collectionviews: Synchronize concurrent ensure_initiated functions

It is possible for concurrent requests to be served by different threads
in the same Python process, resulting in unexpected behaviour if the
wrapper function from ensure_initiated is not finished executing for the
first time. To solve this, wrap the critical code in a lock.

https://phabricator.endlessm.com/T35159

kolibri_explore_plugin/collectionviews.py

@@ -6,6 +6,7 @@
 import time
 from enum import auto
 from enum import IntEnum
+from threading import RLock
 
 from django.utils.translation import gettext_lazy as _
 from kolibri.core.content.errors import InsufficientStorageSpaceError
@@ -72,6 +73,8 @@
     "completed": 1,
 }
 
+ENSURE_INITIATED_RLOCK = RLock()
+
 
 class ChannelNotImported(Exception):
     pass
@@ -763,9 +766,10 @@ def ensure_initiated(api_function):
     """Decorator to initiate only once in the first API call."""
 
     def wrapper(*args, **kwargs):
-        if _collection_download_manager is None:
-            _read_content_manifests()
-        return api_function(*args, **kwargs)
+        with ENSURE_INITIATED_RLOCK:
+            if _collection_download_manager is None:
+                _read_content_manifests()
+            return api_function(*args, **kwargs)
 
     return wrapper