Fixed #18251 -- Removed a deadlock possibility in apploading

Thanks to harm for the report and comments.

django/db/models/loading.py

@@ -6,9 +6,9 @@
 from django.utils.importlib import import_module
 from django.utils.module_loading import module_has_submodule
 
+import imp
 import sys
 import os
-import threading
 
 __all__ = ('get_apps', 'get_app', 'get_models', 'get_model', 'register_models',
         'load_app', 'app_cache_ready')
@@ -39,7 +39,6 @@ class AppCache(object):
         handled = {},
         postponed = [],
         nesting_level = 0,
-        write_lock = threading.RLock(),
         _get_models_cache = {},
     )
 
@@ -54,7 +53,13 @@ def _populate(self):
         """
         if self.loaded:
             return
-        self.write_lock.acquire()
+        # Note that we want to use the import lock here - the app loading is
+        # in many cases initiated implicitly by importing, and thus it is
+        # possible to end up in deadlock when one thread initiates loading
+        # without holding the importer lock and another thread then tries to
+        # import something which also lauches the app loading. For details of
+        # this situation see #18251.
+        imp.acquire_lock()
         try:
             if self.loaded:
                 return
@@ -67,7 +72,7 @@ def _populate(self):
                     self.load_app(app_name)
                 self.loaded = True
         finally:
-            self.write_lock.release()
+            imp.release_lock()
 
     def _label_for(self, app_mod):
         """
@@ -138,7 +143,7 @@ def get_app(self, app_label, emptyOK=False):
         the app has no models in it and 'emptyOK' is True, returns None.
         """
         self._populate()
-        self.write_lock.acquire()
+        imp.acquire_lock()
         try:
             for app_name in settings.INSTALLED_APPS:
                 if app_label == app_name.split('.')[-1]:
@@ -151,7 +156,7 @@ def get_app(self, app_label, emptyOK=False):
                         return mod
             raise ImproperlyConfigured("App with label %s could not be found" % app_label)
         finally:
-            self.write_lock.release()
+            imp.release_lock()
 
     def get_app_errors(self):
         "Returns the map of known problems with the INSTALLED_APPS."