Synchronize entity dynamic imports (#1794)

Co-authored-by: Arkadii Yakovets <ark@cho.red>

AUTHORS

@@ -52,6 +52,7 @@ Heikki Orsila
 Henrik Sozzi
 Hugh McNamara
 Hugo van Kemenade
+Isabelle COWAN-BERGMAN
 Jacky Han
 Jacob Punter
 Jaemin Kim

holidays/registry.py

@@ -11,6 +11,7 @@
 #  License: MIT (see LICENSE file)
 
 import importlib
+from threading import RLock
 from typing import Any, Dict, Iterable, Optional, Tuple, Union
 
 from holidays.holiday_base import HolidayBase
@@ -178,6 +179,11 @@
     "ny_stock_exchange": ("NewYorkStockExchange", "NYSE", "XNYS"),
 }
 
+# A re-entrant lock. Once a thread has acquired a re-entrant lock,
+# the same thread may acquire it again without blocking.
+# https://docs.python.org/3/library/threading.html#rlock-objects
+IMPORT_LOCK = RLock()
+
 
 class EntityLoader:
     """Country and financial holidays entities lazy loader."""
@@ -223,7 +229,10 @@ def __str__(self) -> str:
     def get_entity(self) -> Optional[HolidayBase]:
         """Return lazy-loaded entity."""
         if self.entity is None:
-            self.entity = getattr(importlib.import_module(self.module_name), self.entity_name)
+            # Avoid deadlock due to importlib.import_module not being thread-safe by caching all
+            # the first imports in a dedicated thread.
+            with IMPORT_LOCK:
+                self.entity = getattr(importlib.import_module(self.module_name), self.entity_name)
 
         return self.entity