#3895)

superset/connectors/druid/models.py

@@ -4,7 +4,7 @@
 from datetime import datetime, timedelta
 import json
 import logging
-from multiprocessing import Pool
+from multiprocessing.pool import ThreadPool
 
 from dateutil.parser import parse as dparse
 from flask import escape, Markup
@@ -157,7 +157,7 @@ def refresh(self, datasource_names, merge_flag, refreshAll):
         session.flush()
 
         # Prepare multithreaded executation
-        pool = Pool()
+        pool = ThreadPool()
         ds_refresh = list(ds_map.values())
         metadata = pool.map(_fetch_metadata_for, ds_refresh)
         pool.close()